Here are two simple 12V
DC motor speed controllers that can be built for just a few dollars.
They exploit the fact that the rotational speed of a DC motor is
directly proportional to the mean value of its supply voltage. The first
circuit shows how variable voltage speed control can be obtained via a
potentiometer (VR1) and compound emitter follower (Q1 & Q2). With
this arrangement, the motor’s DC voltage can be varied from 0V to about
12V. This type of circuit gives good speed control and self-regulation
at medium to high speeds but very poor low-speed control and slow
starts. The second circuit uses a switchmode technique to vary motor
Here a quad NOR gate (IC1) acts as a 50Hz
astable multivibrator that generates a rectangular output. The
mark-space ratio of the rectangular waveform is fully variable from 20:1
to 1:20 via potentiometer VR1. The output from the multivibrator drives
the base of Q1, which in turn drives Q2 and the motor. The motor’s mean
supply voltage (integrated over a 50Hz period) is thus fully variable
with VR1 but is applied in the form of high-energy “pulses” with peak
values of about 12V.
This type of circuit gives excellent full-range speed control and
gives high motor torque, even at very low speeds. Its degree of speed
self-regulation is proportional to the mean value of the applied
voltage. Note that for most applications, the power transistor (Q2) in
both circuits will need to be mounted on an appropriate heatsink.