How to make a simple and cheap liquid switch (without floats).

Picture of How to make a simple and cheap liquid switch (without floats).

Many people who have used float switches know that they can be pretty unreliable. In this tutorial you will learn how to make a solid state liquid switch that uses electrodes to sense liquids instead of a float. They are easy to make and are much cheaper than float switches. They are reliable alternatives to float switches and have many applications.

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Step 1: What you need:

Picture of What you need:
There are a few simple and accessible items that you will need to make this switch.
- 1 NPN Transistor - I used the TIP120 from radioshack but just about any NPN will do.
- 1 1Kohm resistor
- 1 capacitor - I used a 25v 1000 microfarad. The capacitor is not necessary but it will allow for a slight delay so a pump will stay on for a bit longer when the liquid is no longer contacting the electrodes.
- A piece of copper or other heat sink. If you are using this for high current operation you will need a bigger heat sink than just a piece of copper.
- misc wire - I'm sure you have some laying around.

Note: For this switch to work the liquid will need to be slightly conductive. It works with tap and purified water but probably not distilled unless an electrolyte is added.
honda154201 year ago
Bro kindly mention this circuit for 18v and 24v plz
biscuit495 (author) 4 years ago
The TIP120 can handle 5A. A typical 750gph 12v bilge pump draws about 3A. It will produce more heat and need a better heat sink. If you use a PNP it will work in reverse but the problem is the continuous current flowing through to keep the transistor off would corrode away the electrodes very quickly. It may be better to use a micro controller that does not constantly run a voltage through the water but just checks once every few minutes. You many be able to modify this circuit with a micro controller or 555 timer to make it only have current flowing for a short time periodically. With a BJT transistor i do not know if a variable resistor will work very well. It takes .7 volts to turn on a silicon based transistor so you would have to adjust it where in an on situation it would be >.7 volts. Thanks for the comment
HarveyH444 years ago
How much of a load will the transistor handle? Nice idea, might give it a shot, but little concerned about whether this will run a pump motor. I'm thinking about a small auto window washer pump. Also, if you use a PNP transistor, will it work in reverse? I mean will it turn on a pump, when the electrodes are dry? Want to make some potted plants self watering. Could a variable resistor be added to adjust sensitivity?