Initial notes:

I got a couple of small water sensor boards in from China some months ago with two applications in mind; weather station rain sensor and hydroponics level control. I’m keen to get these going, mainly the hydroponics controller.

The hydroponics controller must sense water level in a relatively dirty environment; inside tanks and in gravel filled grow beds.

The Chinese boards use a LM393 comparator and apply 5V DC to the electrodes. The problem is electrolysis and erosion of the electrodes, even with the low 100-300uA sense current. Stainless electrodes should last a lot longer than most other materials. There are ways to minimise electrode erosion such as polarity reversal and pulsed short duration sensing.

If the sense current is too low and with lengths of wiring attached, it may be affected by electrical or RF interference. I’m looking at one common and several sense circuits at different levels. Something similar to a 4-20mA circuit would be more robust and allow longer wiring between the sensor and the control electronics when needed. It also simplifies the controller interface to the sensor.

 The greater the distance between the common and sense electrodes, the greater the effect of water quality. For reliable operation it does require a smaller spacing or a common electrode closely positioned with each level sense electrode.

Algae or anything that remains damp on the electrodes may cause false detection. The electrode set must be low-cost, simple, reliable, easily adjusted and cleaned.

I’m thinking of a set of stainless wires that hang down into the water, inside a open tube suspended or attached to the side. The wire bundle can be height adjusted and easily lifted out for cleaning. The trick will be to make it simple, small and keep the wires separated enough for the water to clear the tube and reliably register.

The sensor head would contain some circuitry sealed or potted for unprotected outdoor use.

The controller will be a micro-controller with two relay outputs able to switch a pump or valve, and an alarm output. Operation from a nominal 12VDC supply (9-15VDC). mains (240VAC) rated relays.