Automatic Water Level Circuit.

The circuit is based on a 555 IC for sensing the minimum and maximum water levels and turns a MOSFET on/off which directly controls a 12V DC pump motor.
Circuit Diagram:

Here  we are using the ‘Trigger’ and ‘Threshold’ pins (2 & 6) to detect the maximum and minimum levels, respectively. The two voltage comparator op-amps inside the 555 control the output, turning it on/off.

Looking at the circuit diagram you will notice that the ‘Trigger’ pin (2) is marked ‘HIGH probe’, despite being triggered (output goes HIGH) when the voltage drops below 1/3 of the supply voltage and, the ‘Threshold’ pin (6) is marked ‘LOW probe’ while it is ‘reset’ (output goes LOW) when the voltage rises above 2/3 of the supply voltage. If this appears to you as being upside-down.

Circuit Diagram:

The circuit works as follows:

Three (3) probes are immersed in the vessel. (usually from the top)
One is the ‘GROUND’ probe,  going to the level a little lower than the minimum desired level. This is the ‘common’ (or ‘reference’) probe. The LOW and HIGH probes are set at the desired levels.

Now suppose the vessel is EMPTY.

Resistors R2 and R1 (1M) tie the ‘Trigger’ and ‘Threshold’ pins (2 & 6) to the positive (+) rail (supply). In other words, both pins are HIGH. Remember (from above), to make the output of IC1 go HIGH, the trigger pin (2) needs to drop below 1/3 of the supply voltage. (4V with a 12V supply) Since the trigger pin is still HIGH, the output remains LOW.

We need to fill the vessel when IC1’s output is LOW.

TR1 is OFF. The GATE of the MOSFET switch (TR2) is connected to the supply rail (+12V) with R4 (10k).

TR2 is thus turned on and the pump motor is running.

TR1 (BC547) is connected between the IC1s output (pin 3) and the TR2’s GATE.
Its purpose is phase reversal. It means that when IC1’s output is HIGH, TR1 conducts and pulls its collector/TR2’s GATE junction LOW, so TR2 is OFF. Since the pump (or relay coil) is connected between the positive rail (+12V) and TR2’s DRAIN, the pump/relay coil is NOT energized.

Now, back to the condition when the IC1’s output is low, TR2’s GATE is HIGH (+12V) and conducting. The pump is operating and water is being filled. As the water level rises, a water ‘bridge’ is formed between the GROUND (common) probe and the ‘LOW probe’ (Threshold, pin 6) This ‘bridge’ constitutes a low resistance, relative to the high resistance of R2 (1M), bringing the voltage at this pin to a low level (at least below 1/3 supply but actual voltage depend on the conductivity of the water). However, this is IGNORED by IC1 since its output is already LOW (in the ‘reset’ mode)

When the water level reaches the ‘HIGH probe’, a water ‘bridge’ is formed between it and the GROUND probe. Just as with the LOW probe, this ‘bridge’ constitutes a low resistance, relative to the high value of R1 (1M), bringing the trigger voltage to below the required level (1/3 supply voltage) and IC1 triggers, its output going HIGH. Now Tr1 is turned on, the bias voltage/current of TR2 is removed and the pump STOPS. The filling cycle is completed.