Water Level Sensor Circuit

Figure 1 shows a circuit for sensing the water level in a tank and turning on or off a water pump accordingly. The main component of the circuit is the CD-4011 Quad NAND gate, three gates of which are used as shown in Figure 1 (gates G1, G2, and G3). G1 is configured as an inverter (both inputs are shorted), while G2 and G3 form an RS flip-flop. The level sensors are just copper or stainless steel wires.

When there is no water in the tank, the trigger sensor is floating and the inputs to G1 are pulled 'high' by their pull-up resistor, causing the output of G1 to be 'low'. This, in turn, causes the output of G2 to be high, turning on Q1 which energizes the relay that powers up the water pump. At this point, both of G3's inputs are high, so its output is low.

The water in the tank rises until it reaches the trigger sensor, which 'grounds' G1's inputs, causing G1's output to go 'high'. This does not affect the output of G2, though, since G2's other input (coming from G3) is still 'low'. Thus, at this point, the water pump keeps on filling the tank with water.

When the water level reaches the threshold sensor, G3's pin 9 input is pulled 'low', causing the output of G3 to go 'high'. This means that both of G2's inputs are now 'high', causing G2's output to go 'low'. This turns off Q1, de-energizing the relay and shutting off the water pump. When the water level goes below the trigger sensor, G1's inputs are pulled 'high' again, causing G1's output to go 'low'. This turns on the water pump and the cycle starts all over again.