I am making a automatic water level controller for my home. I am using a MCU to turn ON/OFF submersible water pumps using a float switch to detect water level in overhead tank.

This pump has a start switch (normally open), and a stop switch (normally closed), connected with a contactor.

For my project I want to replace these mechanical switches with a 5 V, 10 A relay, which is controlled using the MCU.

I am stuck with how to interface the contactor (ON/OFF) mechanism with the relay's (NO and NC) contacts. I am planning to use a general purpose relay, something like this but of 10 A capacity: General purpose capacitor.

Here is an block diagram for it.

Block Diagram for interfacing Water Pump

The submersible pump here is 2 hp, 230 V, single-phase. Can anyone suggest me how to do it or direct me to an article that helps?

  • 2
    \$\begingroup\$ why do you need an MCU? can't you just have the float switch control the pump? that would seem to be simpler and more reliable... \$\endgroup\$
    – dandavis
    Commented Jun 6, 2017 at 12:43
  • \$\begingroup\$ Your contactor diagram makes little sense - please provide a link to the data sheet. Regardless, your connection will either produce a disaster or no result, so don't do it. If the contactor is 3-phase, SPST making connections between L and T, there will be no effect. If it is DPDT, with T3 common, there will be no effect. If it is DPDT with either T1 or T2 common, either applying power or activating the contactor will short out your AC and (hopefully) blowing your circuit breaker. So don't do it. \$\endgroup\$ Commented Jun 6, 2017 at 12:46
  • \$\begingroup\$ @dandavis as there will be some other features like buzzer ON/OFF, and may be later I will adding this to complete home automation, so I would need MCU \$\endgroup\$ Commented Jun 6, 2017 at 12:48
  • 2
    \$\begingroup\$ @ArchanaWadhwani: I would advise you to build the system in a way that can still be controlled and monitored by an MCU, but that doesn't rely on the MCU for the ever-important main operation. you still can't beat analog for mission-critical work. \$\endgroup\$
    – dandavis
    Commented Jun 6, 2017 at 12:50
  • \$\begingroup\$ Have a look at the following image search for ideas. I would also sugegst an off the shelf level controller and then adding a float switch as an alarm input if you want to notify your home automation system. - google.com/… \$\endgroup\$
    – KalleMP
    Commented Jun 6, 2017 at 14:28

1 Answer 1


If you want to keep the existing manual Start/Stop buttons (highly recommended!), you will need your MCU to control two relays. The Start relay Normally Open contacts should be wired in parallel with the existing Start button, and the Stop relay Normally Closed contact wired in series with the existing Stop button. The MCU should pulse either relay for 100 mS or so (whatever time is required for reliable operation...)

  • 1
    \$\begingroup\$ What are the ramifications is the pump fails on or off? How do you protect for that? \$\endgroup\$
    – Gil
    Commented Sep 24, 2021 at 3:27

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