Personal air sampler with electric motor driven by intermittent full-power pulses under control, between pulses, of motor's back electromotive force

Constant selected speed running of an electric motor, with a varying mechanical load, is achieved by intermittently pulsing the motor at full power and comparing the back EMF of the motor, between power pulses, with a selected speed voltage, to accordingly control application of the power pulses. A particular control circuit is disclosed which permits operation of a pocket-sized system, powered by a three cell battery, to achieve substantially constant speed, at normally varying loads, throughout a ten hour period, with back EMFs at selected values between 0.01 volt (motor barely turning) and 2.0 volts (high speed). The system is useful in a personal air sampler, used to monitor a workman's exposure to noxious ingredients of breathing air.