Simple Adjustable Timer Circuit with 555 IC


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Using simple 555 timer we can design an adjustable timer switch. This circuit is flexible to adjust required time.

Circuit Diagram

  • 555 timer
  • Electrolytic capacitor – 470 uf
  • ceramic capacitor – 0.1nF
  • Resistors
    • 120k ohm
    • 10k ohm
  • Relay -12v
  • Push button
  • Here 555 timer is operated in monostable mode.
  • When the trigger input is applied,555 timer produces a pulse. This pulse width depends on R and c values.
  • The above proposed circuit is a 1-10 minute timer.When Pot is minimum it gives 1 minute delay,where maximum value of pot can produce 10 minutes.
    • Time period can be calculated using formula

  • When Pot is maximum R is 120K+1.1M ≈ 1.2M (approximately) and C1=470uf
T= 1.2M*470uF = 620 seconds≈10 minutes.This is the maximum time.

  • For minimum time place the pot in least position.Then R= 120k
    • Hence time T=120k*470uf=6 2 seconds~1 minute (approximately).
  • A 12v relay is used to drive the ac load connected at the output.
  • Thus relay will be on for required amount of time set by the user using pot and then it is switched of automatically.
  • This circuit is used in such applications where the load is switched on for sometime and is off for rest of the time.
  • To prevent 555 timer from flyback current in the relay use a diode before the relay.
  • Some 555 versions may get damaged because of this.
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