I made a modification to the water level sensor. For this I used the lid of a margarine tub. I cut it so that it forms straps into which I slid the two probes. The probes are easily adjusted for height.
The probes are connected to a little circuit board which holds a ATtiny.
What the controller board does is to turn on a relay which operates the pump. The controller then measures the resistance between the sensors. As soon as the water touches both sensors, the controller turns off the pump. This cycle repeats every thirty minutes or so.
I opted to fill the growbed with LECA. Only problem is that it is lighter than water. This means that it starts floating when you flood the growbed. I overcome this by putting a layer of rocks about 8 cm from the top. This ensures that the bottom half does not float.
As the growbed is only filled up to a inch from the top of the grow medium, the top portion does not float either.
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