THE 555 MONOSTABLE CIRCUIT IN DETAIL V. Ryan © 2002 - 2019 |
|||
PDF FILE - CLICK HERE FOR PRINTABLE WORKSHEET BASED ON WORK SEEN BELOW |
|||
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. |
|||
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.
|
|
||
|
|||
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. |
||
|