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.
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.