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.
Notes.
* Assemble the circuit on a good quality PCB or common board.
* The circuit can be powered from a 9V PP3 battery or 12V DC power supply.
* The time delay can be varied by varying the values of C1&R1.
* The push button switch S2 is for starting the timer and S1 for stopping the time.
* The appliance can be connected via contacts N1 & N2 of the relay RL1.
* The IC U1 is 2 input quad NAND gate 4011.
