Safety Guard

Protect your home appliances from voltage spikes with this simple time delay circuit. Whenever power to the appliances is switched on or resumes after mains failure, the oscillator starts oscillating and D5 blinks. This continues for three minutes. After that, Q14 output of IC CD4060 goes high to trigger the gate of the SCR through D4. At this moment, the voltage is available at the cathode of the SCR, which energizes the relay coil to activate the appliance and D6 glows. Switch SW1 is used for quick start without waiting for delay.

Schematic:

Parts List:

R1 = 1M

R2 = 470R

R3 = 820R

R4 = 56K

R5 = 470R

R6 = 1K

R7 = 10K

C1 = 1kuF-25V

C2 = 100nF-63V

C3 = 0.02uF-63V

C4 = 10uF-25V

C5 = 10uF-25V

D1 = 1N4007

D2 = 1N4007

D3 = 1N4007

D4 = 1N4148

D5 = Red LEDs

D6 = Red LEDs

RL1 = 12V Relay

IC1 = AN7809

IC2 = CD4060

SW1 = Switch

T1 = 24V-AC Centre Tapped Transformer

Circuit Operation:

At the heart of the circuit is IC CD4060, which consists of two inverter gates for clock generation and a 14-bit binary ripple counter. Here the clock oscillations are governed by resistor R1 and capacitor C1. In this circuit, only two outputs of the IC (Q5 and Q14) have been used. Q5 is connected to an LED (D5) and Q14 is used to trigger the gate of the SCR through D4 as well as reset the counter. The anode of the SCR is connected to +9V and the cathode is connected to the relay coil. The other pin of the relay coil is connected to the negative supply, while its contacts are used for switching on the appliances.