Resistor Light Dimmer

This article is about a simple light dimmer made from a battery, a light bulb, and a high-power variable resistor. The variable resistor is used to limit the current thus making the light bulb more dim.

The first video shows the whole procedure that is presented in this article. The second video only shows how the circuit is working.

Components: big plastic box, 100-ohm high power variable resistor/potentiometer, 1.5 V light bulb - 2 (you might need a spare), light bulb holder, 1.5 V D battery, D battery holder with wires (red and black wire shown in the photo), insulated 1 mm metal wire.

Optional components: solder, insulated wire.

Tools: pliers, scissors, electric drill, drill bit, screwdriver.

Optional tools: wire stripper, soldering iron, component/PCB holder.

I have drawn the circuit in PSpice simulation software version 9.1 (student edition).

This simulation software does not have a light bulb component. Thus I modeled the light bulb as a 5-ohm resistor because the maximum voltage across the bulb is 1.5 V and the maximum current is 300 mA.

Resistance = Voltage / Current = 1.5 V / 0.3 A = 5 ohms

The variable resistance of the potentiometer (variable resistor) changes when I spin the knob clockwise or anticlockwise.

When the variable resistance is maximum, the voltage and current across the light bulb are minimum and the light bulb is dim or OFF. When the variable resistance is minimum or zero, the voltage and current across the light bulb are maximum.

Simulations show that the maximum current across the light bulb is 300 mA. This occurs when the variable resistance is zero.

I used an old high-power 200-ohm variable resistor that I purchased from the local radio club because I did not have a 100-ohm variable resistor. Those high-power variable resistors are very expensive. The average cost is $20. You might be able to use a normal potentiometer. However, it might burn and fail, especially if you use a high-current light bulb or connect a few light bulbs in parallel.

I used the drill and scissors to make the holes. I would spin the scissors in a clockwise and anticlockwise direction to widen the holes. With the potentiometer hole, you need to constantly check if the potentiometer fits in the hole to make sure that the hole is not too wide for the potentiometer.

I used insulated metal wire to attach the components to the box. You can use a non-insulated metal wire but keep in mind that you have to make sure that you do not short the component electrical contacts.

Testing shows that the bulb turns OFF quickly, long before the variable resistor reaches its maximum setting. This could be due to the high value of the variable resistor.

I tried connecting a 100 ohm resistor in parallel with the potentiometer (to reduce the resistance). However, this technique made no difference to the circuit's performance. The main problem is that the light bulb is non-linear and cannot be modeled as a resistor. However, a more complicated solution is beyond the scope of this article.