Electronics Project Sheet 2

Counting Circuits Binary | 4-bit | BCD | Counters | Ripple/Synchronous | Reset | Freq. division | Decoders | Display drivers | Linking counters Next Page: Quantities and Units Also See: 4000 series ICs | 74 series ICs | Logic Gates Binary numbers Electronic circuits count in binary. This is the simplest possible counting system because it uses just two digits, 0 and 1, exactly like logic signals where 0 represents false and 1 represents true. The terms low and high are also used for 0 and 1 respectively as shown in the table. Counting one, two, three, four, five in binary: 1, 10, 11, 100, 101. Binary numbers rapidly become very long as the count increases and this makes them difficult for us to read at a glance. Fortunately it is rarely necessary to read more than 4 binary digits at a time in counting circuits. In a binary number each digit represents a multiple of two (1, 2, 4, 8, 16 etc), in the same way that each digit in decimal represents a multiple of ten (1, 10, 100, 1000 etc). For example 10110110 in binary equals 182 in decimal: Digit value: 128 64 32 16 8 4 2 1 Logic states True False 1 High +Vs On 0 Low 0V Off Seen on a T-shirt: There are 10 kinds of people - those who understand binary, and those who don't. Binary number: 1 0 1 1 0 1 1 0 Decimal value: 128 + 0 + 32 + 16 + 0 + 4 + 2 + 0 = 182 Bits, bytes and nibbles Each binary digit is called a bit, so 10110110 is an 8-bit number. A block of 8 bits is called a byte and it can hold a maximum number of 11111111 = 255 in decimal. Computers and PIC microcontrollers work with blocks of 8 bits. Two (or more) bytes make a word, for example PICs work with a 16-bit word (two bytes) which can hold a maximum number of 65535. A block of 4 bits is called a nibble (half a byte!) and it can hold a maximum number of 1111 = 15 in decimal. Many counting circuits work with blocks of 4 bits because this number of bits is required to count up to 9 in decimal. (The maximum number with 3 bits is only 7). Hexadecimal (base 16) Hexadecimal (often just called 'hex') is base 16 counting with 16 digits. It starts with the decimal digits 0-9, then continues with letters A (10), B (11), C (12), D (13), E (14) and F (15). Each hexadecimal digit is equivalent to 4 binary digits, making conversion between the two systems relatively easy. You may find hexadecimal used with PICs and computer systems but it is not generally used in simple counting circuits. Example: 10110110 binary = B6 hexadecimal = 182 decimal. Top of page | Binary | 4-bit | BCD | Counters | Ripple/Synchronous | Reset | Freq. division | Decoders | Display drivers | Linking 4-bit numbers The table on the right shows the 4-bit numbers and their decimal values. The labels A,B,C,D are widely used in electronics to represent the four bits: Binary Decimal Hex DCBA base 16 0000 0 0 0001 1 1 0010 2 2 0011 3 3 y A = 1, the 'least significant bit' (LSB) 0100 4 4 y B=2 0101 5 5 y C=4 0110 6 6 y D = 8, the 'most significant bit' (MSB) 0111 7 7 1000 8 8 Binary Coded Decimal, BCD 1001 9 9 1010 10 A Binary Coded Decimal, BCD, is a special version of 4-bit binary 1 0 1 1 11 B where the count resets to zero (0000) after the ninth count 1100 12 C (1001). It is used by decade counters and is easily converted to 1 1 0 1 13 D display the decimal digits 0-9 on a 7-segment display. 1110 14 E 1111 15 F Several decade counters using BCD can be linked together to separately count the decimal ones, tens, hundreds, and so on. This is much easier than attempting to convert large binary numbers (such as 10110110) to display their decimal value. Do not confuse BCD which stands for Binary Coded Decimal with the labels A,B,C,D used to represent the four binary digits; it is an unfortunate coincidence that the letters BCD occur in both! Top of page | Binary | 4-bit | BCD | Counters | Ripple/Synchronous | Reset | Freq. division | Decoders | Display drivers | Linking Counters All counters require a 'square wave' clock signal to make them count. This is a digital waveform with sharp transitions between low (0V) and high (+Vs), such as the output from a 555 astable circuit. Most switches bounce when the contacts close giving a rapid series of pulses. Connecting a switch directly to a clock input will usually give several counts when the switch is operated once! One way to 'debounce' the switch is to make it trigger a 555 monostable circuit with a short time period (such as 0.1s) and use the monostable output to drive the clock input. The animated block diagram shows a clock signal driving a 4-bit (0-15) counter with LEDs connected to show the state of the clock and counter outputs QA-QD (Q indicates an output). A square wave clock signal The bouncing output from a switch A 4-bit counter and clock input In this example counting advances on the falling-edge of the clock signal LED on = 1 LED off = 0 The LED on the first output QA flashes at half the frequency of the clock LED. In fact the frequency of each stage of the counter is half the frequency of the previous stage. You can see this pattern too in the table above showing the 4-bit numbers. Notice how output QA changes state every time the clock input changes from high to low (that is when the clock LED turns off), this is called the falling-edge. If you watch the counting closely you can see that QB changes on the falling-edge of QA, QC on the falling-edge of QB and so on. You may be surprised to see the diagram drawn with the input on the right and signals flowing from right to left, the opposite way to the usual convention in electronics! Drawing counter circuits like this means that the outputs are in the correct binary order for us to read easily and I think this is more helpful than rigidly sticking to the usual 'left to right' convention. Ripple and synchronous counters There are two main types of counter: ripple and synchronous. In simple circuits their behaviour appears almost identical, but their internal structure is very different. A ripple counter contains a chain of flip-flops with the output of each one feeding the input of The operation of a flip-flop the next. A flip-flop output changes state every Notice how the output frequency is half the input frequency time the input changes from high to low (on the falling-edge). This simple arrangement works well, but there is a slight delay as the effect of the clock 'ripples' through the chain of flip-flops. In most circuits the ripple delay is not a problem because it is far too short to be seen on a display. However, a logic system connected to ripple counter outputs will briefly see false counts which may produce 'glitches' in the logic system and may disrupt its operation. For example a ripple counter changing from 0111 (7) to 1000 (8) will very briefly show 0110, 0100 and 0000 before 1000! A synchronous counter has a more complex internal structure to ensure that all its outputs change precisely together on each clock pulse, avoiding the brief false counts which occur with ripple counters. Rising-edge and falling-edge clock inputs Counting occurs when the clock input changes state. y y Most synchronous counters count on the rising-edge which is the low to high transition of the clock signal. Most ripple counters count on the falling-edge which is the high to low transition of the clock signal. It may seem odd that ripple counters use the falling-edge, but in fact this makes it easy to link counters because the most significant bit (MSB) of one counter can drive the clock input of the next. This works because the next bit must change state when the previous bit changes from high to low - the point at which a carry must occur to the next bit. Synchronous counters usually have carry out and carry in pins for linking counters without introducing any ripple delays. Top of page | Binary | 4-bit | BCD | Counters | Ripple/Synchronous | Reset | Freq. division | Decoders | Display drivers | Linking Resetting a counter Counters can be reset to zero before their maximum count by connecting one (or more) of their outputs to their reset input, using an AND gate to combine outputs if necessary. If the reset input is 'active-low' a NOT or NAND gate will be required to produce a low output at the desired count. If you see a line drawn above reset it means it is active low, for example: (say 'resetbar'). The reset function normally occurs immediately and you should reset on the next count above the maximum you require. For example to count 0-5 (0000-0101) you should reset on 6 (0110). Some synchronous counters have a synchronous reset which occurs on the next clock pulse rather than immediately. This is important because you must reset on the maximum count you require. For example to count 0-5 (0000-0101), reset on 5 (0101). Presetting Some counters can be preset by presenting a number to their inputs A-D and activating a preset input to load the number into the counter. By making inputs A-D all low you can also use this to reset the counter to zero. Frequency division Counters can be used to reduce the frequency of an input (clock) signal. Each stage of a counter halves the frequency, so for a 4-bit (0-15) counter QA is 1/2, QB is 1/4, QC is 1 /8 and QD is 1/16 of the clock frequency. Division by numbers that are not powers of 2 is possible by resetting counters. Frequency division is one of the main purposes of counters with more than 4 bits and their outputs are usually labelled Q1, Q2 and so on. Qn is the nth stage of the counter, representing 2n. For example Q4 is 24 = 16 (1/16 of clock frequency) and Q12 is 212 = 4096 (1/4096 of clock frequency). Top of page | Binary | 4-bit | BCD | Counters | Ripple/Synchronous | Reset | Freq. division | Decoders | Display drivers | Linking Decoders The most popular type is a 1-of-10 decoder which contains a network of logic gates to make one of its ten outputs Q0-9 become high (or low) in response to the BCD (binary coded decimal) inputs A-D. For example an input of binary 0101 (=5) will activate output Q5. Decoders can be used for a simple counting display and for switching LEDs in sequences. The outputs must never be directly connected together, but diodes can be used to combine them as shown in the diagram. For example using diodes to combine the 2nd (Q1) and 4th (Q3) outputs will make an LED flash twice followed by a longer gap. The top diagram shows this for a decoder where the outputs become low when activated (such as the 7442), and the bottom diagram for a decoder where the outputs become high when activated (such as the 4028). Top of page | Binary | 4-bit | BCD | Counters | Ripple/Synchronous | Reset | Freq. division | Decoders | Display drivers | Linking 7-segment display drivers The inputs A-D of a display driver are connected to the BCD (binary coded decimal) outputs QA-D from a decade counter. A network of logic gates inside the display driver makes its outputs a-g become high or low as appropriate to light the required segments a-g of a 7-segment display. A resistor is required in series with each segment to protect the LEDs, 330 is a suitable value for many displays with a 4.5V to 6V supply. Beware that these resistors are sometimes omitted from circuit diagrams! There are two types of 7-segment displays: y y Common Anode (CA or SA) with all the LED anodes connected together. These need a display driver with outputs which become low to light each segment, for example the 7447. Connect the common anode to +Vs. Common Cathode (CC or SC) with all the cathodes connected together. These need a display driver with Decade counter with display driver and 7-segment display outputs which become high to light each segment, for example the 4511. Connect the common cathode to 0V. The common anode/cathode is often available on 2 pins. Displays also have a decimal point (DP) but this is not controlled by the display driver. The segments of larger displays have two LEDs in series. For display connections please see your supplier's catalogue or manufacturer's datasheet. Multiplexing If there are many 7-segment display digits multiplexing is usually used. This is a system of switching so that of all the decade counters share a single display driver which is connected to all of the displays. The output of each counter is connected in turn to the inputs of the display driver and at the same time the common anode/cathode of the corresponding 7-segment display is connected so that only one display lights at a time.The switching is done very rapidly (typically 400 - 1000Hz) and the segment current is larger than normal so the display appears continuous and of normal brightness. Multiplexing requires ICs to do the switching, but the complete circuit has fewer ICs than having one display driver for each display. Top of page | Binary | 4-bit | BCD | Counters | Ripple/Synchronous | Reset | Freq. division | Decoders | Display drivers | Linking Linking Counters Counters may be linked together in a chain to count larger numbers. It may seem tempting to use a 12-bit or 14-bit counter, but it is not practical to convert their large binary numbers to decimal. You should use a chain of decade (0-9) counters which use BCD (binary coded decimal) to make the conversion to decimal very easy: the first counts the units, the second counts the tens, the third the hundreds and so on. Some dual counter ICs are available with two separate counters on the same IC, the two counters must be linked externally if required (there is no internal link). The way that counters are linked depends on the nature of the counter. The diagrams below show the general arrangements for standard ripple and synchronous counters but it is important to read the detailed information for particular counters, consulting a datasheet if necessary. Linking ripple counters The diagram below