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Learn about the 555 by building the DOCTRONICS Safety Lights Project:

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1. Pin connections

555 timer pin connections

The 555 timer is an extremely versatile integrated circuit which can be used to build lots of different circuits. You can use the 555 effectively without understanding the function of each pin in detail.

Frequently, the 555 is used in astable mode to generate a continuous series of pulses, but you can also use the 555 to make a one-shot or monostable circuit. The 555 can source or sink 200 mA of output current, and is capable of driving wide range of output devices.

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2. Astable circuits

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

Yenka simulation 555 astable circuit

The button allows you to open a simulation of this circuit in Yenka©, the electronics simulation program from Crocodile Clips Limited. To use the program, you need to download and install the Yenka© plug-in.

At the moment, Yenka is available for free download to home users from This is a fantastic offer from the creators of Yenka©: don't miss it!

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:

astable design formula

The period, t, of the pulses is given by:

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 kΩ 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:


With R values in MΩ and C values in μF, the frequency will be in Hz. Alternatively, with R values in kΩ 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 1kΩ, 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 kΩ 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 kΩ and 1MΩ. Suppose you choose C = 10 nF = 0.01 μF:

That is:


This is within the range of practical values and you can choose values from the E12 range of 68 kΩ or 82 kΩ. (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:

down open in a new window pins Prototype board layout for 555 astable circuit

Halloween astable

With the values of R1, R2 and C shown, the LED should flash at around 10 Hz. How would you slow the rate of flashing to 1 Hz? Experiment to find a solution.

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.

Clicking the button under the diagram moves you on to the next prototype board layout on this page. Clicking opens the drawing in a new window which you can maximise to fill the screen: there's no excuse for putting a wire link in the wrong place! Clicking pins opens a small window showing the pin layout for the 555, so that you can remind yourself which pin is which. The pins window remains open and can be brought to the front from the task bar at the bottom of the screen, unless you choose to close it.

Astables turn up in all sorts of places, as you can see from the Halloween toy skull with flashing red LED eyes.

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3. 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 all versions of Windows™ from Windows 95™ through to Windows Vista™ and looks like this:

555 component selection

To download the program (~210K), click on its image. You can save the program to your own computer to use whenever you want.

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4. More astables

4.1. Extended duty cycle astable

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

Yenka simulation Extended duty cycle astable

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:

extended duty cycle astable

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 R1 < R2, the duty cycle will be less than 50% (mark space ratio less than 1.0).

This is the version of the 555 astable is used in the DOCTRONICS safety lights project:

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You can see the extended duty cycle astable in action by building it on prototype board:

down up open in a new window Extended duty cycle astable

The components used here are not the same as those in the test circuit for the normal version of the astable. R1=10 kΩ, R2=47 kΩ, and the timing capacitor C=22 μF.

With the diode in place, the HIGH time, when the LED is ON, should be around 0.69(0.01x22)=0.15 s (0.01 is 10 kΩ converted to MΩ). The LOW time should be visibly longer, 0.69(0.047x22)=0.71 s (0.047 is 47 kΩ converted to MΩ). What happens if you temporarily remove the diode from the circuit?

Replace the diode and then replace R2 with a 100 kΩ or a 1 MΩ potentiometer as indicated in the next prototype board layout:

down up open in a new window Adding a potentiometer

The 1 kΩ in series with the potentiometer, used here as a variable resistor, prevents you from adjusting R2 to zero. You need to solder single core wire to the potentiometer terminals: twisting the wire round the terminal doesn't make an effective connection.

As you adjust R2, the ON time of the LED remains constant, while the OFF time varies. Unlike the normal version of the 555 astable, the ON time can be short compared with the OFF time.

4.2 Minimum component astable

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

Yenka simulation Minimum component astable

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:

design equation

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.

On protoytpe board, the circuit looks like this:

down up open in a new window Minimum component astable

With the values shown, the frequency should be around 2.2 Hz.

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

4.3 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:

Yenka simulation Diminishing frequency astable

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 Ω 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:

down up open in a new window Diminishing frequency astable

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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5. 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:

Yenka simulation Investigating the RESET input

Here is the circuit on prototype board:

down up open in a new window Investigating the RESET input

Use the design formula, or the DOCTRONICS component selector program to calculate the frequency of pulses you would expect to obtain with this circuit. Monitor the output pulses with an oscilloscope to check that your calculation is correct.

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. This is the circuit used in the DOCTRONICS electronic die construction kit:


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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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:

UK police siren sound effect

Here is the prototype board layout:

down up open in a new window UK 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, although it is shown in the prototype board layouts, but this 'bypass' capacitor should be included in your final circuit.

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7. Monostable circuits

A monostable, or one-shot, 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:

Yenka simulation 555 monostable circuit

As you can see, the trigger input is held HIGH by the 10 kΩ 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:

monostable design equation

Remember again about compatible measurement units:


With R1 = 1 MΩ 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:

down up open in a new window pins 555 monostable circuit

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

DOCTRONICS component selection program

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

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8. 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:

Adding a trigger network

As you can see in the V/t graphs below, the voltage at X is normally held HIGH. A positive-going 'spike' would be generated on the rising edge of the Vin signal, but this is suppressed by the diode. On the other hand, the trigger network detects the falling edge at the end of each Vin pulse, producing a short negative-going spike which triggers the monostable:

V/t graphs for monostable with trigger network

The period of the monostable pulse is shorter than the period of the Vin pulses.

