Transistor Relay Driver

Introduction: Transistor Relay Driver

About: Our circuits are designed in Philippines and tested in Australia.

In this Instructable you will make a transistor relay driver.

I used a BJT (Bipolar Junction Transistor) NPN transistor. The advantages of BJTs over MOSFET transistors are being able to handle a high range of input voltages due to Rb resistor without failing. However, BJT transistors are less likely to saturate due to linear characteristics thus causing high power dissipation that might eventually burn the transistor. Also, BJT transistors cost less money are could be easily found in old electronic circuits. MOSFETs are still a bit rare because they are only used for switching applications due to their non-linear characteristics.

In the video, you can see that this circuit also works as a touch switch. However, it is not a reliable touch switch due to low gain because I used only one transistor.

In this instructable (click on the link), I have drawn two diodes because unlike the relay, the motor can spin in a different direction and feedback current in a different direction:

This is not the same for relays. After the transistor turns off. The current direction of the discharging coil remains the same. The capacitor and diode are used to protect the transistor and power supply from those discharging currents that can be very high.


relay/solenoid/coil clicker, NPN BJT power transistor, bolts, nuts, washers, 1 kohm power resistor, 5-ohm power resistor, heat sink, wires.

optional: crocodile clips, box plastic/cardboard, wire terminals for bolts, matrix board, blue tack, wire stripper, solder, soldering iron, electric/mechanical drill (for the plastic box only).

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Step 1: Build the Circuit

I used a very cheap heat sink that might not be sufficient for such a low impedance relay.

I used a capacitor from an old VCR (video cassette recorder).

You do not need a matrix board. You can connect this circuit onto a piece of wood.

Bird nest circuits are considered as dangerous, especially if those are electrical circuits which you see in this instructable.

You are now done.

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