Home > Designing basic digital circuits/circuit wizard > Monostable circuit using a 555 timer

Monostable circuit using a 555 timer

This is a simple monostable circuit designed using circuit wizard.  As explained in the section ‘ What is a digital circuit’ the monostable circuit produces a single output pulse when triggered.  It is monostable because it is stable in just one state which is output low or 0.  When the switch is pressed the LED comes on changing the state to output high or 1.  In this instance the LED would shine for approximately five seconds.  This has been determined by the resistor R2 and capacitor C2.

The time period (the time the LED is switched on) = 1.1 x R2 (resistance in ohms)  x C2 (capacitance in farads)

1.1 is used as the capacitor charges to 2/3 which is 67% so it is a little longer than the time constant which is the time taken to charge 63% which is R2 x C2

  • Beware that electrolytic capacitor values are not accurate, errors of at least 20% are common.
  • Beware that electrolytic capacitors leak charge which substantially increases the time period  if I am using a high value resistor – use the formula as only a very rough guide!
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