CIRCUIT DIAGRAM: August 2010

Electricity and the Electron

What is electricity?

Electricity is the flow of charge around a circuit carrying energy from the battery (or power supply) to components such as lamps and motors.

Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.

The diagram shows a simple circuit of a battery, wires, a switch and a lamp. The switch works by breaking the circuit.

With the switch open the circuit is broken - so electricity cannot flow and the lamp is off.

With the switch closed the circuit is complete - allowing electricity to flow and the lamp is on. The electricity is carrying energy from the battery to the lamp.

We can see, hear or feel the effects of electricity flowing such as a lamp lighting, a bell ringing, or a motor turning - but we cannot see the electricity itself, so which way is it flowing?

Imaginary positive particles
moving in the direction of
the conventional current

Which way does electricity flow?

We say that electricity flows from the positive (+) terminal of a battery to the negative (-) terminal of the battery. We can imagine particles with positive electric charge flowing in this direction around the circuit, like the red dots in the diagram.

This flow of electric charge is called conventional current.

This direction of flow is used throughout electronics and it is the one you should remember and use to understand the operation of circuits.

However this is not the whole answer because the particles that move in fact have negative charge! And they flow in the opposite direction! Please read on...

The electron

When electricity was discovered scientists tried many experiments to find out which way the electricity was flowing around circuits, but in those early days they found it was impossible to find the direction of flow.

They knew there were two types of electric charge, positive (+) and negative (-), and they decided to say that electricity was a flow of positive charge from + to -. They knew this was a guess, but a decision had to be made! Everything known at that time could also be explained if electricity was negative charge flowing the other way, from - to +.

The electron was discovered in 1897 and it was found to have a negative charge. The guess made in the early days of electricity was wrong! Electricity in almost all conductors is really the flow of electrons (negative charge) from - to +.

By the time the electron was discovered the idea of electricity flowing from + to - (conventional current) was firmly established. Luckily it is not a problem to think of electricity in this way because positive charge flowing forwards is equivalent to negative charge flowing backwards.

To prevent confusion you should always use conventional current when trying to understand how circuits work, imagine positively charged particles flowing from + to -.

Books about 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	ISBN and Publisher	Comments
Basic Skills: Electronics by Tom Duncan	ISBN: 0 7195 4449 1 Publisher:John Murray	This is a suitable textbook for a beginner.
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 Electronics by Malcolm Plant	ISBN: 0 3404 2230 0 Publisher:Hodder & Stoughton	A self-study book covering the essentials of electronics.
Electronics - A First Course by Owen Bishop	ISBN: 0 7506 5545 3 Publisher: Newnes	A suitable textbook for GCSE, Intermediate GNVQ and City & Guilds courses. Practical work is introduced almost immediately.
Electronics for Today and Tomorrow by Tom Duncan	ISBN: 0 7195 7413 7 Publisher:John Murray	A suitable textbook for GCSE and AS/A level. No projects or practical exercises.
Success in Electronics by Tom Duncan	ISBN: 0 7195 7205 3 Publisher:John Murray	A self-study textbook for GCSE and AS/A level. No projects or practical exercises.
Electronics Explained by M W Brimicombe	ISBN: 0 17 448303 1 Publisher:Nelson Thornes	An AS/A level textbook with many practical exercises.
Analogue Electronics by John C Morris	ISBN: 0 3407 1925 7 Publisher: Newnes	An AS/A level textbook with many practical investigations to support its discovery-based approach. Transistors, operational amplifiers, thyristors and triacs are covered.
Digital Electronics by John C Morris	ISBN: 0 3405 5638 2 Publisher: Newnes	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.
Reference Books Learn how to use the integrated circuits covered by these books.
IC 555 Projects by E Parr	ISBN: 0 85934 047 3 Publisher: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.
A Beginners Guide to CMOS Digital ICs by R Penfold	ISBN: 0 85934 333 2 Publisher:Bernard Babani	This book explains how to use the 4000 series CMOS logic gates, counters, display drivers and so on. They are ideal for battery powered projects because they use little power and can tolerate a wide range of supply voltages.
A Beginners Guide to TTL Digital ICs by R Penfold	ISBN: 0 85934 332 4 Publisher:Bernard Babani	This book explains how to use the 74 series TTL logic gates, counters, display drivers and so on.
How to Use Op-Amps by E Parr	ISBN: 0 85934 063 5 Publisher:Bernard Babani	Operational amplifiers are very versatile devices and this book thoroughly explains their operation and use, with many circuit designs for the more experienced constructor.
Master IC Cookbook by Clayton Hallmark & Delton Horn	ISBN: 0 8306 6550 1 out of print try Amazon	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.
Practical Books Learn by building projects on breadboard, no soldering is required.
Book Title and Author	ISBN and Publisher	Comments
Adventures with Electronics by Tom Duncan	ISBN: 0 7195 3554 9 Publisher:John Murray	An introduction to electronics by building transistor circuits on S-Dec, a breadboard system which does not require soldering.
Adventures with Micro-Electronics by Tom Duncan	ISBN: 0 7195 3671 5 Publisher:John Murray	Learn about electronics by building integrated circuit ('chip') projects on standard breadboard (no soldering required). This is more advanced than Adventures with Electronics (above).
Adventures with Digital Electronics by Tom Duncan	ISBN: 0 7195 3875 0 out of print try Amazon	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.

