Transistor Light Dimmer

DIY ElectronicsDIY Electronics wrote 06/23/2020 at 11:34 • 3 min read • Like

I created a circuit for this article based on the following links:

Use one transistor to drive an LED:

Used three transistors to drive a 0.3 A light bulb:

My circuit uses two transistors to drive an old 0.1 A (100 mA) 12 V motorbike signalling light bulb with a 9 V battery.

You can see my circuit working in this video:

However, there are also oscillation/PWM light dimmers that are more efficient:

Step 1: Design the Circuit

I drawn the circuit in PSpice simulation software that also allows simulation:

My circuit also allows linear input control that the other circuits (shown in the links) do not allow.

The Q2 PNP transistor acts like a mixer circuit mixing two inputs:

1. Variable resistor control.

2. Input channel (Q1 transistors) control.

Two Q1 transistors are used reduce the maximum power input current (base current) for each transistor that can occur due to high inputs voltages.

The Cs and Rs is a power supply RC low pass filter used to filter power supply oscillations.

Calculate the frequency of the low pass power supply filter:

fl1 = 1 / (2*pi*Rs1*Cs1) = 1 / (2*pi*100 ohms*(470*10^-6 F))

3.38627538493 Hz

For more information on design you can read the links above.

Step 2: Make the Circuit

I only used a soldering iron for the power transistor. I used an old BD140 PNP power transistor from an old VCR (Video Cassette Recorder). It must have been used to drive the tape motors.

I did not implement the power supply filter. This circuit is optional.

Step 3: Encasement

I used a nail to create the hole and widened with scissors:

Step 4: Power Source

9 V battery proved to be sufficient for the 12 V light motorbike signalling bulb.

Step 5: Testing

My circuit has good control over the bulb brightness because I used only two transistors instead of three.