Mains operated Log-Interval Timer

Here is a simple circuit in the series of timer based project “Mains Operated Log-Interval Timer” used to turn ON or OFF the appliance in fixed interval of timer for fixed time period where time is fixed by timer components of this circuit.

Circuit description

As this circuit is mains operated timer circuit so, for more detail we can divide this circuit into two major sections i.e. power supply and timer circuit.

Power supply Circuit:- The primary AC voltage is step down to 12 – 0 – 12 by using transformer X1 which further rectified using bridge rectifier (D1 through D4). The rectified output is regulated by zener diode ZD1 and further filtered using capacitor C1.

Another section is timer section which is build around operational amplifier IC 741, used in comparator mode. The input is given to non-inverting input pin (pin 3) through resistor R3 and R4 which form potential divider. The inverting input is given to pin 2 through resistor R2, capacitor C2 and zener diode ZD2 which combined work as timer components. When Capacitor C2 is charged through resistor R2 up to zener voltage level (5.6V) then positive voltage is more at inverting input (pin2) in compare to non-inverting input (pin3) as a result output voltage is low (lower by 1.5 V). The lower voltage cannot pass through zener diode which cut-off transistor T1 and further off the appliance. The low voltage at output pin is held up-to capacitor is charged. When capacitor is discharge below zener voltage (5.6V) the output is high which drive transistor T1 and further turns on the bulb. We can increase the value of resistor R2 and capacitor C2 up-to 10 Mega-ohm and 1000 micro-farad in order to increase the time period Switch SW1 is used to reset.

circuit diagram of mains operated log interval timer


Resistors (all ¼-watt, ± 5% Carbon)

R1 = 150 KΩ/0.5-watt

R2 = 2.2 MΩ

R3 = 22 KΩ

R4 = 5.6 KΩ

R5 = 1 MΩ

R6 = 2.2 KΩ

R7 = 680 Ω

R8 = 10 KΩ


C1 = 100 µF/25V

C2 = 220 µF/25V


IC1 = LM741 (Op-amp)

T1 = BD115

TRIAC1 = BT136

D1 – D4 = 1N4007

ZD1 = 12V

ZD2 = 5.6V

ZD3 = 3.7V



SW1 = push to on switch

X1 = 230V/50Hz or 110V/60Hz primary to 0 – 15V,10mA secondary transformer

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