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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 speed.

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.

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