What does it do?
| Turns something on, and then off, and keeps repeating this. |
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How does it operate?
The 555 Astable provides an output signal that is high, then low, then high, low...
The time between the signal going from high to low and going from high to low again can be controlled by the values of two resistors.
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Low Frequency pulses
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High Frequency pulses
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555 astable circuit
Click on the circuit diagram to download a Livewire file of the 555 Astable that you can investigate and add to your own circuit. |
The 555 astable is based on the 555 timer IC.
The time that the output signal is high, known as the mark of the pulse, can be calculated:
Time high = 0.693 x (R1 + R2) x C
where R is in M ohms and C is in µF. So, if C = 10µF, R1 + 100k and R2 = 150k, the output signal is high for about 1.7 seconds.
The time that the output signal is low, known as the space of the pulse, can also be calculated:
Time low = 0.693 x R2 x C
So, if C = 10µF and R2 = 150k, the output signal is low for about 1.7 seconds, and the total time period (the time for one complete cycle) is 1 + 1.7 = 2.7 seconds. |
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There is one quite subtle point to watch with a 555. Nearly all the digital process ICs discussed on this web site are 4000 series CMOS devices. The 555 timer is not in this family and there can be problems in mixing families. In particular, a 4000 CMOS device with a supply voltage of 5V needs an input signal voltage of at least 3.5V to guarantee that it is recognised as a logic '1' or 'high'.
However, at the same supply voltage, a 555 produces an output signal voltage of typically 3.3V, and it can be as low as 2.75V. In other words, the 'high' from a 555 may not be recognised as a 'high' if it is fed to a 4000 series process subsystem.
The safest way to deal with this is to use a CMOS version of the 555 e.g. the ICM7555 (which is more expensive) or a pulse unit – which uses 4000 series CMOS. |
A mark : space ratio is the ratio of the ‘mark’ time to the ‘space’ time.
From these two calculations we can obtain the frequency (the number of oscillations per second) of the pulse generated by the 555 astable:
So, if C = 10µF, R1 = 100k and R2 = 150k, the frequency is 0.36 Hz (which is 1/2.7s).
The 555 timer IC works with a d.c. power supply with a voltage between 4.5V and 16V. The 555 timer is able to provide an output current of 100mA and can therefore drive low and medium current output devices directly.
Capacitor types
Ceramic disc capacitors should not be used for the timing capacitor C. They are not sufficiently stable in capacitance to operate properly for timing. Suitable capacitor types are: silver mica, mylar, polycarbonate, polystyrene, tantalum, or similar types.
Possible applications
- Flashing a LED or bulb
- Sounding a buzzer on and off
Making
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Pins of 555 timer IC
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How part of the PCV might look |
Use a Dual In Line (DIL) socket for the IC. Before inserting the IC, connect the power supply and use a voltmeter to check that the voltage on pin 1 is low (0V) and the voltage on pins 4 and 8 is high (the supply voltage).
Connect the negative lead of the capacitor to 0V. Insert the IC the right way round.
Testing
Make sure that the signal going out (on the green PCB track) changes from high to low and that the time period is correct.
Fault finding
If there is a fault, check that:
- The voltage on pin 1 is low (0V)
- The voltage on pins 4 and 8 is high (the supply voltage)
- The capacitor and IC are the right way round
If there is a fault, check the tracks and solder joints.
Alternatives
- Pulse Unit – cheaper but less accurate time period and fixed mark : space ratio.
- PICs – more flexible but more expensive.
Web links
Return to list of datasheets
