Search Site

555 Timer Monostable Circuit – Easy


Building the circuit:




 The components you will need for this circuit are: A pack with all the parts is available here



The Circuit:

 555 Monostable Circuit Diagram

This is the finished schematic of the circuit. You can adapt this circuit by changing the value of the variable resistor so that you have a shorter or longer “on time” by changing the resistor, therefore changing the resistance. This circuit allows you to have a time from 0 seconds to about 50 seconds; therefore if this is not long enough then you can use a 1M variable resistor as this will give you much longer periods for the LED to be on. You can also play around with the value of the capacitor to see what happens when that changes too.


If it would be more beneficial for you to have the LED or buzzer on, when the 555 timer circuit is not set, then connect the LED or buzzer from power to pin 7, instead of pin 3 to ground. This is because pin 7 does the opposite to pin 3 (i.e. when one is off, the other is on). However this does mean that you will need a much larger capacitor and resistor as the capacitor drains faster meaning the circuit doesn’t stay set as long.



Overview / how it works:


This circuit works using a 555 timer. It is set up as a monostable circuit which means unlike the typical Astable which pulses continuously, this must be told to pulse my means of a trigger. The trigger can be set by anything from a switch to a micro-controller as all that is needed is a ‘logic low’ on pin 2, which is what the switch allows when pressed.


The length of a pulse is determined like normal in a 555 timer circuit, by using a capacitor and resistor to change the pulse length, but my having a variable resistor in the circuit, you can tune the time to exactly how long you want.


Once the pulse length has finished, the output from pin 3 will stop, and so to set it again, a trigger will be needed as mentioned above.



Below are some images of the circuit on a breadboard:

555 Monostable Circuit Breadboard Image

555 Monostable Circuit Breadboard Image

555 Monostable Circuit Breadboard Image

555 Monostable Circuit Breadboard Image

Back to top