Here's an extremely simple water-level maintainer that keeps the water level between two predefined levels (see the figure). Due to the high input impedance of the LM311 IC (U1), practically no electrolysis of the metal is observed, and any non-toxic metal could successfully be used as sensors without any deterioration.
The circuit finds use in a variety of places. It can be successfully incorporated in all appliances where water levels are critical, such as chillers, boilers, and radiators. For those industries that must constantly monitor solvents and mixtures for their levels, this circuit can replace monitoring personnel. I tested this circuit for 15 days constantly. It worked perfectly, and the sensors weren't at all electrolyzed in water.
The LM311 was chosen for this circuit because it contains a built-in npn transistor at the output. The user sets the inverting input (pin 3), which is kept at a voltage level determined by the R2-R3 voltage divider. The noninverting input (pin 2) is connected to sensor B through R1 for lower-level detection of the water. The open collector of the built-in transistor (pin 7) is connected to the +12-V terminal. Also, the emitter of the built-in transistor (pin 1) is fed to the SCR's anode. And, the SCR's trigger is connected to sensor A for detecting the water's upper level. The anode, C, which is connected to the +12-V supply, is kept at the lowest water level, below B.
Assume you're starting with a full tank. Sensors A and B and anode C are covered. As sensor B gets a positive potential from anode C through hydrolysis in the water, the noninverting input of U1 is more positive than pin 3. Thus, the output state of the IC is high, and the SCR is also triggered because its gate (connected to sensor A) gets positive potential from anode C through hydrolysis and is in a conducting state. Relay RL1 is energized, and its contacts are open. Therefore, no current flows to the water pump.
Now suppose the water is used and its level drops below sensor A (upper-level sensor). The SCR continues conducting because the lower-level sensor B is still in the water and the IC output is high. As soon as the water level drops below sensor B (lower-level sensor), the output state of the IC changes to low, thereby turning the SCR off. As a result, the relay is de-energized and the water pump is turned on to fill water in the tank.
While the tank is filling with water, sensor B is covered first, which in turn changes the IC's output state to high. But the relay isn't energized because the SCR's gate (connected to the upper-level sensor A) isn't triggered yet. When the water touches the upper sensor A, the SCR is triggered, the relay is energized, the water pump is turned off, and the cycle repeats continuously.