shows how to link standard ripple counters. Notice how the highest output QD of each counter drives the clock (CK) input of the next counter. This works because ripple counters have clock inputs that are 'active-low' which means that the count advances as the clock input becomes low, on the falling-edge. Remember that with all ripple counters there will be a slight delay before the later outputs respond to the clock signal, especially with a long counter chain. This is not a problem in simple circuits driving displays, but it may cause glitches in logic systems connected to the counter outputs. Linking synchronous counters The diagram below shows how to link standard synchronous counters. Notice how all the clock (CK) inputs are linked, and carry out (CO) is used to feed the carry in (CI) of the next counter. This ensures that the entire counter chain is synchronous, with every output changing at the same time. Carry in (CI) of the first counter should be made low or high to suit the particular counter IC being used. Quantities and Units in Electronics Next Page: Books about Electronics Quantity Symbol Voltage Current Charge Resistance Capacitance V I Q R C Usual Unit volt amp* coulomb ohm farad Unit Symbol V A C F Quantities Inductance L henry H Reactance X ohm The table shows electrical quantities which are Impedance Z ohm used in electronics. Power P watt W The relationship between quantities can be Energy E joule J written using words or symbols (letters), but Time t second s symbols are normally used because they are much shorter; for example V is used for Frequency f hertz Hz voltage, I for current and R for resistance: * strictly the unit is ampere, but this is almost always shortened to amp. As a word equation: voltage = current × resistance The same equation using symbols: V = I × R To prevent confusion we normally use the same symbol (letter) for each quantity and these symbols are shown in the second column of the table. Please click on the quantities in the table for further information. Units The first table shows the unit (and unit symbol) which is used to measure each quantity. For example: Charge is measured in coulombs and the symbol for a coulomb is C. Prefix Symbol Value milli micro nano pico m µ n p k M G T 10-3 = 0.001 10-6 = 0.000 001 10-9 = 0.000 000 001 10-12 = 0.000 000 000 001 103 = 1000 106 = 1000 000 109 = 1000 000 000 1012 = 1000 000 000 000 Prefix kilo Some of the units have a convenient size for electronics, but most are either too large mega or too small to be used directly so they are giga used with the prefixes shown in the second tera table. The prefixes make the unit larger or smaller by the value shown. Some examples: 25 mA = 25 × 10-3 A = 25 × 0.001 A = 0.025 A 47µF = 47 × 10-6 F = 47 × 0.000 001 F = 0.000 047 F 270k = 270 × 103 = 270 × 1000 = 270 000 Why not change the units to be better sizes? It might seem a good idea to make the farad (F) much smaller to avoid having to use µF, nF and pF, but if we did this most of the equations in electronics would have to have factors of 1000000 or more included as well as the quantities. Overall it is much better to have the units with their present sizes which are defined logically from the equations. In fact if you use an equation frequently you can use special sets of prefixed units which are more convenient... For example: Ohm's Law, V = I × R the standard units are volt (V), amp (A) and ohm ( ), but you could use volt (V), milliamp (mA) and kilo-ohm (k ) if you prefer. Take care though, you must never mix sets of units: using V, A and k wrong values. in Ohm's Law would give you Soldering | Study | Components | 555 | Symbols | FAQ | Links Books about Electronics For Study and Projects. Next Page: Studying Electronics Books for Studying Electronics The table below shows a selection of books about electronics which may be of interest if you are studying electronics as part of a course at school, or if you have been building projects and wish to learn how the circuits work. Some of the project books explain the operation of their circuits and this can be a good way to learn how they work. Please note that some books are now out of print but you may still be able to obtain them from secondhand bookshops and suppliers such as Amazon. Textbooks Books for beginners, GCSE courses and AS/A level courses. Book Title and Author Basic Skills: ISBN and Publisher ISBN: 0 7195 4449 1 Comments This is a suitable textbook for a beginner. Electronics Publisher: by Tom Duncan John Murray Starting Electronics by Keith Brindley ISBN: 0 7506 4435 4 Publisher: Newnes The practical approach of this book makes it suitable for beginners. Teach Yourself ISBN: 0 3404 2230 0 Electronics Publisher: by Malcolm Hodder & Stoughton Plant Electronics - A ISBN: 0 7506 5545 3 First Course Publisher: Newnes by Owen Bishop Electronics for ISBN: 0 7195 7413 7 Today and Publisher: Tomorrow John Murray by Tom Duncan Success in ISBN: 0 7195 7205 3 Electronics Publisher: by Tom Duncan John Murray Electronics Explained by M W Brimicombe Analogue Electronics by John C Morris Digital Electronics by John C Morris ISBN: 0 17 448303 1 Publisher: Nelson Thornes ISBN: 0 3407 1925 7 Publisher: Newnes A self-study book covering the essentials of electronics. A suitable textbook for GCSE, Intermediate GNVQ and City & Guilds courses. Practical work is introduced almost immediately. A suitable textbook for GCSE and AS/A level. No projects or practical exercises. A self-study textbook for GCSE and AS/A level. No projects or practical exercises. An AS/A level textbook with many practical exercises. An AS/A level textbook with many practical investigations to support its discovery-based approach. Transistors, operational amplifiers, thyristors and triacs are covered. An AS/A level textbook with many practical investigations to support its discovery-based approach. The 555 timer, logic gates, counters, shift registers and displays are covered. ISBN: 0 3405 5638 2 Publisher: Newnes Reference Books Learn how to use the integrated circuits covered by these books. ISBN: 0 85934 047 3 IC 555 Projects Publisher: by E Parr Bernard Babani A Beginners ISBN: 0 85934 333 2 Guide to CMOS Publisher: Digital ICs Bernard Babani The 555 timer IC is used in many projects and this book thoroughly explains its operation and use. There are many circuit diagrams of projects. This book explains how to use the 4000 series CMOS logic gates, counters, display drivers and so on. They are ideal for battery powered by R Penfold A Beginners Guide to TTL Digital ICs by R Penfold ISBN: 0 85934 332 4 Publisher: Bernard Babani projects because they use little power and can tolerate a wide range of supply voltages. This book explains how to use the 74 series TTL logic gates, counters, display drivers and so on. Operational amplifiers are very versatile devices and this book thoroughly explains their operation and use, with many circuit designs for the more experienced constructor. Technical information on many popular integrated circuits (ICs), including the 74 series and 4000 series logic ICs. The book concentrates on the ICs themselves rather than the circuits in which they can be used. How to Use Op- ISBN: 0 85934 063 5 Amps Publisher: Bernard Babani by E Parr Master IC Cookbook by Clayton Hallmark & Delton Horn ISBN: 0 8306 6550 1 out of print try Amazon Practical Books Learn by building projects on breadboard, no soldering is required. Book Title and Author ISBN and Publisher Comments An introduction to electronics by building transistor circuits on S-Dec, a breadboard system which does not require soldering. Learn about electronics by building integrated circuit ('chip') projects on standard breadboard (no soldering required). This is more advanced than Adventures with Electronics (above). Learn about digital electronics by building projects such as traffic lights and a binary 4-bit adder using 4000 series ICs on standard breadboard (no soldering required). This is the most advanced of the three 'Adventures with...' books. Adventures ISBN: 0 7195 3554 9 with Publisher: Electronics John Murray by Tom Duncan Adventures ISBN: 0 7195 3671 5 with MicroPublisher: Electronics John Murray by Tom Duncan Adventures ISBN: 0 7195 3875 0 with Digital out of print Electronics try Amazon by Tom Duncan Rapid Electronics stock a wide range of electronics books including some shown in the table above. Books for Electronics Projects All these books are a good source of circuit diagrams for projects but in most cases you will need to design your own stripboard or PCB layout to build the project. If you plan to build projects from books or magazines that are more than about 10 years old you should check that all the components required are still available. If you want to try designing your own circuits you will need to have a good understanding of electronics. It is best to start by adapting a circuit given in a book. The books for studying electronics include many useful circuit diagrams. Project Books Many of these books just give circuit diagrams. Please be aware that you will need to design your own stripboard or PCB layout to build the project. Book Title and Author IC 555 Projects by E Parr ISBN and Publisher Comments The 555 timer IC is used in many projects ISBN: 0 85934 047 3 and this book thoroughly explains its Publisher: operation and use. There are many circuit Bernard Babani diagrams of projects. Operational amplifiers are very versatile ISBN: 0 85934 063 5 devices and this book thoroughly explains their operation and use, with many circuit Publisher: designs for the more experienced Bernard Babani constructor. ISBN: 0 85934 321 9 Circuit diagrams to help the experienced Publisher: constructor design their own projects. Bernard Babani ISBN: 0 85934 322 7 Circuit diagrams to help the experienced Publisher: constructor design their own projects. Bernard Babani How to Use OpAmps by E Parr Circuit Source Book 1 by R Penfold Circuit Source Book 2 by R Penfold Practical ISBN: 0 85934 384 7 Electronic Model The Publisher: Railway Projects Bernard Babani by R Penfold Electronic Components These pages are intended to help you to identify components, find out their values and learn about their function in circuits. y y y y y y y y y y y y Capacitors Connectors and Cables Diodes including zener diodes Integrated Circuits (Chips) o 4000 series logic ICs (pin connections etc) o 74 series logic ICs (pin connections etc) Lamps LEDs (Light Emitting Diodes) Relays Resistors o Resistor Colour Code Calculator Switches Transistors o Heat sinks for transistors Variable Resistors Other components including LDRs and thermistors Lamps Function | Symbols | Selecting | Types of lamp | Connecting Function and Construction Lamps emit light when an electric current passes through them. All of the lamps shown on this page have a thin wire filament which becomes very hot and glows brightly when a current passes through it. The filament is made from a metal with a high melting point such as tungsten and it is usually wound into a small coil. Filament lamps have a shorter lifetime than most electronic components because eventually the filament 'blows' (melts) at a weak point. Circuit symbols There are two circuit symbols for a lamp, one for a lamp used to provide illumination and another for a lamp used as an indicator. Small lamps such as torch bulbs can be used for both purposes so either circuit symbol may used in simple educational circuits. Lamp used for lighting (for example a car headlamp or torch bulb) Lamp used as an indicator (for example a warning light on a car dashboard) Selecting a Lamp There are three important features to consider when selecting a lamp: y y y Voltage rating - the supply voltage for normal brightness. Power or current rating - small lamps are usually rated by current. Lamp type - please see the table below. The voltage and power (or current) ratings are usually printed or embossed on the body of a lamp. Voltage rating This is the supply voltage required for normal brightness. If a slightly higher voltage is used the lamp will be brighter but its lifetime will be shorter. With a lower supply voltage the lamp will be dimmer and its lifetime will be longer. The light from dim lamps has a yellow-orange colour. Torch lamps pass a relatively large current and this significantly reduces the output voltage of the battery. Some voltage is used up inside the battery driving the large current through the small resistance of the battery itself (its 'internal resistance'). As a result the correct voltage rating for a torch lamp is lower than the normal voltage of the battery which lights it! For example: a lamp rated 3.5V 0.3A is correct for a 4.5V battery (three 1.5V cells) because when the lamp is connected the voltage across the battery falls to about 3.5V. Power or current rating This is the power or current for the lamp when connected to its rated voltage. Low power lamps are usually rated by their current and high power lamps by their power. It is easy to convert between the two ratings: P = I × V where: P = power in watts (W) or I=P/V I = current in amps (A) V = voltage in volts (V) Examples: y y A lamp rated 3.5V 0.3A has a power rating P = I × V = 0.3 × 3.5 = 1.05W A lamp rated 6V 0.06A has a power rating P = I × V = 0.06 × 6 = 0.36W y A lamp rated 12W 2.4W has a current rating I = P / V = 2.4 / 12 = 0.2A Lamp Type Type of Lamp MES Miniature Edison Screw Example These are the standard small lamps. The bulb diameter is usually about 10mm, but tubular bulbs are also available. MES lamps have one contact on the base and the body forms the other contact. They are available with a good range of voltage and power (or current) ratings. Lens ended versions are available to produce a