I have a water tank that I want to fill up with a well. I have a float switch at the top of the tank and another float switch in the middle of the tank. What I want to do is have the pump only turn on when the middle tank switch opens and to keep pumping until the top switch closes.

This is trivial with a microcontroller but I feel there must be a way to do this with solid-state components. I can convert logic-level to what is needed to drive my contactor - I just need to get the actual logic circuit sorted out. I feel I'm missing something obvious and would appreciate any guidance. Thanks!

  • 1
    \$\begingroup\$ you are using an incorrect train of thought ... the requirement should be turn on when the middle tank switch closes and keep pumping until the top switch opens \$\endgroup\$
    – jsotola
    Commented Jun 27, 2019 at 20:26
  • \$\begingroup\$ Part numbers and link to datasheet for the switches, please. What type of contacts do they have and what is the "normal" position of the float for the "normal" position of the contacts? \$\endgroup\$
    – Transistor
    Commented Jun 27, 2019 at 20:37

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. Pump control circuit.

How it works:

  • As the tank empties the TOP switch closes first.
  • When it passes the low-level switch it closes and RLY1 is energised.
  • The pump starts via contact RLY1b.
  • Meanwhile RLY1a bridges out MIDDLE so that the pump continues to run as the water level passes that switch.
  • RLY1 remains latched on until the top switch is opened.

Rate the relay coil for your control voltage. Rate the relay contacts for your pump voltage, AC or DC as required, and pump starting current.

  • \$\begingroup\$ Thanks, this is really helpful and makes perfect sense. I've already ordered the parts and they are on their way. This looks like it will work great! \$\endgroup\$
    – Michael
    Commented Jun 28, 2019 at 15:44

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