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. T

he 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.

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