focused beam of light. LES Lilliput Edison Screw Smaller than MES, these have a bulb diameter of about 5mm. Photograph © Rapid Electronics MCC Miniature Centre Contact These have a bayonet style fitting, like a standard mains lamp in the UK. They have one contact on the base and the body forms the other contact. The bulb diameter is about 10mm. Photograph © Rapid Electronics SBC Small Bayonet Cap These have a bayonet style fitting, like a standard mains lamp in the UK. They have two contacts on the base so the metal body is not connected in the circuit. SBC lamps have high power ratings (24W for example) and their bulbs are large with a diameter of up to about 40mm. Note the two filament arrangements in the lamps shown, horizontal on the left, vertical on the right. Photograph © Rapid Electronics Pre-focus This type of lamp is used in torches and lanterns. The flange at the top of the metal body is used to hold the lamp in place. Lampholders are not readily available so this type is unsuitable for most projects. Photograph © Rapid Electronics Wire ended These are very small lamps with a bulb about 3mm diameter and 6mm long. Take care to avoid snapping the wires where they enter the glass bulb. Photograph © Rapid Electronics Grain of Wheat These are similar to the wire ended lamps above but they have stranded wire leads usually about 150mm long. The bulb is about 3mm diameter and 6mm long - the size of a grain of wheat! Photograph © Rapid Electronics Rapid Electronics stock a wide range of lamps and they have kindly allowed me to use their photographs on this page. The photographs are from their Image Gallery CD-ROM. Connecting and soldering Lamps may be connected either way round in a circuit and the supply may be AC or DC. Most lamps are designed to be used in a lampholder but the small 'wire ended' and 'grain of wheat' lamps have wires which may be soldered directly onto a circuit board. Lampholders usually have screw terminals or solder tags to attach wires. Some small holders have contacts which may be soldered directly to a circuit board. screw terminals solder tags Lampholders Photographs © Rapid Electronics Lamps in Series Several lamps can be successfully connected in series provided they all have identical voltage and power (or current) ratings. The supply voltage is divided equally between identical lamps so their voltage rating must be suitable for this. For example Christmas tree lights may have 20 lamps connected in series to a 240V supply, so each lamp will have 240V ÷ 20 = 12V across it. A disadvantage of connecting lamps in series is that if one lamp blows all of them will go out because the circuit is broken. Christmas tree lamps have a special feature to overcome this problem; they are designed to short circuit (conduct like a wire link) when they blow, so the circuit is not broken and the other lamps remain lit, making it easier to locate the faulty lamp. Sets also include one 'fuse' lamp which blows normally. Light Emitting Diodes (LEDs) Colours | Sizes and shapes | Resistor value | LEDs in series | LED data | Flashing | Displays Example: Circuit symbol: Function LEDs emit light when an electric current passes through them. Connecting and soldering LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method). LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs. Testing an LED Never connect an LED directly to a battery or power supply! It will be destroyed almost instantly because too much current will pass through and burn it out. LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round! For an accurate value please see Calculating an LED resistor value below. Colours of LEDs LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours. The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent. Tri-colour LEDs The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on. The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour. Bi-colour LEDs A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above. Sizes, Shapes and Viewing angles of LEDs LEDs are available in a wide variety of sizes and shapes. The 'standard' LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular. Round cross-section LEDs are frequently used and they are LED Clip very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if Photograph © Rapid Electronics necessary. LED clips are also available to secure LEDs in holes. Other cross-section shapes include square, rectangular and triangular. As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less. Rapid Electronics stock a wide selection of LEDs and their catalogue is a good guide to the range available. Calculating an LED resistor value An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly. The resistor value, R is given by: R = (VS - VL) / I VS = supply voltage VL = LED voltage (usually 2V, but 4V for blue and white LEDs) I = LED current (e.g. 10mA = 0.01A, or 20mA = 0.02A) Make sure the LED current you choose is less than the maximum permitted and convert the current to amps (A) so the calculation will give the resistor value in ohms ( ). To convert mA to A divide the current in mA by 1000 because 1mA = 0.001A. If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright. For example If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A, R = (9V - 2V) / 0.02A = 350 , so choose 390 (the nearest standard value which is greater). Working out the LED resistor formula using Ohm's law Ohm's law says that the resistance of the resistor, R = V/I, where: V = voltage across the resistor (= VS - VL in this case) I = the current through the resistor So R = (VS - VL) / I For more information on the calculations please see the Ohm's Law page. Connecting LEDs in series If you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED. All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for VL. Example calculations: A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal. VL = 2V + 2V + 2V = 6V (the three LED voltages added up). If the supply voltage VS is 9V and the current I must be 15mA = 0.015A, Resistor R = (VS - VL) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200 , so choose R = 220 (the nearest standard value which is greater). Avoid connecting LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor. Reading a table of technical data for LEDs Suppliers' catalogues usually include tables of technical data for components such as LEDs. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows typical technical data for some 5mm diameter round LEDs with diffused packages (plastic bodies). Only three columns are important and these are shown in bold. Please see below for explanations of the quantities. Type Standard Standard Standard Standard High intensity Super bright Colour Red Bright red IF max. VF VF VR typ. max. max. 5V 5V 5V 5V 5V 5V Luminous intensity 5mcd @ 10mA 80mcd @ 10mA 32mcd @ 10mA 32mcd @ 10mA 60mcd @ 20mA 500mcd @ 20mA Viewing Wavelength angle 60° 60° 60° 60° 50° 60° 660nm 625nm 590nm 565nm 430nm 660nm 30mA 1.7V 2.1V 30mA 2.0V 2.5V Yellow 30mA 2.1V 2.5V Green Blue Red 25mA 2.2V 2.5V 30mA 4.5V 5.5V 30mA 1.85V 2.5V Low current IF max. VF typ. Red 30mA 1.7V 2.0V 5V 5mcd @ 2mA 60° 625nm VF max. VR max. Luminous intensity Viewing angle Wavelength Maximum forward current, forward just means with the LED connected correctly. Typical forward voltage, VL in the LED resistor calculation. This is about 2V, except for blue and white LEDs for which it is about 4V. Maximum forward voltage. Maximum reverse voltage You can ignore this for LEDs connected the correct way round. Brightness of the LED at the given current, mcd = millicandela. Standard LEDs have a viewing angle of 60°, others emit a narrower beam of about 30°. The peak wavelength of the light emitted, this determines the colour of the LED. nm = nanometre. Flashing LEDs Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit. LED Displays LED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). The pictures below illustrate some of the popular designs: Bargraph 7-segment Starburst Dot matrix Photographs © Rapid Electronics Pin connections of LED displays There are many types of LED display and a supplier's catalogue should be consulted for the pin connections. The diagram on the right shows an example from the Rapid Electronics catalogue. Like many 7-segment displays, this example is available in two versions: Common Anode (SA) with all the LED anodes connected together and Common Cathode (SC) with all the cathodes connected together. Letters Pin connections diagram © Rapid Electronics a-g refer to the 7 segments, A/C is the common anode or cathode as appropriate (on 2 pins). Note that some pins are not present (NP) but their position is still numbered Relays Choosing a relay | Protection diodes | Reed relays | Advantages & disadvantages Also see: Switches | Diodes Circuit symbol for a relay A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram. Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification. Relay showing coil and switch contacts Relays Photographs © Rapid Electronics Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches. Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil. The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT. The relay's switch connections are usually labelled COM, NC and NO: y y y y y COM = Common, always connect to this, it is the moving part of the switch. NC = Normally Closed, COM is connected to this when the relay coil is off. NO = Normally Open, COM is connected to this when the relay coil is on. Connect to COM and NO if you want the switched circuit to be on when the relay coil is on. Connect to COM and NC if you want the switched circuit to be on when the relay coil is off. Choosing a relay You need to consider several features when choosing a relay: 1. Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue. 2. Coil voltage The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value. 3. Coil resistance The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current: Relay coil current = supply voltage coil resistance 4. For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current. 5. Switch ratings (voltage and current) The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC". 6. Switch contact arrangement (SPDT, DPDT etc) Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches. Protection diodes for relays Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection. Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs. Reed relays Reed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch. Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of Reed Relay supply voltages (9-20V for example). They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch Photograph © Rapid Electronics low currents (500mA maximum for example). The reed relay shown in the photograph will plug into a standard 14-pin DIL socket ('IC holder'). For further information about reed switches please see the page on switches. Relays and transistors compared Like relays, transistors can be used as an electrically operated switch. For switching small DC currents (< 1A) at low voltage they are usually a better choice than a relay. However, transistors cannot switch AC (such as mains electricity) and in simple circuits they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! The main advantages and disadvantages of relays are listed below: Advantages of relays: y y y y Relays can switch AC and DC, transistors can only switch DC. Relays can switch higher voltages than standard transistors. Relays are often a better choice for switching large currents (> 5A). Relays can switch many contacts at once. Disadvantages of relays: y y y y Relays are bulkier than transistors for switching small currents. Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. Relays use more power due to the current flowing through their coil. Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil. Connectors and Cables Connectors: Battery clips | Terminal blocks | Croc clips | 4mm & 2mm | DC power Audio & communication: Jack | Phono | Coax | BNC | DIN | D | IDC & RJ45 Cables: Single-core | Stranded | 'Figure 8' | Signal | Screened | Co-axial | Mains flex Battery clips and holders The standard battery clip fits a 9V PP3 battery and many battery holders such as the 6 × AA cell holder shown. Battery holders are also available with wires attached, with pins for PCB mounting, or as a complete box with lid, switch and wires. Many small electronic projects use a 9V PP3 battery but if you wish to use the project for long periods a better choice is a battery holder with 6 AA cells. This has the same voltage but a much longer battery life and it will work out cheaper in the long run. Photographs © Rapid Electronics Larger battery clips fit 9V PP9 batteries but these are rarely used now. Terminal blocks and PCB terminals Terminal blocks are usually supplied in 12-way lengths but they can be cut into smaller blocks with a sharp knife, large wire cutters or a junior hacksaw. They are sometimes called PCB 'chocolate blocks' because of the way they can terminal be easily cut to size. block Terminal block Photographs © Rapid Electronics PCB mounting terminal blocks provide an easy way of making semi-permanent connections to PCBs. Many are designed to interlock to provide more connections. Crocodile clips The 'standard' crocodile clip has no cover and a screw contact. However, miniature insulated crocodile clips are more suitable for many purposes including test leads. They have a solder contact and lugs which fold down to grip the cable's insulation, Crocodile clips increasing the strength of the joint. Remember to Photographs © Rapid Electronics feed the cable through the plastic cover before soldering! Add and remove the cover by fully opening the clip, a piece of wood can be used to hold the jaws open. 4mm plugs, sockets and terminals These are the standard single pole connectors used on meters and other electronic equipment. They are capable of passing high currents (typically 10A) and most designs are very robust. Shrouded plugs and sockets are available for use with high voltages where there is a risk of electric shock. A wide variety of colours is available from most suppliers. 