This timer circuit can be used to switch
OFF a particular device after around 35
minutes. The circuit can be used to
switch OFF devices like radio, TV, fan,
pump etc after a preset time of 35
minutes. Such a circuit can surely save a
lot of power.
The circuit is based on quad 2 input
CMOS IC 4011 (U1).The resistor R1 and
capacitor C1 produces the required long
time delay. When pushbutton switch S2
is pressed, capacitor C1 discharges and
input of the four NAND gates are pulled
to zero. The four shorted outputs of U1
go high and activate the transistor Q1 to
drive the relay. The appliance connected
via the relay is switched ON. When S2 is
released the C1 starts charging and when
the voltage at its positive pin becomes
equal to ½ the supply voltage the outputs
of U1 becomes zero and the transistor is
switched OFF. This makes the relay
deactivated and the appliance connected
via the relay is turned OFF. The timer
can be made to stop when required by
pressing switch S1.
Circuit diagram with Parts list.
1-Assemble the circuit on a good quality
PCB or common board.
2-The circuit can be powered from a 9V
PP3 battery or 12V DC power supply.
3-The time delay can be varied by varying
the values of C1&R1.
4-The push button switch S2 is for starting
the timer and S1 for stopping the time.
5-The appliance can be connected via
contacts N1 & N2 of the relay RL1.
6-The IC U1 is 2 input quad NAND gate 4011.