Forrest M. Mims Jameco Puzzler 5

What's the Missing Component?

by Forrest M. Mims III

Puzzler 5

The Ultra-Simple Sensors Company assigned its engineering staff to design a circuit that would trigger an LED when a few millimeters of liquid water is present in a basement or boat. The winning circuit is shown in Fig. 1 – less its sensor.



Figure 1. Simple liquid water detector.


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

The LTC1050 in Fig. 1 is connected as a comparator that switches on an LED when liquid water is present at the sensor. Otherwise the LED is off.

What is the water sensor behind the puzzle piece?

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