Forrest M. Mims Jameco Puzzler 4

What's the Missing Component?

by Forrest M. Mims III

Puzzler 4

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.

Figure 1. Simple rotation detector.

What is the component concealed behind the puzzle piece? Before revealing the answer, here are some hints:

Figure 1 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.

What is the rotation detector behind the puzzle piece?

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