90 Second Timer

90 Second Timer

Summary
The circuit describes can be used to on and off the device for fix interval of time in this it is 90 seconds. This means device periodically become on for preset time and remain off for same period of time and it can be set with the help of R3 and C1

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Description
Circuit is based on LM324 which is 14 pin IC consisting of 4 independent operational amplifier in a single package. The output voltage of LM324 is many times higher than the voltage difference between the input terminals of an opamp.
In this pin 12 and 13 are input pin and pin 14 is output pin 4 is for supply and  pin 11 is ground. Working of the circuit is very simple it compares the voltage on pin 13 with respect to pin 12.When power supply is applied output voltage is low as pin 12 is low compare to pin 13 as a result transistor connected to it does not work and relay when remain off.
After some time capacitor starts discharging and voltage at pin 13 become high in comparison to pin12 as a result out pin 14 become high transistor starts conducting and device connected to relay start operating on and off for the time set.
Time period can be set by C1 and R3 when you increase the value time period will increase when you reduce the value time period will decrease.
Components used

IC
IC1(LM324)	1
Resistor
R1-R2,R4(82K)	3
R3(10M)	1
R5(470K)	1
R6(1K)	1
Capacitor
C1(10uF)	1
Miscellaneous
Relay	1
LED	1
T1(BC547)	1