Electronics Puzzler Solution: Rotation Detector

By Forrest M. Mims III

What's the Missing Component?

While watching his son do wheelies with his radio-controlled car, Bob Wheels realized that small DC motors are very tough gadgets. They also generate a voltage having a polarity dependent on the direction of rotation. So he built a test circuit to determine if an inexpensive motor could be used as a rotation detector for the video game he was designing.

How It Works

The figure below shows the circuit with the missing component displayed. Both LEDs glow when the motor's armature is still or when it is rotated in a clockwise direction. When the armature is rotated in a counter-clockwise direction, the green LED switches off and only the red LED glows.

Rotation Detector

The mystery component is a DC Motor.


My first project that used a small DC motor to detect rotation was an airspeed indicator for a homemade wind tunnel. Details are here: An $11 Wind Tunnel Design, Model Rocketry, July 1970, pp.13-15.

Going Further

Any general purpose, single-supply op amp should work in this circuit. Different effects can be had by connecting a small capacitor between pin 3 of the op amp and the motor.

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