By means of a Relay, employed to drive a water pump, this circuit provides automatic level control of a water reservoir or well. The shorter steel rod is the "water high" sensor, whereas the longer is the "water low" sensor. When the water level is below both sensors, IC1C output (pin #10) is low; if the water becomes in contact with the longer sensor the output remains low until the shorter sensor is reached. At this point IC1C output goes high, Q1 conducts, the Relay is energized and the pump starts operating.
Now, the water level begins to decrease and the shorter sensor will be no longer in contact with the water, but IC1C output will be hold high by the signal return to pin #5 of IC1B, so the pump will continue its operation. But when the water level falls below the longer sensor, IC1C output goes low and the pump will stop. SW1 is optional and was added to provide reverse operation. Switching SW1 in order to connect R3 to pin #11 of IC1D, the pump will operate when the reservoir is nearly empty and will stop when the reservoir is full. In this case, the pump will be used to fill the reservoir and not to empty it as in the default operating mode.
- R1 = 15K – 1/4W Resistors
- R2 = 15K – 1/4W Resistors
- R3 = 10K – 1/4W Resistor
- R4 = 1K – 1/4W Resistor
- D1 = LED – any type and color
- D2 = 1N4148 – 75V 150mA Diode
- Q1 = BC337 – 45V 800mA NPN Transistor
- IC1 = 4001 Quad 2 Input NOR Gate CMos IC
- SW = SPDT Toggle or Slide Switch (Optional)
- RL1 = Relay with SPDT 2A @ 230V switch
- Coil Voltage 12V – Coil resistance 200-300 Ohm
- Two steel rods of appropriate length
- The two steel rods must be supported by a small insulated (wooden or plastic) board.
- The circuit can be used also with non-metal tanks, provided a third steel rod having about the same height of the tank will be added and connected to the circuit’s negative ground.