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From research it appears that water level detection circuits, such as the one I have included below as a diagram, work on the principle that when the water is high enough to submerge both the +ve and -ve terminal of a voltage source, a circuit is completed and the LED indicator comes on. enter image description here

Suppose you wanted to run a 7-segment display powered by +9V that displays a message when the water level has risen high enough to complete the circuit. Would it be safe to stick the +ve and gnd rails you are using as a source for the display (or other electronics) in the water? Would the water deliver a stable enough voltage to drive the display, and should a significant voltage drop be expected?

Cheers

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    \$\begingroup\$ It's good to be inventive, but you don't need to reinvent the wheel. There's already proper sensor's on the market. Also, you will have issues because you will perform electrolysis which I don't think you want to do. \$\endgroup\$ – Harry Svensson Feb 20 '18 at 9:20
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I suspect it wouldn't work very well. Water itself is not conductive, it's the dissolved ions in it that conduct. So it has a fairly high resistance which gets higher if it's purified. I doubt there would be enough current flow to drive a display.

Another issue with powering things through water is that running current through a solution is the basic mechanism of electroplating. The more current flows the more one of the electrodes will erode, and the other may oxidise. Can be minimised with gold-plated electrodes (e.g. PCB ENIG finish).

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the resistance of water is quite high to drive a 7 segment display even if there are salts/ions. what you could do is similar to what is done in the image you mentioned. use the transistor as a switch where the gate of the transistor receives voltage when water 'shorts' it to the power source. and instead of driving the led+resister you can use the 7 segment display. based on type of 7 segment you can directly replace the resistor+led or add a smaller resistor to limit current.

and as mentioned by pjc50, you will face the issue of electrolysis if you run it for long. you can either use inert electrode like graphite or goldplated ones or use a more complete circuit that will provide an oscillating signal instead of a constant 9V dc.

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