I have a swamp cooler that does not turn off automatically when the water level evaporates below the safe level, and if I fail to turn it off even once I could damage it.

The sensor simply needs to know if it is A) covered in liquid or B) not covered in liquid. I am not interested in how much liquid is left in the swamp cooler.

I have heard that people have used sensors with the Arduino to measure the frequency of bubbles by using a sensor to detect liquid.

What options are available for the RPi?

The first result from Google is a eTape Liquid Level for $40 which can measure how much water remains in the swamp cooler. This would be interesting, but its $40 and I would be paying for a feature I do not need.

This project on Fritzing looks cool and very cheap. It requires the construction of a homemade sensor using copper. I was however hoping to purchase a sensor, but I'll default to this option if necessary.

  • I wonder if the simple Fritzing solution would change state too late, after damage had been done.
    – joan
    Commented Oct 5, 2014 at 18:52
  • @joan What do you mean? Why would it not change state as soon as the water level dropped below the copper pieces? Commented Oct 5, 2014 at 19:08
  • That rather depends on how you plan to build and fit the sensor. As described it would be on the floor.
    – joan
    Commented Oct 5, 2014 at 19:24
  • @joan The swamp cooler I own has a line that indicates the level which the water should never go below while the unit is running. I thought I could hang it exactly at that line. Commented Oct 5, 2014 at 19:54
  • 3
    You are way over-thinking this... just use a simple float switch.
    – Tyson
    Commented Oct 6, 2014 at 14:36

4 Answers 4


I found this Raspberry Pi water level meter - should answer the question is there water or not , but also a level indicator , like a measuring cup.

12" eTape Liquid Level Sensor https://www.adafruit.com/product/464


I found this project on Reddit you can modify for your purpose:


It uses the ultrasonic sensor HC-SR04 and I think it's the simplest system you can use.


If you have the room I recommend using an off the shelf Bilge Pump Float Sensor that is designed to trigger a bilge pump in a boat. They are often high quality (if it fails, it could sink your boat) and cheap (walmart). They are essentially single pole switches which are closed when the water level is higher than it, and open when the water level is below it. Many of these use magnetic reed switches that never touch the water even when fully submerged. (it's tough)

Bilge Pump Float Sensor


  • cheap
  • reliable
  • simple to interface


  • Size (maybe)
  • Has to touch the water

enter image description here


What about something like this?

enter image description here

This is simplified drawing to show you idea.

Maybe you should build something like toilet reservoir valve:

enter image description here

(image source: link)

If you need digital signal (when specyfic level is reached) - you can use simple switch. If you need analog value - use potentiometer.

  • This is actually a very robust way, a bit bulky but its simple. I have seen these used to cut off pumps in ground water pumps in Africa. Virtually maintenance free.
    – Piotr Kula
    Commented Nov 5, 2014 at 20:25

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