With the constant growth of the process control industry, the need and applications of On/Off delay timers is ever increasing. There are many industrial processes and tasks that would not be possible without the application of Relay based Timer Switches. A Relay based Timer timer switch is a process control device that starts or ends a process with respect to the preset time defined by the RC time constant of the circuit. It is generally used in controlling commercial or industrial-grade equipment such as HVAC equipment, traffic lighting, and large pumps. In today's tutorial, we will design a 12V Relay based Timer Switch Circuit Using a BC547 Transistor & a small number of other components.

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Hardware Table

S.NoComponentValueQty
1)RelaySPDT/5V1
2)AC - DC Converter220V/12V1
3)NPN TransistorBC5471
3)Block Connector-1
4)Potentiometer10K1
5)Pushbutton-1
6)Capacitor1000uF1
7)Diode1N40071
8)Wall outlet with Plug220V1
9)Soldering Iron45W - 65W1
10)Soldering Wire with Flux-1
11)Veroboard-1
12)Jumper wires-As per need

Useful Steps

1) Solder the 1000uF capacitor on the PCB board.

2) After that, solder the pushbutton on the PCB board.

3) Solder the BC547 NPN transistor on the PCB board.

4) After that, solder the 10K pot on the PCB board.

Circuit Diagram For 12V Relay based Timer Switch

Working Explanation

The working of this circuit is actually pretty simple. Here, a 10K pot sets the preset resistance for the timer circuit. . On powering on the circuit and pressing the pushbutton, the 1000uF capacitor charges & discharges as per the resistive preset delay introduced by the 10K preset pot.

The output from the 10K pot functions as a control signal on the base of the BC547 transistor. Here, we are using a flyback diode (1N4007) to protect the relay against any undesirable feedback due to shorts. The collector output from the transistor energizes the coil terminal of the 5V relay. This makes the relay and powers up any external circuit that is connected between the NC and COM terminal of the relay. Always choose a relay that electrically conforms with the input supply voltage and the output load connected to it.