This article is about a variable resistor motor driver. It is a straightforward circuit. A variable resistor is connected in series with the motor to the power source (3 V dual 1.5 V AA battery power supply).

You can see the circuit working in the first video. The second video is 8 minutes long. You can watch it if you do not like reading this article.

Supplies

I purchased a plastic jiffy box at the hardware store. I picked up the variable resistor (potentiometer) from the local radio club. I purchased the dual-switched AA battery holder from the local electronics hobby store (Jaycar). You will also need a motor, pliers, a circuit holder, a permanent marker, scissors, wires, a wire stripper, an electric drill, and a soldering iron.

Step 1: Design the Circuit

This circuit is straightforward. Unfortunately, the circuit does not have short-circuit protection because of its simplicity. If the circuit had short circuit protection, then it would not be published on our website (Easy Components). Thus, you need to be careful not to short the output.

Calculate the maximum motor current:

Imax = Vs / Rm

Where: Vs = Power Supply Voltage (V)

Rm = Motor Resistance

Imax = (Vs1 + Vs2) / Rm

Imax = (1.5 V + 1.5 V) / 10 ohms

= 3 V / 10 ohms

= 0.3 A

= 300 mA

Calculate the minimum motor current:

Imin = Vs / (Rm + Rvar)

= 3 V / (10 ohms + 200 ohms)

= 3 V / 210 ohms

= 0.0142857 A

= 14.2857 mA

This is a very small current for the motor because the variable resistor value is too high. Unfortunately, I did not have a smaller variable resistor in stock. This circuit is suitable for low-current motors.

Step 2: Simulations

I used PSpice student edition version 9.1 because this software works fast on my computer.

Simulations show the current when the variable resistor is set to 10 % of the maximum value (20 ohms is 10 % of 200 ohms). The total resistance is thus 30 ohms (Rm + Rvar = 10 ohms + 20 ohms). 3 V / 30 ohms = 0.1 A = 100 mA. At this current, the motor will spin very slowly.

Step 3: Drill the Holes

I drilled a small hole with an electric drill. I widened the hole with scissors.

Step 4: Make the Circuit

I soldered the red wire of the 3V power supply to the potentiometer and connected the red output wire to the potentiometer. I connected the black wire of the power supply to the black output wire.

Step 5: Testing

You can see the circuit working in the video.