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.
Step 1: What Do You Need?
wire (preferable not too thick)
transistor BC457C or something like that
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!!!