This circuit, shown in the figure, is also suggested by Texas Instruments and shows how to control the level of a water tank (or other transparent liquid) using flow sensors. What we have are two sensors that are positioned at different levels of the tank, focused on light sources, which in this case are LEDs. The cutting of the light beam by a float causes the sensor circuits to act on a TTL flip-flop assembled with gates from a 7402, changing its state. Thus, we have the production of signals at levels A and B, which serve both to turn on a pump to fill the tank at level B and to turn it off when the float reaches level A. The circuit must be powered with 5 V and the high sensitivity requires that the light sources of the LEDs be strictly pointed at the sensors, which cannot receive light from other sources.