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

Triggering from a rising edge

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

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9. Retriggerable monostable

Another version of the monostable circuit allows you to initiate a new monstable pulse before any ongoing pulse has been completed:

Retriggerable monostable

As you can see, a PNP transistor, BC557B, has been added to the trigger circuit. To understand how the circuit works click 'play' in the animation:

Waveforms for retriggerable monostable

The first trigger pulse results in a normal ouput pulse with a period determined by t=1.1RC. Note that the timing capacitor is fully discharged during the trigger pulse: the timing period starts when the trigger input goes HIGH.

The second trigger pulse is followed by a third and then a fourth before the normal output period has been completed. The additional trigger pulses discharge the timing capacitor, giving an extended output pulse.

To see this circuit in action, the prototype board layout for the standard 555 monostable can be modified, as follows:

up open in a new window pins Retriggerable monostable circuit

Operate the push switch. The normal output of the monostable should be around 5 s. When the LED switches OFF, press the push switch again and then press is once more after 2-3 s.

Provided you keep pressing the switch at intervals of less than 5 s, the output LED remains ON.

If the input voltage divider is replaced by a source of pulses, this circuit can be used as a 'missing pulse' or 'low rate' detector. Any decrease in the frequency of the input pulses below the design level, will allow the monostable to complete its cycle, driving the output LOW.

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10. 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 source or sink up to 200 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:

555 as a transducer driver

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

In a Schmitt trigger circuit there are two different switching thresholds. If Vin is slowly increased starting from 0 V, the output voltage snaps from HIGH to LOW when Vin reaches a level equal to 2/3 of the power supply voltage. Once this level has been exceeded, decreasing Vin does not affect the output until Vin 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:

555 as a transducer driver

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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11. Inside the 555

You can use the 555 timer without knowing anything at all about its internal circuit. On the other hand, you might like to know something about what is going on inside.

The diagram below shows the 555 timer in astable mode with the internal circuit shown in block diagram form:

555 Internal block diagram

Inside the 555 you can see three resistors, labelled R. These resistors are equal in value and form a voltage divider, providing reference voltages at 1/3 and 2/3 of the power supply voltage, VCC.

The reference voltages are connected to one input of each of two comparators, which in turn control the logic state of a bistable, or flip flop stage.

Pin 2 of the 555 is the trigger input. When the voltage connected to pin 2 is less than 1/3 of the power supply voltage, the output of the lower comparator forces the logic state of the flip flop to LOW. The output stage has an inverting action. In other words, when the output of the flip flop is LOW, the output of the 555 goes HIGH.

Now think about what happens when the power supply is first connected to the astable circuit. Initially, timing capacitor C is discharged. The voltage at pin 2 is 0 V and the output of the 555 is driven HIGH. C starts to charge through resistors R1 and R2. Note that C is also connected to pin 6, the threshold input of the 555.

When the voltage across C goes past 1/3 of the power supply voltage, the output of the lower comparator snaps a new level. This doesn't change the logic state of the flip flop: its output remains LOW.

The inputs to the second comparator are the voltage at pin 6, the threshold input, and 2/3 VCC from the internal voltage divider.

When the voltage across C goes past 2/3 of the power supply voltage, the output of the second comparator snaps to a new level, the flip flop changes state, its output becomes HIGH and the output of the 555 goes from HIGH to LOW.

Inside the 555, the flip flop is connected to an NPN transistor, the collector of which is connected to pin 7, the discharge pin of the 555. When the output of the flip flop goes HIGH, the transistor is switched ON, providing a low resistance path from the discharge pin to 0 V. The timing capacitor, C, starts to empty through R2 and the voltage across it decreases.

Note that the capacitor charges through R1 and R2, but discharges only through R2.

When the voltage across C decreases below 1/3 of the power supply voltage, the lower comparator snaps to a new level, the flip flop changes state and the output of the 555 goes HIGH once again.

The graph below shows how the voltage across the timing capacitor, VC , changes with the output voltage of the 555, Vout:

555 astable voltages

The initial ouptut pulse is longer than subsequent pulses because C is completely discharged when the power supply is first connected. Subsequent HIGH and low times correspond to half-charge/discharge times, either from 1/3 to 2/3 of the power supply voltage, or from 2/3 to 1/3 of the power supply voltage.

The HIGH time is given by:

HIGH time

Remember, C charges through both R1 and R2.

The LOW time is given by:

LOW time

C discharges only through R2.

The period, t, of the 555 astable is given by:


The frequency, f, is given by:


Try 1÷0.69 on your calculator to confirm that it does equal 1.44.

The design formula for the 555 astable follows from the behaviour of RC networks and from the two switching thresholds of the voltage divider inside the device.

In a 555 monostable, only the upper threshold is used to determine the period, so the formula corresponds to a 2/3 charge time:

monostable design equation

You can find further details about the internal circuit of the 555 in the data sheets provided in the next section.


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12. Links

A great little book
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10. 1 Data sheets

The links below allow you to download documents in Adobe Acrobat ©, PDF, format. In the unlikely event that you don't already have Acrobat Reader, you can download the latest version direct from Adobe:

open document 555 data sheet (Phillips Semiconductors, 1994)

open document 555 application note (Philips Semiconductors AN 170, 1988)

10. 2 Internet resources

open website 555 Timer Tutorial (Tony van Roon, University of Guelph, Canada)

open website 555 timer IC (from Wikipedia, the free encyclopaedia)

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