4mm terminal and solder tag Photographs © Rapid Electronics Plugs Plugs may have a screw or solder terminal to hold the cable. Check if you need to thread the cable through the cover before connecting it. Some plugs, such as those illustrated, are 'stackable' which means that they include a socket to accept another plug, allowing several plugs to be connected to the same point - a very useful feature for test leads. Sockets These are usually described as 'panel mounting' because they are designed to be fitted to a case. Most sockets have a solder contact but the picture shows other options. Fit the socket in the case before attaching the wire otherwise you will be unable to add the mounting nut. Terminals In addition to a socket these have provision for attaching a wire by threading it through a hole (or wrapping it around the post) and tightening the top nut by hand. They usually have a threaded stud to fit a solder tag inside the case. 2mm plugs and sockets These are smaller versions of the 4mm plugs and sockets described above, but terminals are not readily available. The plugs illustrated are stackable. Despite their small size these connectors can pass large currents and some are rated at 10A. Photograph © Rapid Electronics DC power plugs and sockets These 2-pole plugs and sockets ensure that the polarity of a DC supply cannot be accidentally reversed. The standard sizes are 2.1 and 2.5mm plug diameter. Standard plugs have a 10mm shaft, 'long' plugs have a 14mm shaft. Sockets are available for PCB or chassis mounting and most Photographs © Rapid Electronics include a switch on the outer contact which is normally used to disconnect an internal battery when a plug is inserted. Miniature versions with a 1.3mm diameter plug are used where small size is essential, such as for personal cassette players. Jack plugs and sockets These are intended for audio signals so mono and stereo versions are available. The sizes are determined by the plug diameter: ¼" (6.3mm), 3.5mm and 2.5mm. The 2.5mm size is only available for mono. Screened plugs have metal bodies connected to the COM contact. Most connections are soldered, remember to thread cables through plug covers before soldering! Sockets are designed for PCB or chassis mounting. ¼" plug connections are similar to those for 3.5mm plugs shown below. ¼" socket connections are COM, R and L in that order from the mounting nut, ignore R for mono use. Most ¼" sockets have switches on all contacts which open as the plug is inserted so they can be used to isolate internal speakers for example. ¼" (6.3mm) jack plug and socket 3.5mm jack plug and socket 3.5mm jack line socket (for fitting to a cable) Photographs © Rapid Electronics The connections for 3.5mm plugs and sockets are shown below. Plugs have a lug which should be folded down to grip the cable's insulation and increase the strength of the joint. 3.5mm mono sockets have a switch contact which can be used to switch off an internal speaker as the plug is inserted. Ignore this contact if you do not require the switching action. L = left channel signal R = right channel signal COM = common (0V, screen) 3.5mm jack plug and socket connections (the R connection is not present on mono plugs) Do not use jack plugs for power supply connections because the contacts may be briefly shorted as the plug is inserted. Use DC power connectors for this. Phono plugs and sockets These are used for screened cables carrying audio and video signals. Stereo connections are made using a pair of phono plugs and sockets. The centre contact is for the signal and the outer contact for the screen (0V, common). Screened plugs have metal bodies connected to the outer contact to give the signal additional protection from electrical noise. Sockets are available for PCB or chassis mounting, singly for mono, or in pairs for stereo. Line sockets are available for making extension leads. Photographs © Rapid Electronics Construction of a screened cable Coax plugs and sockets These are similar to the phono plugs and sockets described above but they are designed for use with screened cables carrying much higher frequency signals, such as TV aerial leads. They provide better screening because at high frequencies this is essential to reduce electrical noise. Photographs © Rapid Electronics BNC plugs and sockets These are designed for screened cables carrying high frequency signals where an undistorted and noise free signal is essential, for example oscilloscope leads. BNC plugs are connected with a push and twist action, to disconnect you need to twist and pull. Plugs and sockets are rated by their impedance BNC plug, photograph © Rapid Electronics (50 or 75 ) which must be the same as the cable's impedance. If the connector and cable impedances are not matched the signal will be distorted because it will be partly reflected at the connection, this is the electrical equivalent of the weak reflection which occurs when light passes through a glass window. DIN plugs and sockets These are intended for audio signals but they can be used for other low-current purposes where a multi-way connector is required. They are available from 3 way to 8 way. 5 way is used for stereo audio connections. The contacts are numbered on the connector, but they are not in numerical order! For audio use the 'common' (0V) wire is connected to contact 2. 5 way plugs and sockets are available in two versions: 180° and 270° (the angle refers to the arc formed by the contacts). Plastic covers of DIN plugs (and line sockets) are removed by depressing the retaining lug with a small screwdriver. You may also need small pliers to extract the body from the cover but do not pull on the pins themselves to avoid damage. Remember to thread the cable through the cover before starting to solder the connections! DIN plug 5 way 180° DIN socket (chassis mounting) Soldering DIN plugs is easier if you clamp the insert with Photographs © Rapid Electronics the pins. Wires should be pushed into the hollow pins - first 'tin' the wires (coat them with a thin layer of solder) then melt a little solder into the hollow pin and insert the wire while keeping the solder molten. Take care to avoid melting the plastic base, stop and allow the pin to cool if necessary. Mini-DIN connectors are used for computer equipment such as keyboards and mice but they are not a good choice for general use unless small size is essential. D connectors These are multi-pole connectors with provision for screw fittings to make semi-permanent connections, for example on computer equipment. The D shape prevents incorrect connection. Standard D-connectors have 2 rows of contacts (top picture); 9, 15 and 25-way versions are the most popular. High Density D-connectors have 3 rows of contacts (bottom picture); a 15-way version is used to connect computer monitors for example. Note that covers (middle picture) are usually sold separately because both plugs and sockets can be fitted to cables by fitting a cover to a chassis mounted connector. PCB mounting versions of plugs and sockets are also available. The contacts are usually numbered on the body Photographs © Rapid Electronics of the connector, although you may need a magnifying glass to see the very small markings. Soldering Dconnectors requires a steady hand due to the closeness of the contacts, it is easy to accidently unsolder a contact you have just completed while attempting to solder the next one! IDC communication connectors These multi-pole insulation displacement connectors are used for computer and telecommunications equipment. They automatically cut through the insulation on wires when installed and special tools are required to fit them. They are available as 4, 6 and 8-way versions. The 8-way RJ45 is the standard connector for modern computer networks. If you regularly use these you may be interested in our network lead tester project. Photographs © Rapid Electronics Standard UK telephone connectors are similar in style but a slightly different shape. They are called BT (British Telecom) connectors. Cables Cable... flex... lead... wire... what do all these terms mean? y y y y A cable is an assembly of one or more conductors (wires) with some flexibility. A flex is the proper name for the flexible cable fitted to mains electrical appliances. A lead is a complete assembly of cable and connectors. A wire is a single conductor which may have an outer layer of insulation (usually plastic). Single core equipment wire This is one solid wire with a plastic coating available in a wide variety of colours. It can be bent to shape but will break if repeatedly flexed. Use it for connections which will not be disturbed, for example links between points of a circuit board. Typical specification: 1/0.6mm (1 strand of 0.6mm diameter), maximum current 1.8A. Stranded wire This consists of many fine strands of wire covered by an outer plastic coating. It is flexible and can withstand repeated bending without breaking. Use it for connections which may be disturbed, for example wires outside cases to sensors and switches. A very flexible version ('extra-flex') is used for test leads. Typical specifications: 10/0.1mm (10 strands of 0.1mm diameter), maximum current 0.5A. 7/0.2mm (7 strands of 0.2mm diameter), maximum current 1.4A. 16/0.2mm (16 strands of 0.2mm diameter), maximum current 3A. 24/0.2mm (24 strands of 0.2mm diameter), maximum current 4.5A. 55/0.1mm (55 strands of 0.1mm diameter), maximum current 6A, used for test leads. 'Figure 8' (speaker) cable 'Figure 8' cable consists of two stranded wires arranged in a figure of 8 shape. One wire is usually marked with a line. It is suitable for low Photograph © Rapid Electronics voltage, low current (maximum 1A) signals where screening from electrical interference is not required. It is a popular choice for connecting loudspeakers and is often called 'speaker cable'. Signal cable Signal cable consists of several colour-coded cores of stranded wire housed within an outer plastic sheath. With a typical maximum current of 1A per core it is suitable for low voltage, low current signals where screening from electrical interference is not required. The picture shows 6-core cable, but 4-core and 8core are also readily available. Photograph © Rapid Electronics Screened cable The diagram shows the construction of screened cable. The central wire carries the signal and the screen is connected to 0V (common) to shield the signal from electrical interference. Screened cable is used for audio signals and dual versions are available for stereo. Screened cable (mono) Screened cable (stereo) Screened cable (stereo) Construction of a screened cable Photographs © Rapid Electronics Co-axial cable Photograph © Rapid Electronics This type of screened cable (see above) is designed to carry high frequency signals such as those found in TV aerials and oscilloscope leads. Mains flex Flex is the proper name for the flexible cable used to connect appliances to the mains supply. It contains 2 cores (for live and neutral) or 3 cores (for live, neutral and earth). Mains flex has Photograph © Rapid Electronics thick insulation for the high voltage (230V in UK) and it is available with various current ratings: 3A, 6A and 13A are popular sizes in the UK. Mains flex is sometimes used for low voltage circuits which pass a high current, but please think carefully before using it in this way. The distinctive colours of mains flex should act as a warning of the mains high voltage which can be lethal; using mains flex for low voltage circuits can undermine this warning. Rapid Electronics stock a wide range of connectors and they have kindly allowed me to use their photographs on this page. The photographs are from their Image Gallery CD-ROM Diodes Signal diodes | Rectifier diodes | Bridge rectifiers | Zener diodes Also see: LEDs | AC and DC | Power Supplies Example: symbol: Circuit Function Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves. Forward Voltage Drop Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph). Reverse Voltage When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown. Ordinary diodes can be split into two types: Signal diodes which pass small currents of 100mA or less and Rectifier diodes which can pass large currents. In addition there are LEDs (which have their own page) and Zener diodes (at the bottom of this page). Connecting and soldering Diodes must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is marked by a line painted on the body. Diodes are labelled with their code in small print, you may need a magnifying glass to read this on small signal diodes! Small signal diodes can be damaged by heat when soldering, but the risk is small unless you are using a germanium diode (codes beginning OA...) in which case you should use a heat sink clipped to the lead between the joint and the diode body. A standard crocodile clip can be used as a heat sink. Rectifier diodes are quite robust and no special precautions are needed for soldering them. Testing diodes You can use a multimeter or a simple tester (battery, resistor and LED) to check that a diode conducts in one direction but not the other. A lamp may be used to test a rectifier diode, but do NOT use a lamp to test a signal diode because the large current passed by the lamp will destroy the diode! Signal diodes (small current) Signal diodes are used to process information (electrical signals) in circuits, so they are only required to pass small currents of up to 100mA. General purpose signal diodes such as the 1N4148 are made from silicon and have a forward voltage drop of 0.7V. Germanium diodes such as the OA90 have a lower forward voltage drop of 0.2V and this makes them suitable to use in radio circuits as detectors which extract the audio signal from the weak radio signal. For general use, where the size of the forward voltage drop is less important, silicon diodes are better because they are less easily damaged by heat when soldering, they have a lower resistance when conducting, and they have very low leakage currents when a reverse voltage is applied. Protection diodes for relays Signal diodes are also used to protect transistors and ICs from the brief high voltage produced when a relay coil is switched off. The diagram shows how a protection diode is connected 'backwards' across the relay coil. Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs. Diode Maximum Maximum Reverse Current Voltage Rectifier diodes (large current) Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode. 1N4001 1N4002 1N4007 1N5401 1N5408 1A 1A 1A 3A 3A 50V 100V 1000V 100V 1000V All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. The table shows maximum current and maximum reverse voltage for some popular rectifier diodes. The 1N4001 is suitable for most low voltage circuits with a current of less than 1A. Also see: Power Supplies Bridge rectifiers There are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is one of them and it is available in special packages containing the four diodes required. Bridge rectifiers are rated by their maximum current and maximum reverse voltage. They have four leads or terminals: the two DC outputs are labelled + and -, the two AC inputs are labelled . The diagram shows the operation of a bridge rectifier as it converts AC to DC. Notice how alternate pairs of diodes conduct. Also see: Power Supplies Various types of Bridge Rectifiers Note that some have a hole through their centre for attaching to a heat sink Photographs © Rapid Electronics Zener diodes Example: symbol: a = anode, k = cathode Zener diodes are used to maintain a fixed voltage. They are designed to 'breakdown' in a reliable and nondestructive way so that they can be used in reverse to maintain a fixed voltage across their terminals. The diagram shows how they are connected, with a resistor in series to limit the current. Zener diodes can be distinguished from ordinary diodes by their code and breakdown voltage which are printed on them. Zener diode codes begin BZX... or BZY... Their breakdown voltage is printed with V in place of a decimal point, so 4V7 means 4.7V for example. Zener diodes are rated by their breakdown voltage and maximum power: y y Circuit The minimum voltage available is 2.4V. Power ratings of 400mW and 1.3W are common. Integrated Circuits (Chips) Pin numbers | IC holders | Static | Datasheets | Sinking/sourcing | Combining outputs | 555 and 556 Timers | Logic ICs | 4000 Series | 74 Series | PIC microcontrollers Also see: 4000 Series ICs | 74 Series ICs | 555 and 556 Timer Circuits Integrated Circuits are usually called ICs or chips. They are complex circuits which have been etched onto tiny chips of semiconductor (silicon). The chip is packaged in a plastic holder with pins spaced on a 0.1" (2.54mm) grid which will fit the holes on stripboard and breadboards. Very fine wires inside the package link the chip to the pins. Pin numbers The pins are numbered anti-clockwise around the IC (chip) starting near the notch or dot. The diagram shows the numbering for 8-pin and 14-pin ICs, but the principle is the same for all sizes. IC holders (DIL sockets) ICs (chips) are easily damaged by heat when soldering and their short pins cannot be protected with a heat sink. Instead we use an IC holder, strictly called a DIL socket (DIL = Dual In-Line), which can be safely soldered onto the circuit board. The IC is pushed into the holder when all soldering is complete. IC holders are only needed when soldering so they are not used on breadboards. Commercially produced circuit boards often have ICs soldered directly to the board without an IC holder, usually this is done by a machine which is able to work very quickly. Please don't attempt to do this yourself because you are likely to destroy the IC and it will be difficult to remove without damage by desoldering. Removing an IC from its holder If you need to remove an IC it can be gently prised out of the holder with a small flatblade screwdriver. Carefully lever up each end by inserting the screwdriver blade between the IC and its holder and gently twisting the screwdriver. Take care to start lifting at both ends before you attempt to remove the IC, otherwise you will bend and possibly break the pins. Static precautions Many ICs are static sensitive and can be damaged when you touch them because your body may have become charged with static electricity, from your clothes for example. Static sensitive ICs will be supplied in antistatic packaging with a warning label and they should be left in this packaging until you are ready to use them. Antistatic bags for ICs It is usually adequate to earth your hands by touching a Photograph © Rapid Electronics metal water pipe or window frame before handling the IC but for the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces. You can make an earthed work surface with a sheet of aluminium kitchen foil and using a crocodile clip to connect the foil to a metal water pipe or window frame with a 10k resistor in series. Datasheets Datasheets are available for most ICs giving detailed information about their ratings and functions. In some cases example circuits are shown. The large amount of information with symbols and abbreviations can make datasheets seem overwhelming to a beginner, but they are worth reading as you become more confident because they contain a great deal of useful information for more experienced users designing and testing circuits. Datasheets are available as PDF files from: y y y PDF files To view and print PDF files you need an Acrobat Reader which may be downloaded free for Windows, Mac, RISC OS, or UNIX/Linux computers. If you are not sure which type of computer you have it is probably Windows. DatasheetArchive.com Datasheets.org.uk DatasheetCatalog.com Sinking and sourcing current IC outputs are often said to 'sink' or 'source' current. The terms refer to the direction of the current at the IC's output. If the IC is sinking current it is flowing into the output. This means that a device connected between the positive supply (+Vs) and the IC output will be switched on when the output is low (0V). If the IC is sourcing current it is flowing out of the output. This means that a device connected between the IC output and the negative supply (0V) will be switched on when the output is high (+Vs). It is possible to connect two devices to an IC output so that one is on when the output is low and the other is on when the output is high. This arrangement is used in the Level Crossing project to make the red LEDs flash alternately. The maximum sinking and sourcing currents for an IC output are usually the same but there are some exceptions, for example 74LS TTL logic ICs can sink up to 16mA but only source 2mA. Using diodes to combine outputs The outputs of ICs must never be directly connected together. However, diodes can be used to combine two or more digital (high/low) outputs from an IC such as a counter. This can be a useful way of producing simple logic functions without using logic gates! The diagram shows two ways of combining outputs using diodes. The diodes must be capable of passing the output current. 1N4148 signal diodes are suitable for low current devices such as LEDs. For example the outputs Q0 - Q9 of a 4017 1-of-10 counter go high in turn. Using diodes to combine the 2nd (Q1) and 4th (Q3) outputs as shown in the bottom diagram will make the LED flash twice followed by a longer gap. The diodes are performing the function of an OR gate. Example projects: Traffic Light | Dice | Model Lighthouse The 555 and 556 Timers The 8-pin 555 timer IC is used in many projects, a popular version is the NE555. Most circuits will just specify '555 timer IC' and the NE555 is suitable for these. The 555 output (pin 3) can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply LEDs, relay coils and low current lamps. To switch larger currents you can connect a transistor. The 556 is a dual version of the 555 housed in a 14-pin package. The two timers (A and B) share the same power supply pins. Low power versions of the 555 are made, such as the ICM7555, but these should only be used when specified (to increase battery life) because their maximum output current of about 20mA (with 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555. For further information please see the page on 555 and 556 timer circuits. Logic ICs (chips) Logic ICs process digital signals and there are many devices, including logic gates, flipflops, shift registers, counters and display drivers. They can be split into two groups according to their pin arrangements: the 4000 series and the 74 series which consists of various families such as the 74HC, 74HCT and 74LS. For most new projects the 74HC family is the best choice. The older 4000 series is the only family which works with a supply voltage of more than 6V. The 74LS and 74HCT families require a 5V supply so they are not convenient for battery operation. The table below summarises the important properties of the most popular logic families: Property Technology Power Supply 4000 Series CMOS 3 to 15V 74 Series 74HC Highspeed CMOS 2 to 6V 74 Series 74HCT 74 Series 74LS High-speed CMOS TTL LowTTL compatible power Schottky 5V ±0.5V Very high impedance. Unused inputs must be connected to +Vs or 0V. Compatible with 74LS (TTL) outputs. Can sink and source about 20mA, enough to light an LED. To switch larger currents use a transistor. 5V ±0.25V 'Float' high to logic 1 if unconnected. 1mA must be drawn out to hold them at logic 0. Can sink up to 16mA (enough to light an LED), but source only about 2mA. To switch larger currents use a transistor. Inputs Very high impedance. Unused inputs must be connected to +Vs or 0V. Inputs cannot be reliably driven by 74LS outputs unless a 'pull-up' resistor is used (see below). Can sink and source about 5mA (10mA with 9V supply), enough to light an LED. To switch larger currents use a transistor. One output can drive up to 50 Can sink and source about 20mA, enough to light an LED. To switch larger currents use a transistor. Outputs Fan-out One output can drive up to 50 CMOS, 74HC One output can or 74HCT inputs, but only 10 74LS inputs. drive up to 10 CMOS, 74HC or 74HCT inputs, but only one 74LS input. 74LS inputs or 50 74HCT inputs. Maximum Frequency about 1MHz about 25MHz about 25MHz about 35MHz Driving 4000 or 74HC inputs from a 74LS output using a pull-up resistor. Power consumption of the IC itself A few µW. A few µW. A few µW. A few mW. Mixing Logic Families It is best to build a circuit using just one logic family, but if necessary the different families may be mixed providing the power supply is suitable for all of them. For example mixing 4000 and 74HC requires the power supply to be in the range 3 to 6V. A circuit which includes 74LS or 74HCT ICs must have a 5V supply. A 74LS output cannot reliably drive a 4000 or 74HC input unless a 'pull-up' resistor of 2.2k is connected between the +5V supply and the input to correct the slightly different logic voltage ranges used. Note that a 4000 series output can drive only one 74LS input. Quick links to individual ICs 4000 4001 4002 4011 4012 4060 4068 4069 4070 4071 4000 Series CMOS This family of logic ICs is numbered from 4000 onwards, and from 4500 onwards. They have a B at the end of the number (e.g. 4001B) which refers to an improved design introduced some years ago. Most of them are in 14-pin or 16-pin packages. They use CMOS circuitry which means they use very little power and can tolerate a wide range of power supply voltages (3 to 15V) making them ideal for battery powered projects. CMOS is pronounced 'see-moss' and stands for Complementary Metal Oxide Semiconductor. 4017 4020 4023 4024 4025 4026 4028 4029 4030 4040 4049 4050 4072 4073 4075 4077 4081 4082 4093 4510 4511 4516 4518 4520 However the CMOS circuitry also means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. For the more sensitive (and expensive!) ICs special equipment is available, including earthed wrist straps and earthed work surfaces. For further information, including pin connections, please use the quick links on the right or go to 4000 Series ICs. 74 Series: 74LS, 74HC and 74HCT There are several families of logic ICs numbered from 74xx00 onwards with letters (xx) in the middle of the number to indicate the type of circuitry, eg 74LS00 and 74HC00. The original family (now obsolete) had no letters, eg 7400. The 74LS (Low-power Schottky) family (like the original) uses TTL (Transistor-Transistor Logic) circuitry which is fast but requires more power than later families. The 74HC family has High-speed CMOS circuitry, combining the speed of TTL with the very low power consumption of the 4000 series. They are CMOS ICs with the same pin arrangements as the older 74LS family. Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead. The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower Quick links to individual ICs 7400 7402 7403 7404 7405 7408 7409 7410 7411 7412 7414 7420 7421 7427 7430 7432 7442 7447 7486 7490 7493 74132 74160 74161 74162 74163 74192 74193 74390 74393 74HC4017 74HC4020 74HC4040 74HC4060 74HC4511 immunity to noise, but this is unlikely to be a problem in most situations. Beware that the 74 series is often still called the 'TTL series' even though the latest ICs do not use TTL! For further information, including pin connections, please use the quick links on the right or go to 74 series ICs. The CMOS circuitry used in the 74HC and 74HCT series ICs means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. PIC microcontrollers A PIC is a Programmable Integrated Circuit microcontroller, a 'computer-on-a-chip'. They have a processor and memory to run a program responding to inputs and controlling outputs, so they can easily achieve complex functions which would require several conventional ICs. Programming a PIC microcontroller may seem daunting to a beginner but there are a number of systems designed to make this easy. The PICAXE system is an excellent example because it uses a standard computer to program (and re-program) the PICs; no specialist equipment is required other than a low-cost download lead. Programs can be written in a simple version of www.picaxe.co.uk BASIC or using a flowchart. The PICAXE programming software and extensive documentation is available to download free of charge, making the system ideal for education and users at home. For further information (including downloads) please see www.picaxe.co.uk 4000 series CMOS Logic ICs Gates: 2-input | 3-input | 4-input | 8-input | 4069 NOT | 4049 NOT | 4050 Buffer | 4000 Decade and 4-bit counters: 4017 | 4026 | 4029 | 4510 | 4516 | 4518 | 4520 7-bit, 12-bit & 14-bit counters: 4020 | 4024 | 4040 | 4060 Decoders and display drivers: 4028 | 4511 Also see: 74 Series | Logic Gates | Counting Circuits | ICs (chips) (with summary of logic ICs) General characteristics y y Supply: 3 to 15V, small fluctuations are tolerated. Inputs have very high impedance (resistance), this is good because it means they will not affect the part of the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This is likely to make the IC behave erratically and it will significantly increase the supply current. To prevent problems all unused inputs MUST be connected to the supply (either +Vs or 0V), this applies even if that part of the IC is not being used in the circuit! Outputs can sink and source only about 1mA if you wish to maintain the correct output voltage to drive CMOS inputs. If there is no need to drive any inputs the maximum current is about 5mA with a 6V supply, or 10mA with a 9V supply (just enough to light an LED). To switch larger currents you can connect a transistor. Fan-out: one output can drive up to 50 inputs. Gate propagation time: typically 30ns for a signal to travel through a gate with a 9V supply, it takes a longer time at lower supply voltages. Frequency: up to 1MHz, above that the 74 series is a better choice. Power consumption (of the IC itself) is very low, a few µW. It is much greater at high frequencies, a few mW at 1MHz for example. Quick links to individual ICs 4000 4001 4002 4011 4012 4017 4020 4023 4024 4025 4026 4028 4029 4030 4040 4049 4050 4060 4068 4069 4070 4071 4072 4073 4075 4077 4081 4082 4093 4510 4511 4516 4518 4520 y y y y y There are many ICs in the 4000 series and this page only covers a selection, concentrating on the most useful gates, counters, decoders and display drivers. For each IC there is a diagram showing the pin arrangement and brief notes explain the function of the pins where necessary. The notes also explain if the IC's properties differ substantially from the standard characteristics listed above. If you are using another reference please be aware that there is some variation in the terms used to describe input pins. I have tried to be logically consistent so the term I have used describes the pin's function when high (true). For example 'disable clock' on the 4026 is often labelled 'clock enable' but this can be confusing because it enables the clock when low (false). An input described as 'active low' is like this, it performs its function when low. If you see a line drawn above a label it means it is active low, for example: (say 'reset-bar'). Datasheets are available from: y y y DatasheetArchive.com Datasheets.org.uk DatasheetCatalog.com Static precautions The CMOS circuitry means that 4000 series ICs are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them. Gates Quad 2-input gates y y y y y y y y 4001 quad 2-input NOR 4011 quad 2-input NAND 4030 quad 2-input EX-OR (now obsolete) 4070 quad 2-input EX-OR 4071 quad 2-input OR 4077 quad 2-input EX-NOR 4081 quad 2-input AND 4093 quad 2-input NAND with Schmitt trigger inputs The 4093 has Schmitt trigger inputs to provide good noise immunity. They are ideal for slowly changing or noisy signals. The hysteresis is about 0.5V with a 4.5V supply and almost 2V with a 9V supply. Triple 3-input gates y y y y 4023 triple 3-input NAND 4025 triple 3-input NOR 4073 triple 3-input AND 4075 triple 3-input OR Notice how gate 1 is spread across the two ends of the package. Dual 4-input gates y y y y 4002 dual 4-input NOR 4012 dual 4-input NAND 4072 dual 4-input OR 4082 dual 4-input AND NC = No Connection (a pin that is not used). 4068 8-input NAND/AND* gate This gate has a propagation time which is about 10 times longer than normal so it is not suitable for high speed circuits. NC = No Connection (a pin that is not used). * = The AND output (pin 1) is not available on some versions of the 4068. 4069 hex NOT (inverting buffer) 4049 hex NOT and 4050 hex buffer y y 4049 hex NOT (inverting buffer) 4050 hex non-inverting buffer Inputs: These ICs are unusual because their gate inputs can withstand up to +15V even if the power supply is a lower voltage. Outputs: These ICs are unusual because they are capable of driving 74LS gate inputs directly. To do this they must have a +5V supply (74LS supply voltage). The gate output is sufficient to drive four 74LS inputs. NC = No Connection (a pin that is not used). Note the unusual arrangement of the power supply pins for these ICs! 4000 dual 3-input NOR gate and NOT gate Two 3-input NOR gates and a single NOT gate in one package. NC = No Connection (a pin that is not used). Decade and 4-bit Counters 4017 decade counter (1-of-10) The count advances as the clock input becomes high (on the rising-edge). Each output Q0-Q9 goes high in turn as counting advances. For some functions (such as flash sequences) outputs may be combined using diodes. The reset input should be low (0V) for normal operation (counting 0-9). When high it resets the count to zero (Q0 high). This can be done manually with a switch between reset and +Vs and a 10k resistor between reset and 0V. Counting to less than 9 is achieved by connecting the relevant output (Q0-Q9) to reset, for example to count 0,1,2,3 connect Q4 to reset. The disable input should be low (0V) for normal operation. When high it disables counting so that clock pulses are ignored and the count is kept constant. The ÷10 output is high for counts 0-4 and low for 5-9, so it provides an output at 1/10 of the clock frequency. It can be used to drive the clock input of another 4017 (to count the tens). Example projects: Heart-shaped badge | Network Lead Tester | Traffic Light | Dice | Model Lighthouse 4026 decade counter and 7segment display driver The count advances as the clock input becomes high (on the rising-edge). The outputs a-g go high to light the appropriate segments of a commoncathode 7-segment display as the count advances. The maximum output current is about 1mA with a 4.5V supply and 4mA with a 9V supply. This is sufficient to directly drive many 7-segment LED displays. The table below shows the segment sequence in detail. The reset input should be low (0V) for normal operation (counting 0-9). When high it resets the count to zero. The disable clock input should be low (0V) for normal operation. When high it disables counting so that clock pulses are ignored and the count is kept constant. The enable display input should be high (+Vs) for normal operation. When low it makes outputs a-g low, giving a blank display. The enable out follows this input but with a brief delay. The ÷10 output (h in table) is high for counts 0-4 and low for 5-9, so it provides an output at 1/10 of the clock frequency. It can be used to drive the clock input of another 4026 to provide multi-digit counting. The not 2 output is high unless the count is 2 when it goes low. Example project: 'Random' flasher for 8 LEDs This project uses the 4026 in an unconventional way, the outputs a-g and the ÷10 output (h) are used to flash individual LEDs in a complex pattern which appears random if not studied too closely! 4029 up/down synchronous counter with preset The 4029 is a synchronous counter so its outputs change precisely together on each clock pulse. This is helpful if you need to connect the outputs to logic gates because it avoids the glitches which occur with ripple counters. The count occurs as the clock input becomes high (on the rising-edge). The up/down input determines the direction of counting: high for up, low for down. The state of up/down should be changed when the clock is high. For normal operation (counting) preset, and carry in should be low. The binary/decade input selects the type of counter: 4-bit binary (0-15) when high; decade (0-9) when low. The counter may be preset by placing the desired binary number on the inputs A-D and briefly making the preset input high. There is no reset input, but preset can be used to reset the count to zero if inputs A-D are all low. Connecting synchronous counters in a chain: please see 4510/16 below. 4510 up/down decade (0-9) counter with preset 4516 up/down 4-bit (0-15) counter with preset These are synchronous counters so their outputs change precisely together on each clock pulse. This is helpful if you need to connect their outputs to logic gates because it avoids the glitches which occur with ripple counters. The count occurs as the clock input becomes high (on the rising-edge). The up/down input determines the direction of counting: high for up, low for down. The state of up/down should be changed when the clock is high. For normal operation (counting) preset, reset and carry in should be low. When reset is high it resets the count to zero (0000, QA-QD low). The clock input should be low when resetting. The counter may be preset by placing the desired binary number on the inputs A-D and briefly making the preset input high, the clock input should be low when this happens. Connecting synchronous counters in a chain The diagram below shows how to link synchronous counters, notice how all the clock (CK) inputs are linked. Carry out (CO) feeds carry in (CI) of the next counter. Carry in (CI) of the first counter should be low for 4029, 4510 and 4516 counters. 4518 dual decade (0-9) counter 4520 dual 4-bit (0-15) counter These contain two separate synchronous counters, one on each side of the IC. Normally a clock signal is connected to the clock input, with the enable input held high. Counting advances as the clock signal becomes high (on the rising-edge). Special arrangements are used if the 4518/20 counters are linked in a chain, as explained below. For normal operation the reset input should be low, making it high resets the counter to zero (0000, QAQD low). Counting to less than the maximum (9 or 15) can be achieved by connecting the appropriate output(s) to the reset input, using an AND gate if necessary. For example to count 0 to 8 connect QA (1) and QD (8) to reset using an AND gate. Connecting 4518 and 4520 counters in a chain The diagram below shows how to link 4518 and 4520 counters. Notice how the normal clock inputs are held low, with the enable inputs being used instead. With this arrangement counting advances as the enable input becomes low (on the falling-edge) allowing output QD to supply a clock signal to the next counter. The complete chain is a ripple counter, although the individual counters are synchronous! If it is essential to have truly synchronous counting a system of logic gates is required, please see a 4518/20 datasheet for further details. 7-bit, 12-bit and 14-bit counters 4020 14-bit (÷16,384) ripple counter The 4020 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse. The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means a counter output can directly drive the clock input of the next counter in a chain. Output Qn is the nth stage of the counter, n 4 1 representing 2 , for example Q4 is 2 = 16 ( /16 of 14 1 clock frequency) and Q14 is 2 = 16384 ( /16384 of clock frequency). Note that Q2 and Q3 are not available. The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low). Also see: 4040 (12-bit) and 4060 (14-bit with internal oscillator). 4024 7-bit (÷128) ripple counter The 4024 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse. The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means a counter output can directly drive the clock input of the next counter in a chain. Output Qn is the nth stage of the counter, representing 2 , for example Q4 is 2 = 16 ( /16 of clock frequency) and Q7 is 27 = 128 (1/128 of clock frequency). The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low). n 4 1 4040 12-bit (÷4096) ripple counter The 4040 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse. The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means a counter output can directly drive the clock input of the next counter in a chain. Output Qn is the nth stage of the counter, n 4 1 representing 2 , for example Q4 is 2 = 16 ( /16 of 12 1 clock frequency) and Q12 is 2 = 4096 ( /4096 of clock frequency). The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low). Also see these 14-bit counters: 4020 and 4060 (includes internal oscillator). 