Home > Circuits > Lamp Dimmer
Modified:21:13, 22 October 2013
A very simple circuit that can effectively dim any filament lamp, LED display or even small motors up to about 30 Watts with a moderately small heatsink. The circuit will work from 6 to 15 volts and up to 65 Watts with a large enough heatsink.

The circuit is based around the standard 555 astable circuit, with a couple of alterations, namely the wiper of VR1 is connected to pin 7 and the addition of D1. This configuration means that the frequency can stay constant but the mark/space ration (on/off time) can be altered quite dramatically, from about 1% to 99% thus making an ideal PWM voltage controller.

Q1 is a Darlington PNP type with up to 5A of current drive, however you must use a large heatsink which will get hot, and do not exceed 65 Watts. This can be replaced with a higher rated device such as TIP147 for up to 125W!! This is enough to operate PCB drills!!

Note: The output is never fully off or on but this has very little effect on the operation of the device being controlled. Also be aware that since this is a very basic circuit, there is no polarity or overload protection.

Below is graph simulations of the output.

Blue trace = voltage at C2
Red trace = voltage at output

VR1 at minimum

VR1 at halfway

VR1 at maximum


The circuit fits nicely on a small PCB and is light enough for the whole PCB to be secured using VR1 panel mount fixing thus removing the need to drill holes in the PCB.

PCB Size: 40mm x 38mm

Below the circuit assembled with heatsink facing outwards for better cooling air flow.

A kit for this project is available in our shop.

Designed and Written by Phil Townshend 2008
Back to Circuits page...

www.edutek.ltd.uk - Working Electronics For Students & Teachers