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I want to do a simple project where an LED turns off when it is submerged in water.

I thought this would be simple because when you short the two leads of an LED with wire, it just shorts the power supply turning, it off. Water is very insulative, though, and even after adding a ton of salt and lemon juice, it won't short the LED. Can I make a simple circuit that can do this?

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  • \$\begingroup\$ You shouldn't be putting an LED underwater. If you put it in really dark water it will look like it has turned off. \$\endgroup\$
    – Andy aka
    Sep 25 at 17:57
  • \$\begingroup\$ You can buy children's bath toys that do this for a dollar or two. \$\endgroup\$ Sep 25 at 18:11
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A small n-channel MOSFET such as a 2N7000, two resistors, and the LED. Plus a battery, waterproofing of the components (except for the sensing area), etc. Is this within your skill set? If so, I'll update this answer with a schematic.

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  • \$\begingroup\$ For my own education: Please post the schematic \$\endgroup\$ Sep 26 at 19:26
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Yes - sure you can. You could detect the short with a logic-level circuit and control the power rail with a relay under feet.

  1. Simple short-circuit 'Sensor' builds from two wires-stubs or a dedicated PCB. Feed your 'Sensor' with V+ rail through a high value series resistor (10k). Connect the other end of the 'sensor' to an input pin of e.g. an Arduino. Pull down this input with a larger resistor to V- (100k). As soon as the 'Sensor' is inserted into water the current flow should be high enough to change the input value to high. Then switch the relay / FET - Channel as you desire.

  2. Build some sort of capacity based sensor. Connect an open wire to an output Out1 through a series resistor (10k). Connect an input to the wire too with a high value pulldown (100k). Also connect a second output Out2 to the wire stub with a low value series resistor (470). The wire-stub works as capacitor here. Charge it by turning on the Out1 output until the input reads back a high value. Measure the charging-duration. As soon as the apacitor is charged turn off out1 and discharge the capacitor by pulling out2 low. Do this for a fixed time (1ms). Now repeat the logic at a certain rate - e.g pwm the outputs. Make sure out1/2 remain in HIGH-Z mode while not required. When the duration changes significantly the wire stub is submerged in water. If not (surrounded by air) everything is okay.

  3. You can integrate both solutions without the need for a controller board at all. Just use discrete components. Solution 1. will be much easier. Use a PNP-Transistor and build an inverse logic switch.

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