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	ISBN and Publisher	Comments
IC 555 Projects by E Parr	ISBN: 0 85934 047 3 Publisher: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.
How to Use Op-Amps by E Parr	ISBN: 0 85934 063 5 Publisher:Bernard Babani	Operational amplifiers are very versatile devices and this book thoroughly explains their operation and use, with many circuit designs for the more experienced constructor.
Circuit Source Book 1 by R Penfold	ISBN: 0 85934 321 9 Publisher:Bernard Babani	Circuit diagrams to help the experienced constructor design their own projects.
Circuit Source Book 2 by R Penfold	ISBN: 0 85934 322 7 Publisher:Bernard Babani	Circuit diagrams to help the experienced constructor design their own projects.
Practical Electronic Model Railway Projects by R Penfold	ISBN: 0 85934 384 7 Publisher:Bernard Babani	The projects include stripboard layouts, so this is a good book for the beginner.
How to Design and Make Your Own PCBs by R Penfold	ISBN: 0 85934 096 1 Publisher:Bernard Babani	This book is ideal for the home constructor and contains many practical tips.

Quantities and Units in Electronics

Quantity	Usual Symbol	Unit	Unit Symbol
Voltage	V	volt	V
Current	I	amp*	A
Charge	Q	coulomb	C
Resistance	R	ohm
Capacitance	C	farad	F
Inductance	L	henry	H
Reactance	X	ohm
Impedance	Z	ohm
Power	P	watt	W
Energy	E	joule	J
Time	t	second	s
Frequency	f	hertz	Hz
* strictly the unit is ampere, but this is almost always shortened to amp.

Quantities

The table shows electrical quantities which are used in electronics.

The relationship between quantities can be written using words or symbols (letters), but symbols are normally used because they are much shorter; for example V is used for voltage, I for current and R for resistance:

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.

Prefix	Prefix Symbol	Value
milli	m	10^-3	= 0.001
micro	µ	10^-6	= 0.000 001
nano	n	10^-9	= 0.000 000 001
pico	p	10^-12	= 0.000 000 000 001
kilo	k	10³	= 1000
mega	M	10⁶	= 1000 000
giga	G	10⁹	= 1000 000 000
tera	T	10¹²	= 1000 000 000 000

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.

Some of the units have a convenient size for electronics, but most are either too large or too small to be used directly so they are used with the prefixes shown in the second 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

in Ohm's Law would give you wrong values.

Counting Circuits

Binary numbers

Logic states
True	False
1	0
High	Low
+Vs	0V
On	Off

Seen on a T-shirt:There are 10 kinds of
people - those who
understand binary,
and those who don't.

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
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.

4-bit numbers

Binary D C B A	Decimal	Hex base 16
0 0 0 0 0 0 0 1 0 0 1 0 0 0 1 1 0 1 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 0 0 1 0 0 1 1 0 1 0 1 0 1 1 1 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1	0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15	0 1 2 3 4 5 6 7 8 9 A B C D E F

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:

A = 1, the 'least significant bit' (LSB)
B = 2
C = 4
D = 8, the 'most significant bit' (MSB)

Binary Coded Decimal, BCD

Binary Coded Decimal, BCD, is a special version of 4-bit binary where the count resets to zero (0000) after the ninth count (1001). It is used by decade counters and is easily converted to display the decimal digits 0-9 on a 7-segment display.

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!

Counters

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

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).

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

The operation of a flip-flop
Notice how the output frequency
is half the input frequency

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 next. A flip-flop output changes state every 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.

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 havecarry out and carry in pins for linking counters without introducing any ripple delays.

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 'reset-bar').

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).

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 the4028).

7-segment display drivers

decade counter, display driver and 7-segment display

Decade counter with display
driver and 7-segment display

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:

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 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.

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.
connecting synchronous counters

555 and 556 Timer Circuits

Introduction

Example circuit symbol (above)Actual pin arrangements (below)

There is more information about
555 timers and their circuits on the
Electronics in Meccano website.

