Instructables user diy_bloke built this op-amp controlled pump for his garden, using the classic 741 operational amplifier.
He wrote a very detailed how-to including showing how to etch a PCB, as well as how to make a humidity sensor out of gypsum:
The simplest form of a soil humidity sensor is made from two galvanized nails that are stuck in the soil. Though technically other metals or conducting materials can be used, stainless steel and galvanized iron seem to be the best resistent against corrosion in the soil. Stainless steel however is very difficult to solder, therefore galvanized material is a better choice.
4 thoughts on “Irrigating Your Garden With an Op Amp”
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Dont solder, Crimp. you can easily crimp your wires to Stainless steel. notch the stainless with a file in several locations wher eyou crimp the copper ferrule around it with the wire and then put a piece of glue lined heat shrink over all of it to make it a water tight and corrosion resistant connection.
There’s no hysterestis in the op-amp comparator, so it’s going to go into oscillation right around the point where it should switch on or off. I see there’s a lowpass filter at the output so the pump doesn’t rattle on and off rapidly, but that’s not a very good solution as it forces the pump to run a fixed amount of time in any case. That means you may be needlessly flooding your garden, or spilling water out of the flowerpot, or not watering it enough to last and forcing the pump to start and stop several times a day. Regular plastic impeller pumps don’t like that.
Adding hysterestis to the comparator is a matter of adding a single resistor for feedback: http://www.ecircuitcenter.com/Circuits/op_comp/op_comp.htm
That way there’s an amount of “slop” in the switching. When the comparator output goes up, it returns some of the signal back to the input and changes the reference level so the device switches off at a different point than it turns on. That way you can precisely adjust how wet you want the soil before turning the pump off, or how dry you’ll let it become before turning it back on. The greater the difference, the less frequently your pump has to run.