The circuit below was developed to guard the fish pond. In this case to prevent that the pump sucks just air when the waterlevel get below the pump. When the waterfilters get saturated and dirty, the water level behind the filter gets to an unacceptable level. You can see this when the pump also produces airbubbles in the water.
Because you are not all day peeking if this is the case, I connected the pump via a Solid State Relais, which acts as a power switch mounted in one of the AC wires and is controlled by the circuitry below.
The sensor is fabricated using two sturdy solid *copperwires which are mounted approximately 1cm () apart in the water after the waterfilter. The conductivity of the water is sufficient to pull the input of the first IC (IC1c) high. The output of IC1d will then also be high. (* see note)
This output signals the R-S Flip-Flop formed by IC1a and IC1b.
Often the condition is correct when you power up, which is indicated by the green led. However, if the red led is lit, just press the Reset switch to put the circuit in the proper operational condition. The current flowing through the green led is also fed through the diode in the Solid State Relay, activating the relays and the starting the pump.
If for some reason the water level is getting low and the copper wires no longer touch the water, the input of the first IC is pulled low and consequently also the ouput of the IC behind that. The R-S Flip-Flop flips to the other condition and the green led goes out and also the pump and the red led will be lit to indicate 'something' is wrong. In this case the waterlevel.
When you're ready after the filters have been cleaned, the only you have to do is press the Reset button to activate the pump again. This way unnecessary damage to the pump is prevented.
The copper wires will need regular cleaning to make sure they conduct.
If you have questions about this circuit, please direct them to Jan Hamer or visit his website in the Netherlands (if you can read Dutch).
Published & Translated from Dutch into English with permission of Jan Hamer, The Netherlands.