The 8-pin 555 timer must be one of the most useful ICs ever made and it is used in many projects. With just a few external components it can be used to build many circuits, not all of them involve timing!

A popular version is the NE555 and this is suitable in most cases where a '555 timer' is specified. 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. The circuit diagrams on this page show a 555, but they could all be adapted to use one half of a 556.

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 a 9V supply) is too low for many standard 555 circuits. The ICM7555 has the same pin arrangement as a standard 555.

The circuit symbol for a 555 (and 556) is a box with the pins arranged to suit the circuit diagram: for example 555 pin 8 at the top for the +Vs supply, 555 pin 3 output on the right. Usually just the pin numbers are used and they are not labelled with their function.

The 555 and 556 can be used with a supply voltage (Vs) in the range 4.5 to 15V (18V absolute maximum).

Standard 555 and 556 ICs create a significant 'glitch' on the supply when their output changes state. This is rarely a problem in simple circuits with no other ICs, but in more complex circuits a smoothing capacitor (eg 100µF) should be connected across the +Vs and 0V supply near the 555 or 556.

The input and output pin functions are described briefly below and there are fuller explanations covering the various circuits:

Astable - producing a square wave
Monostable - producing a single pulse when triggered
Bistable - a simple memory which can be set and reset
Buffer - an inverting buffer (Schmitt trigger)

Datasheets are available from:

Inputs of 555/556

Trigger input: when < ¹/₃ Vs ('active low') this makes the output high (+Vs). It monitors the discharging of the timing capacitor in an astable circuit. It has a high input impedance > 2M

Threshold input: when > ²/₃ Vs ('active high') this makes the output low (0V)*. It monitors the charging of the timing capacitor in astable and monostable circuits. It has a high input impedance > 10M

.
* providing the trigger input is > ¹/₃ Vs, otherwise the trigger input will override the threshold input and hold the output high (+Vs).

Reset input: when less than about 0.7V ('active low') this makes the output low (0V), overriding other inputs. When not required it should be connected to +Vs. It has an input impedance of about 10k

Control input: this can be used to adjust the threshold voltage which is set internally to be ²/₃ Vs. Usually this function is not required and the control input is connected to 0V with a 0.01µF capacitor to eliminate electrical noise. It can be left unconnected if noise is not a problem.

The discharge pin is not an input, but it is listed here for convenience. It is connected to 0V when the timer output is low and is used to discharge the timing capacitor in astable and monostable circuits.

connecting a loudspeaker to 555 and 556 outputs

Output of 555/556

The output of a standard 555 or 556 can sink and source up to 200mA. This is more than most ICs and it is sufficient to supply many output transducers directly, including LEDs (with a resistor in series), low current lamps, piezo transducers, loudspeakers (with a capacitor in series), relay coils (with diode protection) and some motors (with diode protection). The output voltage does not quite reach 0V and +Vs, especially if a large current is flowing.

To switch larger currents you can connect a transistor.

The ability to both sink and source current means that two devices can be connected to the output so that one is on when the output is low and the other is on when the output is high. The top diagram shows two LEDs connected in this way. This arrangement is used in the Level Crossing project to make the red LEDs flash alternately.

Loudspeakers

A loudspeaker (minimum resistance 64

) may be connected to the output of a 555 or 556 astable circuit but a capacitor (about 100µF) must be connected in series. The output is equivalent to a steady DC of about ½Vs combined with a square wave AC (audio) signal. The capacitor blocks the DC, but allows the AC to pass as explained incapacitor coupling.

Piezo transducers may be connected directly to the output and do not require a capacitor in series.

Relay coils and other inductive loads

Like all ICs, the 555 and 556 must be protected from the brief high voltage 'spike' produced when an inductive load such as a relay coil is switched off. The standardprotection diode must be connected 'backwards' across the the relay coil as shown in the diagram.

However, the 555 and 556 require an extra diode connected in series with the coil to ensure that a small 'glitch' cannot be fed back into the IC. Without this extra diode monostable circuits may re-trigger themselves as the coil is switched off! The coil current passes through the extra diode so it must be a 1N4001 or similar rectifier diode capable of passing the current, a signal diode such as a 1N4148 is usually not suitable.

555/556 Astable

555 astable output, a square wave
(Tm and Ts may be different)

555 astable circuit

An astable circuit produces a 'square wave', this is a digital waveform with sharp transitions between low (0V) and high (+Vs). Note that the durations of the low and high states may be different. The circuit is called an astable because it is not stable in any state: the output is continually changing between 'low' and 'high'.

