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Pin connections	CONTROL VOLTAGE input
Astable circuits	Monostable circuits
Astable component selection	More about triggering
More astables	555 as a transducer driver
RESET input	LINKS . . .

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Pin connections

You can use the 555 effectively without understanding the function of each pin in detail.

The 555 timer is an extremely versatile integrated circuit which can be used to build lots of different circuits.

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Astable circuits

Astable circuits produce pulses. The circuit most people use to make a 555 astable looks like this:

As you can see, the frequency, or repetition rate, of the output pulses is determined by the values of two resistors, R1 and R2 and by the timing capacitor, C.

The design formula for the frequency of the pulses is:

The HIGH and LOW times of each pulse can be calculated from:

The duty cycle of the waveform, usually expressed as a percentage, is given by:

An alternative measurement of HIGH and LOW times is the mark space ratio:

Before calculating a frequency, you should know that it is usual to make R1=1 kW because this helps to give the output pulses a duty cycle close to 50%, that is, the HIGH and LOW times of the pulses are approximately equal.

Remember that design formulae work in fundamental units. However, it is often more convenient to work with other combinations of units:

resistance	capacitance	period	frequency
	F	s	Hz
	µF	s	Hz
	µF	ms	kHz

With R values in MW and C values in µF, the frequency will be in Hz. Alternatively, with R values in kW and C values in µF, frequencies will be in kHz.

Suppose you want to design a circuit to produce a frequency of approximately 1 kHz for an alarm application. What values of R1, R2 and C should you use?

R1 should be 1kW, as already explained. This leaves you with the task of selecting values for R2 and C. The best thing to do is to rearrange the design formula so that the R values are on the right hand side:

Now substitute for R1 and f :

You are using R values in kW and f values in kHz, so C values will be in µF.

To make further progress, you must choose a value for C. At the same time, it is important to remember that practical values for R2 are between 1 kW and 1MW. Suppose you choose C = 10 nF = 0.01 µF:

that is:

and:

This is within the range of practical values and you can choose values from the E12 range of 68 kW or 82 kW. (The E12 range tells you which values of resistor are manufactured and easily available from suppliers.)

A test circuit can be set up on prototype board, as follows:

With the values of R1, R2 and C shown, the LED should flash at around 10 Hz.

What happens if you replace R2 with an LDR or a thermistor? This gives an astable which changes frequency in response to light intensity, or with temperature.

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Astable component selection

With a little practice, it is quite easy to choose appropriate values for a 555 timer astable. To make things even easier, you might like to download the DOCTRONICS 555 timer component selection program.

The program works with Windows 95 and looks like this:

To download the program (~210K), click on its image.

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More astables

Extended duty cycle astable:

An extremely useful variation of the standard astable circuit involves adding a diode in parallel with R2:

This simple addition has a dramatic effect on the behaviour of the circuit. The timing capacitor, C, is now filled only through R1 and emptied only through R2.

The design equation for the output pulse frequency is:

HIGH and LOW times are calculated from:

With this circuit, the duty cycle can be any value you want. If R1 > R2, the duty cycle will be greater than 50% (equivalent to a mark space ratio of more than 1.0). On the other hand, if R2 > R1, the duty cycle will be less than 50% (mark space ratio less than 1.0).

This version of the 555 astable is used in the cyclist/pedestrian safety lights project.

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Minimum component astable:

This is a cheap and cheerful astable using just one resistor and one capacitor as the timing components:

Note that there is no connection to pin 7 and that R1 is linked to the output, pin 3.

The design equation for the circuit is:

The HIGH and LOW times are supposed to be equal, giving a duty cycle of 50% (equivalent to a mark space ratio of 1.0).

However, if you build this circuit, it is probable that the HIGH time will be longer than the LOW time. (This happens because the maximum voltage reached by the output pulses is less than the power supply voltage.) Things will get worse if the output current increases.

If you need an astable circuit which can be adjusted to give an accurate frequency, this circuit is not the one to choose.

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Diminishing frequency astable:

The excitement and realism of electronic games, including roulette, can be increased using an astable circuit which is triggered to produce rapid pulses initially, but which then slows down and eventually stops altogether.

It is easy to modify the basic 555 astable circuit to produce this result:

When the 'go' button is pressed, the 47 µF capacitor in parallel with the timing network, R1, R2 and C, charges up very quickly through the 100 W resistor. When the button is released, the astable continues to oscillate but the charge stored slowly leaks away, with the result that it takes longer and longer to charge up the timing capacitor. To trigger the next pulse, the voltage across C must increase to two thirds of the power supply voltage. After a while, the voltage across the 47 µF drops below this value and the pulses stop.

With the values shown, the initial frequency is about 100 Hz and the output pulses coast to a stop after around 40 seconds.

The initial frequency can be calculated from the design equation for the basic 555 astable. To give a realistic coasting time, you should use large values for the resistors R1 and R2. The coasting time is determined by the 47 µF capacitor. Experiment with different values until you get the effect you want.

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RESET input

If the RESET input, pin 4, is held HIGH, a 555 astable circuit functions as normal. However, if the RESET input is held LOW, output pulses are stopped. You can investigate this effect by connecting a switch/pull down resistor voltage divider to pin 4:

Here is the circuit on prototype board:

Use the design formula, or the DOCTRONICS component selector program to calculate the frequency of pulses you would expect to obtain with this circuit.

