Sensing water

Next step is the sensors for soil moisture of the plants and water level in the tank. I don’t really need very high accuracy in either case, just some indication of the levels.

So I tried a very simple solution. I have been using wireless thermometers from UPM which have very cheap remote sensors sending on the 433MHz band. The actual temperature sensor is some kind of resistive NTC-sensor, and the resistance of the soil will wary with the water content, so I tried a very straight on solution: I cut off the sensor and mounted some random connector I had lying around, (just to get two metal pins with a fixed distance) and stuck that in the soil. And I was lucky – the resistance of the soil with different water content levels matches resistance range of the temperature sensor!

The NetHome-server can read the raw values from the wireless sensor and I get a range from a raw value of 1936 (soil saturated with water) down to 950 (soil dry). So just by cutting a cable I get a wireless soil moisture sensor! I know that you really should use AC current for this kind of measurement and that the resistance may be affected by a lot of other factors, but as ballpark figure this will probably do fine.


  1. Hi,
    I am trying to make soil moisture sensors out of one of my temp/humidity sensors that looks exactly like the one you used but from ClasOhlsson. I have connected wires from the board to nails in the soil. The problem I face is that the readings are HI or LO, I never get into the right resistance window. How do I figure out the resistance to use for the temp circuit and humidity circuit?

  2. Hi. I currently just use the temperature sensor for soil moisture measurement. The "sensors" I use now are thin metal spikes (legs from resistors) that I have put shrinkable tubing (krympslang) around so only about 5 mm of the metal is exposed in the end. I place them about 10 cm apart in the pot, but I adjust the distance so the impedance becomes correct. Hope this helps!

  3. I got it going. I missd a solder point and did not relise recognize that the humidity sensor did not work unless temperature circuit was reading correctly. I also quickly learned that the humidity readings vary with temperature readings..