The time period (T) of the square wave is the time for one complete cycle, but it is usually better to consider frequency (f) which is the number of cycles per second.

T = 0.7 × (R1 + 2R2) × C1 and f =	1.4
	(R1 + 2R2) × C1

T = time period in seconds (s)
f = frequency in hertz (Hz)
R1 = resistance in ohms (

)
R2 = resistance in ohms (

)
C1 = capacitance in farads (F)

The time period can be split into two parts: T = Tm + Ts
Mark time (output high): Tm = 0.7 × (R1 + R2) × C1
Space time (output low): Ts = 0.7 × R2 × C1

Many circuits require Tm and Ts to be almost equal; this is achieved if R2 is much larger than R1.

For a standard astable circuit Tm cannot be less than Ts, but this is not too restricting because the output can both sink and source current. For example an LED can be made to flash briefly with long gaps by connecting it (with its resistor) between +Vs and the output. This way the LED is on during Ts, so brief flashes are achieved with R1 larger than R2, making Ts short and Tm long. If Tm must be less than Ts a diode can be added to the circuit as explained under duty cycle below.

Choosing R1, R2 and C1

555 astable frequencies
C1	R2 = 10k R1 = 1k	R2 = 100k R1 = 10k	R2 = 1M R1 = 100k
0.001µF	68kHz	6.8kHz	680Hz
0.01µF	6.8kHz	680Hz	68Hz
0.1µF	680Hz	68Hz	6.8Hz
1µF	68Hz	6.8Hz	0.68Hz
10µF	6.8Hz	0.68Hz (41 per min.)	0.068Hz (4 per min.)

R1 and R2 should be in the range 1k

to 1M

. It is best to choose C1 first because capacitors are available in just a few values.

Choose C1 to suit the frequency range you require (use the table as a guide).
Choose R2 to give the frequency (f) you require. Assume that R1 is much smaller than R2 (so that Tm and Ts are almost equal), then you can use:

R2 = 0.7

f × C1
Choose R1 to be about a tenth of R2 (1k min.) unless you want the mark time Tm to be significantly longer than the space time Ts.
If you wish to use a variable resistor it is best to make it R2.
If R1 is variable it must have a fixed resistor of at least 1k in series
(this is not required for R2 if it is variable).

Astable operation

With the output high (+Vs) the capacitor C1 is charged by current flowing through R1 and R2. The threshold and trigger inputs monitor the capacitor voltage and when it reaches ²/₃Vs (threshold voltage) the output becomes low and the discharge pin is connected to 0V.

The capacitor now discharges with current flowing through R2 into the discharge pin. When the voltage falls to ¹/₃Vs (trigger voltage) the output becomes high again and the discharge pin is disconnected, allowing the capacitor to start charging again.

This cycle repeats continuously unless the reset input is connected to 0V which forces the output low while reset is 0V.

An astable can be used to provide the clock signal for circuits such as counters.

A low frequency astable (< 10Hz) can be used to flash an LED on and off, higher frequency flashes are too fast to be seen clearly. Driving a loudspeaker or piezo transducer with a low frequency of less than 20Hz will produce a series of 'clicks' (one for each low/high transition) and this can be used to make a simple metronome.

An audio frequency astable (20Hz to 20kHz) can be used to produce a sound from a loudspeaker or piezo transducer. The sound is suitable for buzzes and beeps. The natural (resonant) frequency of most piezo transducers is about 3kHz and this will make them produce a particularly loud sound.

Duty cycles

Duty cycle

The duty cycle of an astable circuit is the proportion of the complete cycle for which the output is high (the mark time). It is usually given as a percentage.

For a standard 555/556 astable circuit the mark time (Tm) must be greater than the space time (Ts), so the duty cycle must be at least 50%:

Duty cycle =	Tm	=	R1 + R2
	Tm + Ts		R1 + 2R2

555 astable circuit with diode across R2

To achieve a duty cycle of less than 50% a diode can be added in parallel with R2 as shown in the diagram. This bypasses R2 during the charging (mark) part of the cycle so that Tm depends only on R1 and C1:

Tm = 0.7 × R1 × C1 (ignoring 0.7V across diode)
Ts = 0.7 × R2 × C1 (unchanged)

Duty cycle with diode =	Tm	=	R1
	Tm + Ts		R1 + R2

Use a signal diode such as 1N4148.

Example projects using 555 astable: Flashing LED | Dummy Alarm | Heart-shaped Badge | 'Random' Flasher

555/556 Monostable

555 monostable output, a single pulse

555 monostable circuit with manual trigger

A monostable circuit produces a single output pulse when triggered. It is called a monostable because it is stable in just one state: 'output low'. The 'output high' state is temporary.

