Electronics Puzzler: Rotation Detector

By Forrest M. Mims III
Rotation Detector

What's the Missing Component?

Video game designer Bob Wheels needed an inexpensive, counter-clockwise rotation detector. An optical rotation sensor he tried was too fragile to last long in the busy hands of a teenaged game player, as he realized while watching his son play with a radio-controlled car. That experience also gave him an idea for a simple rotation detector that can withstand lots of punishment.

Puzzler Hints:

Image above shows a test version of the circuit designed to detect rotation. The LTC1050 op amp is connected in its single-supply mode.

When the rotation detector is not rotating or is rotated in a clockwise direction, both LEDs glow. Only the red LED glows when the rotation detector is rotated in a counter-clockwise direction.

The rotation detector has only one moving component. It is unaffected by light.

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