4060 14-bit (÷16,384) ripple counter with internal oscillator The 4060 is a ripple counter so beware that glitches may occur in any logic gate systems connected to its outputs due to the slight delay before the later counter outputs respond to a clock pulse. The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means a counter output can directly drive the clock input of the next counter in a chain. The clock can be driven directly, or connected to the internal oscillator (see below). Output Qn is the nth stage of the counter, representing 2n, for example Q4 is 24 = 16 (1/16 of clock frequency) and Q14 is 214 = 16384 (1/16384 of clock frequency). Note that Q1-3 and Q11 are not available. The reset input should be low for normal operation (counting). When high it resets the count to zero (all outputs low). The 4060 includes an internal oscillator. The clock signal may be supplied in three ways: y From an external source to the clock input, as for a normal counter. In this case there should be no connections to external C and external R (pins 9 and 10). RC oscillator as shown in the diagram. The oscillator drives the clock input with an approximate frequency f = 1/(2×R1×C) (it partly depends on the supply voltage). R1 should be at least 50k if the supply voltage is less than 7V. R2 should be between 2 and 10 times R1. Crystal oscillator as shown in the diagram, note that there is no connection to pin 9. The 32768 Hz crystal will give a 2Hz signal at the last output, Q14. y y Also see: 4020 (14-bit) and 4040 (12-bit), neither have internal oscillators. Example projects: Christmas Decoration | Valentine Heart Decoders 4028 BCD to decimal (1 of 10) decoder The appropriate output Q0-9 becomes high in response to the BCD (binary coded decimal) input. For example an input of binary 0101 (=5) will make output Q5 high and all other outputs low. The 4028 is a BCD (binary coded decimal) decoder intended for input values 0 to 9 (0000 to 1001 in binary). With inputs from 10 to 15 (1010 to 1111 in binary) all outputs are low. Note that the 4028 can be used as a 1-of-8 decoder if input D is held low. Also see: 4017 (a decade counter and 1-of-10 decoder in a single IC). 7-segment Display Drivers 4511 BCD to 7-segment display driver The appropriate outputs a-g become high to display the BCD (binary coded decimal) number supplied on inputs A-D. The outputs a-g can source up to 25mA. The 7-segment display segments must be connected between the outputs and 0V with a resistor in series (330 with a 5V supply). A common cathode display is required. Display test and blank input are active-low so they should be high for normal operation. When display test is low all the display segments should light (showing number 8). When blank input is low the display will be blank (all segments off). The store input should be low for normal operation. When store is high the displayed number is stored internally to give a constant display regardless of any changes which may occur to the inputs A-D. The 4511 is intended for BCD (binary coded decimal). Inputs values from 10 to 15 (1010 to 1111 in binary) will give a blank display (all segments off). Switches Switch Contacts - pole, throw etc. Standard Switches - SPST, SPDT, DPST, DPDT. Special Switches - multiway, key, tilt, reed etc. Also see: Relays | Series and Parallel Connections - Switches Selecting a Switch There are three important features to consider when selecting a switch: y y y Circuit symbol for a simple on-off switch Contacts (e.g. single pole, double throw) Ratings (maximum voltage and current) Method of Operation (toggle, slide, key etc.) Switch Contacts Several terms are used to describe switch contacts: y y y y y y Pole - number of switch contact sets. Throw - number of conducting positions, single or double. Way - number of conducting positions, three or more. Momentary - switch returns to its normal position when released. Open - off position, contacts not conducting. Closed - on position, contacts conducting, there may be several on positions. For example: the simplest on-off switch has one set of contacts (single pole) and one switching position which conducts (single throw). The switch mechanism has two positions: open (off) and closed (on), but it is called 'single throw' because only one position conducts. Switch Contact Ratings Switch contacts are rated with a maximum voltage and current, and there may be different ratings for AC and DC. The AC values are higher because the current falls to zero many times each second and an arc is less likely to form across the switch contacts. For low voltage electronics projects the voltage rating will not matter, but you may need to check the current rating. The maximum current is less for inductive loads (coils and motors) because they cause more sparking at the contacts when switched off. Standard Switches Type of Switch ON-OFF Single Pole, Single Throw = SPST A simple on-off switch. This type can be used to switch the power supply to a circuit. When used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral. Photograph © Rapid Electronics Circuit Symbol Example SPST toggle switch (ON)-OFF Push-to-make = SPST Momentary A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around ON. This is the standard doorbell switch. Photograph © Rapid Electronics Push-to-make switch ON-(OFF) Push-to-break = SPST Momentary A push-to-break switch returns to its normally closed (on) position when you release the button. Photograph © Rapid Electronics Push-to-break switch ON-ON Single Pole, Double Throw = SPDT This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position. SPDT toggle switch A SPDT toggle switch may be used as a simple on-off switch by connecting to COM and one of the A or B terminals shown in the diagram. A and B are interchangeable so switches are usually not labelled. ON-OFF-ON SPDT Centre Off A special version of the standard SPDT switch. It has a third switching position in the centre which is off. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released. Photographs © Rapid Electronics SPDT rocker switch SPDT slide switch (PCB mounting) Dual ON-OFF Double Pole, Single Throw = DPST A pair of on-off switches which operate together (shown by the dotted line in the circuit symbol). A DPST switch is often used to switch mains electricity because it can isolate both the live and neutral connections. Photograph © Rapid Electronics DPST rocker switch Dual ON-ON Double Pole, Double Throw = DPDT A pair of on-on switches which operate together (shown by the dotted line in the circuit symbol). A DPDT switch can be wired up as a reversing switch for a motor as shown in the diagram. DPDT slide switch ON-OFF-ON DPDT Centre Off A special version of the standard SPDT switch. It has a third switching position in the centre which is off. This can be Wiring for Reversing Switch very useful for motor control because you have forward, off and reverse positions. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released. Photograph © Rapid Electronics Rapid Electronics stock a wide range of switches and they have kindly allowed me to use their photographs on this page. The photographs are from their Image Gallery CD-ROM. Special Switches Type of Switch Push-Push Switch (e.g. SPST = ON-OFF) This looks like a momentary action push switch but it is a standard on-off switch: push once to switch on, push again to switch off. This is called a latching action. Photograph © Rapid Electronics Example Microswitch (usually SPDT = ON-ON) Microswitches are designed to switch fully open or closed in response to small movements. They are available with levers and rollers attached. Photograph © Rapid Electronics Keyswitch A key operated switch. The example shown is SPST. Photograph © Rapid Electronics Tilt Switch (SPST) Tilt switches contain a conductive liquid and when tilted this bridges the contacts inside, closing the switch. They can be used as a sensor to detect the position of an object. Some tilt switches contain mercury which is poisonous. Photograph © Rapid Electronics Reed Switch (usually SPST) The contacts of a reed switch are closed by bringing a small magnet near the switch. They are used in security circuits, for example to check that doors are closed. Standard reed switches are SPST (simple on-off) but SPDT (changeover) versions are also available. Warning: reed switches have a glass body which is easily broken! For advice on handling please see the Electronics in Meccano website. Photograph © Rapid Electronics DIP Switch (DIP = Dual In-line Parallel) This is a set of miniature SPST on-off switches, the example shown has 8 switches. The package is the same size as a standard DIL (Dual In-Line) integrated circuit. This type of switch is used to set up circuits, e.g. setting the code of a remote control. Photograph © Rapid Electronics Multi-pole Switch The picture shows a 6-pole double throw switch, also known as a 6-pole changeover switch. It can be set to have momentary or latching action. Latching action means it behaves as a push-push switch, push once for the first position, push again for the second position etc. Photograph © Rapid Electronics Multi-way Switch Multi-way switches have 3 or more conducting positions. They may have several poles (contact sets). A popular type has a rotary action and it is available with a range of contact arrangements from 1-pole 12-way to 4-pole 3 way. The number of ways (switch positions) may be reduced by adjusting a stop under the fixing nut. For example if you need a 2-pole 5-way switch you can buy the 2-pole 6-way version and adjust the stop. Contrast this multi-way switch (many switch positions) with the multipole switch (many contact sets) described above. Photograph © Rapid Electronics Transistors This page covers practical matters such as precautions when soldering and identifying leads. The operation and use of transistors is covered by the Transistor Circuits page. Types | Connecting | Soldering | Heat sinks | Testing | Codes | Choosing | Darlington pair Also see: Heat sinks | Transistor Circuits Function Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage. A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on). The amount of current amplification is called the current gain, symbol hFE. For further information please see the Transistor Circuits page. Types of transistor There are two types of standard transistors, NPN and PNP, with different circuit symbols. The letters refer to the layers of semiconductor material used to make the transistor. Most transistors used today are NPN because this is the easiest type to make from silicon. If you are new to electronics it is best to start by learning how to use NPN transistors. The leads are labelled base (B), collector (C) and emitter Transistor circuit symbols (E). These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels! A Darlington pair is two transistors connected together to give a very high current gain. In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page. Connecting Transistors have three leads which must be connected the correct way round. Please take care with this because a wrongly connected transistor may be damaged instantly when you switch on. If you are lucky the orientation of the transistor will be clear from the PCB or stripboard layout diagram, otherwise you will need to refer to a supplier's catalogue to identify the leads. The drawings on the right show the leads for some of the most common case styles. Please note that transistor lead diagrams Transistor leads for some common case styles. show the view from below with the leads towards you. This is the opposite of IC (chip) pin diagrams which show the view from above. Please see below for a table showing the case styles of some common transistors. Soldering Transistors can be damaged by heat when soldering so if you are not an expert it is wise to use a heat sink clipped to the lead between the joint and the transistor body. A standard crocodile clip can be used as a heat sink. Crocodile clip Photograph © Rapid Electronics. Do not confuse this temporary heat sink with the permanent heat sink (described below) which may be required for a power transistor to prevent it overheating during operation. Heat sinks Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a Heat sink heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air. Photograph © Rapid Electronics For further information please see the Heat sinks page. Testing a transistor Transistors can be damaged by heat when soldering or by misuse in a circuit. If you suspect that a transistor may be damaged there are two easy ways to test it: 1. Testing with a multimeter Use a multimeter or a simple tester (battery, resistor and LED) to check each pair of leads for conduction. Set a digital multimeter to diode test and an analogue multimeter to a low resistance range. Test each pair of leads both ways (six tests in total): y y y The base-emitter (BE) junction should Testing an NPN transistor behave like a diode and conduct one way only. The base-collector (BC) junction should behave like a diode and conduct one way only. The collector-emitter (CE) should not conduct either way. The diagram shows how the junctions behave in an NPN transistor. The diodes are reversed in a PNP transistor but the same test procedure can be used. 2. Testing in a simple switching circuit Connect the transistor into the circuit shown on the right which uses the transistor as a switch. The supply voltage is not critical, anything between 5 and 12V is suitable. This circuit can be quickly built on breadboard for example. Take care to include the 10k resistor in the base connection or you will destroy the transistor as you test it! If the transistor is OK the LED should light when the