The duration of the pulse is called the time period (T) and this is determined by resistor R1 and capacitor C1:

time period, T = 1.1 × R1 × C1

T = time period in seconds (s)
R1 = resistance in ohms (

)
C1 = capacitance in farads (F)
The maximum reliable time period is about 10 minutes.

Why 1.1? The capacitor charges to ²/₃ = 67% so it is a bit longer than the time constant (R1 × C1) which is the time taken to charge to 63%.

Choose C1 first (there are relatively few values available).
Choose R1 to give the time period you need. R1 should be in the range 1k to 1M, so use a fixed resistor of at least 1k in series if R1 is variable.
Beware that electrolytic capacitor values are not accurate, errors of at least 20% are common.
Beware that electrolytic capacitors leak charge which substantially increases the time period if you are using a high value resistor - use the formula as only a very rough guide!
For example the Timer Project should have a maximum time period of 266s (about 4½ minutes), but many electrolytic capacitors extend this to about 10 minutes!

Monostable operation

The timing period is triggered (started) when the trigger input (555 pin 2) is less than ¹/₃ Vs, this makes the output high (+Vs) and the capacitor C1 starts to charge through resistor R1. Once the time period has started further trigger pulses are ignored.

The threshold input (555 pin 6) monitors the voltage across C1 and when this reaches ²/₃ Vs the time period is over and the outputbecomes low. At the same time discharge (555 pin 7) is connected to 0V, discharging the capacitor ready for the next trigger.

The reset input (555 pin 4) overrides all other inputs and the timing may be cancelled at any time by connecting reset to 0V, this instantly makes the output low and discharges the capacitor. If the reset function is not required the reset pin should be connected to +Vs.

Power-on reset or
trigger circuit

Power-on reset or trigger

It may be useful to ensure that a monostable circuit is reset or triggered automatically when the power supply is connected or switched on. This is achieved by using a capacitor instead of (or in addition to) a push switch as shown in the diagram.

The capacitor takes a short time to charge, briefly holding the input close to 0V when the circuit is switched on. A switch may be connected in parallel with the capacitor if manual operation is also required.

This arrangement is used for the trigger in the Timer Project.

Edge-triggering

edge-triggering circuit

If the trigger input is still less than ¹/₃ Vs at the end of the time period the output will remain high until the trigger is greater than ¹/₃ Vs. This situation can occur if the input signal is from an on-off switch or sensor.

The monostable can be made edge triggered, responding only to changes of an input signal, by connecting the trigger signal through a capacitor to the trigger input. The capacitor passes sudden changes (AC) but blocks a constant (DC) signal. For further information please see the page on capacitance. The circuit is 'negative edge triggered' because it responds to a sudden fall in the input signal.

The resistor between the trigger (555 pin 2) and +Vs ensures that the trigger is normally high (+Vs).

Example projects using 555 monostable: Adjustable Timer | Electronic 'Lock' | Light-sensitive Alarm

555/556 Bistable (flip-flop) - a memory circuit

555 bistable circuit

The circuit is called a bistable because it is stable in two states: output high and output low. It is also known as a 'flip-flop'.

It has two inputs:

Trigger (555 pin 2) makes the output high.
Trigger is 'active low', it functions when < ¹/₃ Vs.
Reset (555 pin 4) makes the output low.
Reset is 'active low', it resets when < 0.7V.

The power-on reset, power-on trigger and edge-triggering circuits can all be used as described above for the monostable.

Example projects using 555 bistable: Quiz | Model Railway Signal

555/556 Inverting Buffer (Schmitt trigger) or NOT gate

555 inverting buffer circuit
(a NOT gate)

NOT gate symbol

The buffer circuit's input has a very high impedance (about 1M

) so it requires only a few µA, but the output can sink or source up to 200mA. This enables a high impedance signal source (such as an LDR) to switch a low impedance output transducer (such as a lamp).

It is an inverting buffer or NOT gate because the output logic state (low/high) is the inverse of the input state:

Input low (< ¹/₃ Vs) makes output high, +Vs
Input high (> ²/₃ Vs) makes output low, 0V

When the input voltage is between ¹/₃ and ²/₃ Vs the output remains in its present state. This intermediate input region is a deadspace where there is no response, a property called hysteresis, it is like backlash in a mechanical linkage. This type of circuit is called a Schmitt trigger.

If high sensitivity is required the hysteresis is a problem, but in many circuits it is a helpful property. It gives the input a high immunity to noise because once the circuit output has switched high or low the input must change back by at least ¹/₃ Vs to make the output switch back.

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