The 555 Monostable Circuit


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Electronic timers are central to school projects. You will find as you develop your circuits that the timer circuit can be adapted to suit many purposes. There are several reliable timers but the 555 timer is the most common. Whether you are putting together an alarm or a circuit to activate a computer, a timer is the common component. The 555 timer IC (integrated circuit) is very stable, relatively cheap and reliable. It may be used as monostable or astable.
MONOSTABLE

Monostable means that once the circuit is switched on it will time once and then stop. In order to start it again it must be switched on manually a second time.

		In the circuit drawn opposite, the 555 timer is set to turn on the buzzer when the push switch is pressed; the buzzer sounds for approximately 8 seconds. This is a monostable circuit as it works only once. The switch must be pressed again for the buzzer to sound again.
On the diagram above if the components 'boxed in' by the dotted line are removed and the alternative components (shown on the right) are added - the 555 timer circuit can be used to energise a relay. The timer can now be used to trigger a relay which then allows another circuit to work. In this case the timer holds the relay closed for a preset amount of time allowing the second circuit to work and then switches the relay open, which stops the secondary circuit.






WHAT THE 'PINS' OF THE 555 ACTUALLY DO

		The pin (leg) that triggers the 555 IC is leg two. In other words leg two starts the timing sequence once a voltage is applied to it and after the 555 timer has ended it’s timing sequence a signal (output) is sent down leg three. In the circuit at the top of this page, the signal down leg three starts the buzzer. The variable resistor VR1 can be used to increase or decrease the timing cycle.