Water Switch

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Introduction: Water Switch

Be careful and never use this idea working with the mains (or high voltage)!!!

Just use batteries with low voltage and the worst thing that can happen is that your battery drowns in the water...

Having made soft and paper buttons, a student wanted to have a bathing suit with LED's. When jumping into the water the LED's should light up.
Besides from the problems with electronics and water, it looked simple: two wires into the water should make a switch. The battery and LED's should be in a plastic bag and sealed.
But it didn't work!

The water, apparently being too clean, had a resistance of several K...

Solving this problem is easy and the things you can do with it are fun. Make your own rain meter, make a jacket which lights up in rain, make your milk bottle interactive etc.

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Step 1: What Do You Need?

Tools:
soldering iron
solder
wire (preferable not too thick)
scissors
pliers
tape
(battery holder)

parts:
transistor BC457C or something like that
3V cell
a LED, around 3V
a resistor around 4K

Step 2: Schematics

The schematics is very simple, the main thing is to get the transistor and the LED right:

The principle of the transistor is simple:
it lets current through when the base (middle) is positive.
otherwise it does not conducts current so there is not action.

You can position the transistor by looking at its shape, a half round cylinder.

The next thing to position right is the LED:
long leg to the positive side of the battery, short to the negative side.

(A LED is a diode: it will light up only when the current is coming from the right side.)

Step 3: Soldering and Testing

So put the two long wires into the water and: miracle! the LED lights up!

Variations 1: bathing suit, rain coat, umbrella's.

Variations 2: use LED's which swicth on and of from themselves, the so called flashing LED's, use RGB LED's which change color.

Variations 3: use more LED's together: you can solder the second LED parallel to the first, it will only drown your battery twice as fast...

Variation 4: make a normal switch between the battery and the positive (or negative side), you can switch off when your bathing suit is in the closet...

This is basically a switch, so you can switch on (or of) with water what you can switch with a normal switch: arduino, relais, etc

Last warning: never never  never  never do this with high voltage!!!

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10 Discussions

Either my water is too clean or I did something wrong. Circuit works if I touch the leads together but not in water. The liquid I need to have a switch in is maple sap which does have minerals and a small amount of sugar. Any suggestions?

Are you certain that the transistor has the same pinout as the one listed here, LED +/- correct direction, faulty LED? Maybe a faulty transistor??? Maple sap should work much better than water.

Do you know if works with alcohol? Or just with water?

This is not quite the same circuit. The LED is at the wrong place, it should be between the transistor and the positive battery terminal(since this is an NPN transistor). Also, the symbol you used for the transistor is wrong (it's actually the symbol for a logic and gate).

The transistor doesn't make the current through the water any bigger, it simply pulls a high current through the LED when a small current goes through the water (when wired as suggested by the author).

Much better indeed, although the BC457 is an NPN transistor (as opposed to the PNP symbol in your schematic). With the diode wired on the negative batter terminal, a PNP such as the BC557 should be used. If you wire the LED to the positive terminal, the NPN transistors should be used.

The essential diffrence between the two is that one ( PNP ) is controlled by the current pulled out of it while the other (NPN) is controlled by the current pushed into it.

if you put a relay where the led was it could handle high voltage

This is great if you are just learning to use transistors. And of course might be useful anyways.

Great!