This electronic motor-speed control circuit is designed to operate in an electrically noisy environment. The circuit includes an optoelectronic pickup device. whichis placed inside the motor housing to provide a speed feedback signal. The circuit automatically maintains the speed of the motor at the commanded value. The pickup device contains an infrared LED and a phototransistor. The radiation from the diode is chopped into pulses by the motor fan blades. which are detected by the phototransistor.
Closed-loop-motor-speed-control - schematic

The train of pulses from the phototransistor is fed to a frequency-to-voltage converter, the output of which is a voltage proportional to the speed of the motor. This voltage is low-pass filtered, amplified, and compared with a manually-adjustable control voltage that represents the commanded speed. The difference between the speed-measurement and speed-command signals is amplified and fed as a control voltage to an external power amplifier that drives the motor. A selector switch at the output of the final amplifier of this circuit also enables the operator to bypass the circuit and manually set the control voltage for the external amplifier.

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