Home > Circuit Bricks > Monstable -Timer
UPDATED:.21:09 21 October 2013


  • This monostable is based upon a 555 timer IC.
  • A monostable produces an output pulse (switches on for a length of time, then off).
  • The time of the pulse can be set according to 2 components.
  • It is usually triggered by an input (switch or logic gate).
  • Useful for switching on devices for up to 30 minutes
  • For shorter pulses, use a logic gate monostable.

Circuit Diagram
  • Trigger
    This is activated by connecting it to 0v (active low). If the trigger stays low, the output will stay on. If connected to a switch then it must have a pull-up resistor connected to it.
  • Reset
    When connected to 0v, the timer will cease operating.

The output will go high then low for the pre-determined time. The output can only sink/source about 25mA, enough for an LED, buzzer or low wattage bulb.

You will need to add a driver to drive bigger devices such as lamps and motors.

The duration of the pulse is calculated as follows:

Time (secs) = 1.1 x C1 x R1

It can be tricky to choose values to start with, so try the following:

Choose one of the following values for the capacitor, depending on the time you require:

Less than 1 sec. = 1uF,
Up to 120 secs. = 100uF,
Over 120 secs. = 1000uF

Now rearrange the formula to work out the required resistor value R1

R1 = Time / (1.1 x C1)

To ensure that the caluclations are correct, use either Mohms & uF or Ohms & Farads as units

Try using a preset or a potentiometer for R1. Then you can adjust the time manually.
The 555 timer comes in an 8pin DIL package.


When using ICs, it is always best to use an IC socket so the IC can be removed easily if needed.

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