Electronics Puzzler Solution: Water DetectorBy Forrest M. Mims III
The mystery component is two input leads and nothing else.How It Works
Figure 2 shows the circuit with the two input leads serving as probes. The LTC1050 op amp is connected as a comparator. R3 is adjusted until the LED is off. When both probes are immersed in water, the comparator switches on the LED.
The mystery component is a pair of bare wire leads.
Many different methods can be used to detect liquid water. The simplest depend on the conductivity of water.
The water probes can be arranged in many different ways so long as the space between them is well insulated. Any general purpose, single-supply op amp should work in this circuit. Note that the circuit can also be triggered simply by touching the two sensor probes.