how do water-detecting motors work?

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My dad recently bought an electric fishing lure that will only turn on when submerged in water, but you charge it by connecting electrodes to the outer metal hoops. How does it not short-circuit under water? Does the water act as a “connector” that closes the circuit somehow to turn on the motor?

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2 Answers

Anonymous 0 Comments

Water is a lot more conductive than air.

Generally the designer use two circuits. One for detection and another for the motor.

Only a small portion of the energy is spent for detection.

Then this small detection signal is used and amplified to power the motor.

It is explained here:

https://www.wellpcb.com/water-detector-circuit.html

Anonymous 0 Comments

There are several ways to detect water. It’s conductivity may even be part of it. Potentially the water is not short-circuiting in a bad way, but is actually being used to close the circuit and allow current to flow, thus, sensing water. If you dunk it in distilled or even better, deionized water and it doesn’t do anything, then there’s your answer.

If it still moves in deionized water, it’s likely capacitive. Capacitor’s are basically two conductive plates that don’t touch. A capacitor’s capacitance depends on the distance between the plates, the area of the plates, and the electrical permittivity of the insulator that separates the plates. If the thing isn’t changing size or shape, then neither area nor distance change, but if water gets between the plates, it can still change the capacitance. This is how automatic water bottle fillers stop when the water level rises above the hole behind the bottle, there’s a capacitance being measures.

The easiest way to measure a capacitor is by giving it a charge. Capacitor’s work somewhat like batteries in that they store charge, but the charge builds up over milliseconds or at most seconds instead of hours. So you could measure the time it takes to charge up the capacitor with any number of methods.