In an electronic die, provided the output pulses are fast enough, it is impossible to 'cheat' by holding down the button for a definite length of time.

Think about how you could use this circuit together with a bistable as part of a burglar alarm. Under normal conditions, the output of the bistable is LOW and the astable is stopped. If the alarm is triggered, the output of the bistable goes HIGH and the pulses start, sounding the alarm.

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CONTROL VOLTAGE input

By applying a voltage to the CONTROL VOLTAGE input, pin 5, you can alter the timing characteristics of the device. In the astable mode, the control voltage can be varied from 1.7 V to the power supply voltage, producing an output frequency which can be higher or lower than the frequency set by the R1, R2, C timing network.

The CONTROL VOLTAGE input can be used to build an astable with a frequency modulated output. In the circuit below, one astable is used to control the frequency of a second, giving a 'police siren' sound effect.

In most applications, the CONTROL VOLTAGE input is not used. It is usual to connect a 10 nF capacitor between pin 5 and 0 V to prevent interference. You don't need to do this in building a test circuit, but this 'bypass' or 'decoupling' capacitor should be included in your final circuit.

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Monostable circuits

A monostable circuit produces a single pulse when triggered. The two questions about monostables you immediately need to ask are:

• How can the circuit be triggered to produce an output pulse?
• How is the duration, or period, of the output pulse determined?

The circuit used to make a 555 timer monostable is:

As you can see, the trigger input is held HIGH by the 10 kW pull up resistor and is pulsed LOW when the trigger switch is pressed. The circuit is triggered by a falling edge, that is, by a sudden transition from HIGH to LOW.

The trigger pulse, produced by pressing the button, must be of shorter duration than the intended output pulse.

The period, t, of the output pulse can be calculated from the design equation:

Remember again about compatible measurement units:

resistance	capacitance	period
	F	s
	µF	s
	µF	ms

With R1 = 1 MW and C = 1 µF, the output pulse will last for 1.1 s.

You can build a test version of the 555 monostable as follows:

By clicking on the monostable tab, the 555 component selection program can be used to investigate the effect of different R1 and C values:

To download the program (~210K), click on its image.

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More about triggering

For a simple 555 monostable, the trigger pulse must be shorter than the output pulse. Sometimes you want to trigger the monostable from a longer pulse:

The trigger network detects the falling edge at the end of each V_in pulse, producing a short 'spike' which triggers the monostable at the appropriate time. The period of the monostable pulse is shorter than the period of the V_in pulses.

If you want to trigger the monostable from a rising edge, you need to add a transistor NOT gate to the trigger circuit:

If you build these circuits, it is interesting to investigate the action of the trigger network using an oscilloscope.

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555 as a transducer driver

A transducer is a subsytem which converts energy from one form into another, where one of the forms is electrical. In an output transducer, for example, electrical energy can be converted into light, sound, or movement.

The output of a 555 timer can deliver more than 100 mA of current. This means that output transducers including buzzers, filament lamps, loudspeakers and small motors can be connected directly to the output of the 555, pin 3.

You can use the 555 as a transducer driver, that is, as an electronic switch which turns the transducer ON or OFF:

This circuit has an inverting Schmitt trigger action. The 'inverting' part of this description means that when V_in is LOW, the output is HIGH, and when V_in is HIGH, the output is LOW.

In a 'Schmitt trigger' circuit there are two different switching thresholds. If V_in is slowly increased starting from 0 V, the output voltage snaps from HIGH to LOW when V_in reaches a level equal to ²/₃ of the power supply voltage. Once this level has been exceeded, decreasing V_in does not affect the output until V_in drops below ¹/₃ of the power supply voltage. (If an input change in one direction produces a different result from a change in the opposite direction, the circuit is said to show hysteresis.)

If a filament lamp is connected between the positive power supply rail and the output, as shown above, current flows through the lamp when the output voltage is LOW. In other words, the lamp lights when the input voltage is HIGH.

If you connect the lamp between the output and 0 V, the circuit will still work, but the lamp will light when the input voltage is LOW:

Note that, in both versions of the circuit pins 2 and 6 are joined together. The circuit can be simplified by omitting the 10  nF bypass capacitor, and will continue to work when the RESET input, pin 4 is left unconnected.

Some people are very fond of this circuit and use it whenever a transducer driver is required. However, with a HIGH/LOW digital input signal the same result can be achieved more obviously and at lower cost using a transistor switch circuit.

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Links

Cross references in the Beastie Zone:

Cross references in Design Electronics:

Books: 110 IC Timer Projects for the Home Constructor, Jules H Gilder, Newnes: ISBN 0-408-00480-0
    IC 555 Projects, E.A. Parr, Babani: ISBN 0-85934-047-3

External links: 555 timer

Data sheets: The links below allow you to download documents in Adobe Acrobat, format. You will need Acrobat Reader. You can install this from the cover CD of many computer magazines, or download it direct from Adobe:

    555 data sheet (Phillips Semiconductors, 1994)

    555 application note (Philips Semiconductors AN 170, 1988)

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