switch is pressed and not light when the switch is released. To test a PNP transistor use the same circuit but reverse the LED and the supply voltage. A simple switching circuit to test an NPN transistor Some multimeters have a 'transistor test' function which provides a known base current and measures the collector current so as to display the transistor's DC current gain hFE. Transistor codes There are three main series of transistor codes used in the UK: y Codes beginning with B (or A), for example BC108, BC478 The first letter B is for silicon, A is for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. The rest of the code identifies the particular transistor. There is no obvious logic to the numbering system. Sometimes a letter is added to the end (eg BC108C) to identify a special version of the main type, for example a higher current gain or a different case style. If a project specifies a higher gain version (BC108C) it must be used, but if the general code is given (BC108) any transistor with that code is suitable. y Codes beginning with TIP, for example TIP31A TIP refers to the manufacturer: Texas Instruments Power transistor. The letter at the end identifies versions with different voltage ratings. y Codes beginning with 2N, for example 2N3053 The initial '2N' identifies the part as a transistor and the rest of the code identifies the particular transistor. There is no obvious logic to the numbering system. Choosing a transistor Most projects will specify a particular transistor, but if necessary you can usually substitute an equivalent transistor from the wide range available. The most important properties to look for are the maximum collector current IC and the current gain hFE. To make selection easier most suppliers group their transistors in categories determined either by their typical use or maximum power rating. To make a final choice you will need to consult the tables of technical data which are normally provided in catalogues. They contain a great deal of useful information but they can be difficult to understand if you are not familiar with the abbreviations used. The table below shows the most important technical data for some popular transistors, tables in catalogues and reference books will usually show additional information but this is unlikely to be useful unless you are experienced. The quantities shown in the table are explained below. NPN transistors Code BC107 BC108 BC108C BC109 BC182 BC182L BC547B BC548B BC549B 2N3053 BFY51 Structure NPN NPN NPN NPN NPN NPN NPN NPN NPN NPN NPN Case style IC max. VCE hFE max. min. Ptot max. Category (typical use) Audio, low power General purpose, low power General purpose, low power Audio (low noise), low power General purpose, low power General purpose, low power Audio, low power General purpose, low power Audio (low noise), low power General purpose, low power BC184 BC549 Possible substitutes BC182 BC547 BC108C BC183 BC548 TO18 100mA 45V 110 300mW TO18 100mA 20V 110 300mW TO18 100mA 20V 420 600mW TO18 200mA 20V 200 300mW TO92C 100mA 50V 100 350mW TO92A 100mA 50V 100 350mW TO92C 100mA 45V 200 500mW TO92C 100mA 30V 220 500mW TO92C 100mA 30V 240 625mW TO39 700mA 40V TO39 1A 30V 50 40 500mW BC107 BC182L BC107 BC182 BC107B BC108B BC109 BFY51 General purpose, 800mW medium power BC639 BC639 TIP29A TIP31A TIP31C TIP41A 2N3055 NPN NPN NPN NPN NPN NPN TO92A TO220 TO220 TO220 TO220 TO3 1A 1A 3A 3A 6A 15A 80V 60V 60V 40 40 10 800mW 30W 40W 40W 65W 117W General purpose, medium power General purpose, high power General purpose, high power General purpose, high power General purpose, high power General purpose, high power BFY51 TIP31C TIP41A 100V 10 60V 60V 15 20 TIP31A TIP41A Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data. PNP transistors Code BC177 BC178 BC179 BC477 BC478 TIP32A TIP32C Structure PNP PNP PNP PNP PNP PNP PNP Case style IC max. VCE hFE max. min. Ptot max. Category (typical use) Audio, low power General purpose, low power Audio (low noise), low power Audio, low power General purpose, low power General purpose, high power General purpose, high power BC177 BC178 Possible substitutes BC477 BC478 TO18 100mA 45V 125 300mW TO18 200mA 25V 120 600mW TO18 200mA 20V 180 600mW TO18 150mA 80V 125 360mW TO18 150mA 40V 125 360mW TO220 TO220 3A 3A 60V 25 40W 40W TIP32C 100V 10 TIP32A Please note: the data in this table was compiled from several sources which are not entirely consistent! Most of the discrepancies are minor, but please consult information from your supplier if you require precise data. Structure This shows the type of transistor, NPN or PNP. The polarities of the two types are different, so if you are looking for a substitute it must be the same type. There is a diagram showing the leads for some of the most common case styles in the Connecting section above. This information is also available in suppliers' catalogues. Maximum collector current. Maximum voltage across the collector-emitter junction. You can ignore this rating in low voltage circuits. Case style IC max. VCE max. hFE This is the current gain (strictly the DC current gain). The guaranteed minimum value is given because the actual value varies from transistor to transistor - even for those of the same type! Note that current gain is just a number so it has no units. The gain is often quoted at a particular collector current IC which is usually in the middle of the transistor's range, for example '100@20mA' means the gain is at least 100 at 20mA. Sometimes minimum and maximum values are given. Since the gain is roughly constant for various currents but it varies from transistor to transistor this detail is only really of interest to experts. Why hFE? It is one of a whole series of parameters for transistors, each with their own symbol. There are too many to explain here. Ptot max. Maximum total power which can be developed in the transistor, note that a heat sink will be required to achieve the maximum rating. This rating is important for transistors operating as amplifiers, the power is roughly IC × VCE. For transistors operating as switches the maximum collector current (IC max.) is more important. This shows the typical use for the transistor, it is a good starting point when looking for a substitute. Catalogues may have separate tables for different categories. Category Possible substitutes These are transistors with similar electrical properties which will be suitable substitutes in most circuits. However, they may have a different case style so you will need to take care when placing them on the circuit board. Darlington pair This is two transistors connected together so that the amplified current from the first is amplified further by the second transistor. This gives the Darlington pair a very high current gain such as 10000. Darlington pairs are sold as complete packages containing the two transistors. They have three leads (B, C and E) which are equivalent to the leads of a standard individual transistor. You can make up your own Darlington pair from two transistors. For example: y y For TR1 use BC548B with hFE1 = 220. For TR2 use BC639 with hFE2 = 40. The overall gain of this pair is hFE1 × hFE2 = 220 × 40 = 8800. The pair's maximum collector current IC(max) is the same as TR2. Other Components LDR | Thermistor | Piezo transducer | Loudspeaker | Buzzer & Bleeper | Inductor (coil) Light Dependent Resistor (LDR) An LDR is an input transducer (sensor) which converts brightness (light) to resistance. It is made from cadmium sulphide (CdS) and the resistance decreases as the brightness of light falling on the LDR increases. A multimeter can be used to find the resistance in darkness and bright light, these are the typical results for a standard LDR: y y Darkness: maximum resistance, about 1M . Very bright light: minimum resistance, about 100 . Photograph © Rapid Electronics For many years the standard LDR has been the ORP12, now the NORPS12, which is about 13mm diameter. Miniature LDRs are also available and their diameter is about 5mm. An LDR may be connected either way round and no special precautions are required when soldering. circuit symbol Thermistor A thermistor is an input transducer (sensor) which converts temperature (heat) to resistance. Almost all thermistors have a negative temperature coefficient (NTC) which means their resistance decreases as their temperature increases. It is possible to make thermistors with a positive temperature coefficient (resistance increases as temperature increases) but these are rarely used. Always assume NTC if no information is given. A multimeter can be used to find the resistance at various temperatures, these are some typical readings for example: y y y Icy water 0°C: high resistance, about 12k . Room temperature 25°C: medium resistance, about 5k . Boiling water 100°C: low resistance, about 400 . Photograph © Rapid Electronics Suppliers usually specify thermistors by their resistance at 25°C (room temperature). Thermistors take several seconds to respond to a sudden temperature change, small thermistors respond more rapidly. circuit symbol A thermistor may be connected either way round and no special precautions are required when soldering. If it is going to be immersed in water the thermistor and its connections should be insulated because water is a weak conductor; for example they could be coated with polyurethane varnish. Piezo transducer Photograph © Rapid Electronics Piezo transducers are output transducers which convert an electrical signal to sound. They require a driver circuit (such as a 555 astable) to provide a signal and if this is near their natural (resonant) frequency of about 3kHz they will produce a particularly loud sound. circuit symbol Piezo transducers require a small current, usually less than 10mA, so they can be connected directly to the outputs of most ICs. They are ideal for buzzes and beeps, but are not suitable for speech or music because they distort the sound. They are sometimes supplied with red and black leads, but they may be connected either way round. PCB-mounting versions are also available. Piezo transducers can also be used as input transducers for detecting sudden loud noises or impacts, effectively behaving as a crude microphone. Photograph © Rapid Electronics capacitor in series to block DC Loudspeaker Loudspeakers are output transducers which convert an electrical signal to sound. Usually they are called 'speakers'. They require a driver circuit, such as a 555 astable or an audio amplifier, to provide a signal. There is circuit symbol a wide range available, but for many electronics projects a 300mW miniature loudspeaker is ideal. This type is about 70mm diameter and it is usually available with resistances of 8 and 64 . If a project specifies a 64 speaker you must use this higher resistance to prevent damage to the driving circuit. Most circuits used to drive loudspeakers produce an audio (AC) signal which is combined with a constant DC signal. The DC will make a large current flow through the speaker due to its low resistance, possibly damaging both the speaker and the driving circuit. To prevent this happening a large value electrolytic capacitor is connected in series with the speaker, this blocks DC but passes audio (AC) signals. See capacitor coupling. Loudspeakers may be connected either way round except in stereo circuits when the + and - markings on their terminals must be observed to ensure the two speakers are in phase. Correct polarity must always be observed for large speakers in cabinets because the cabinet may contain a small circuit (a 'crossover network') which diverts the high frequency signals to a small speaker (a 'tweeter') because the large main speaker is poor at reproducing them. Miniature loudspeakers can also be used as a microphone and they work surprisingly well, certainly good enough for speech in an intercom system for example. Buzzer (about 400Hz) Bleeper (about 3kHz) Photographs © Rapid Electronics Buzzer and Bleeper These devices are output transducers converting electrical energy to sound. They contain an internal oscillator to produce the sound which is set at about 400Hz for buzzers and about 3kHz for bleepers. circuit symbol Buzzers have a voltage rating but it is only approximate, for example 6V and 12V buzzers can be used with a 9V supply. Their typical current is about 25mA. Bleepers have wide voltage ranges, such as 3-30V, and they pass a low current of about 10mA. Buzzers and bleepers must be connected the right way round, their red lead is positive (+). Inductor (coil) An inductor is a coil of wire which may have a core of air, iron or ferrite (a brittle material made from iron). Its electrical property is called inductance and the unit for this is the henry, symbol H. 1H is very large so mH and µH are used, 1000µH = 1mH and 1000mH = 1H. Iron and ferrite cores increase the inductance. Inductors are mainly used in tuned circuits and to block high frequency AC signals (they are sometimes called chokes). They pass DC easily, but block AC signals, this is the opposite of capacitors. Inductor (miniature) Inductors are rarely found in simple projects, but one exception is the tuning coil of a radio receiver. This is an inductor which you may have to make yourself by neatly winding enamelled copper wire around a ferrite rod. Enamelled copper wire has very thin insulation, allowing the turns of the coil to be close together, but this makes it impossible to strip in the usual way - the best method is to gently pull the ends of the wire through folded emery paper. Warning: a ferrite rod is brittle so treat it like glass, not iron! Ferrite rod Photographs © Rapid Electronics circuit symbol An inductor may be connected either way round and no special precautions are required when soldering.