Simple Adjustable Timer Circuit with 555 IC

[guclusat]

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

Circuit Diagram

Components

• 555 timer
• Electrolytic capacitor – 470 uf
• ceramic capacitor – 0.1nF
• Resistors
  • 120k ohm
  • 10k ohm
• Relay -12v
• Push button

Working

• 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

T=(R1+R2)*C1.seconds

• 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.

Note

• 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.