This page is about a simple motor controller made with one dual gang potentiometer.

You can see my circuit working in this video:

Step 1: Design the Circuit

The dual gang potentiometer is represented with two variable resistors in my circuit:

I did not include the diode in series with the 9 V battery because this is a Cheap Components website. This diode is needed to protect the power source. Do not connect this circuit to USB output.

The motor will spin faster if the difference between the potentiometer settings increases. You might say that when we rotate the dual gang potentiometer the difference between resistance settings (levels) of first level and lower level are almost zero. However, this is not true if I switch the polarity of the power supply to the second potentiometer. Then the difference is zero only at mid point setting.

Also, you can make the circuit with two single gang potentiometers instead of one dual gang potentiometer. This circuit will be more reliable. There is an article published on this topic: https://weirdcircuits.site123.me

Step 2: Simulations

Simulations show a that there is a high potential difference between the two potentiometer outputs:

Yet the load current is low:

You can increase the load current by reducing the value of the Rm resistors (Rm1, Rm2 and Rm3) or connecting more Rm resistors in parallel.

Step 3: Make the Circuit

You need to use a high current potentiometer that costs a lot of money. 

I used a cheap 2 kohm dual gang potentiometer from eBay.

Step 4: Testing

You can see that my motor is not spinning that fast:

Conclusion

The optimum way to increase the speed of the motor is to eliminate the Rm resistor (replace with short circuit) and select a low resistance value (100 ohms), high power dual gang potentiometer. However, this decision is risky and will cost a lot of money. Nowadays there are many low cost PWM (Pulse Width Modulation) motor controllers.