Simple Adjustable Timer Circuit
with 555 ICSimple Adjustable Timer Circuit
with 555
Circuit Diagram







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.

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.