Currently i'm experimenting with 12VDC energy meters latching relays and at least for me the feature of detecting status of contacts ( required by some clients ) which is built in as in the diagram below is quite interesting. Now i need to interface it to a microcontroller safely. At the top of my head i may think of using an optocoupler but my understanding is i need to isolate the grounds ? thus making the design a bit pain in the kneck, is this the only feasible method or are there other approaches ?. enter image description here

TL;DR pins 4 and 5 switch on / off depeding on status of relay, how do i sense/ measure status of the latch with those pins?

  • \$\begingroup\$ Could you narrow down what you have in mind? The simplest thing would be to use a comparator circuit to see if the voltages are the similar. A more complex expensive thing would to send a RF coupled signal and then detect with a lock in amplifier. \$\endgroup\$
    – Voltage Spike
    Commented Oct 22, 2019 at 19:01
  • \$\begingroup\$ I edited the post please tell me if that is more clear \$\endgroup\$ Commented Oct 22, 2019 at 19:15

1 Answer 1


If the Auxiliary contacts are not currently used, you just connect them to a microcontroller digital input as you would any other switch - something like so:


simulate this circuit – Schematic created using CircuitLab

If the auxiliary contact is already used, you may have to use an opto-coupler or perhaps an additional relay, depending on what is connected to the auxiliary contacts of the contactor.

  • \$\begingroup\$ I'm not sure what is meant by "used" , the pins are not connected to anything (4,5) but are effected by the coills when they are activated/deactivated. Assuming i connect pin 4 with my digital ground, this is considered safe to the mcu pins right? \$\endgroup\$ Commented Oct 22, 2019 at 19:27
  • 1
    \$\begingroup\$ By "used" I meant "currently connected to something". If the auxiliary contacts are not connected to anything now, you can use them as my schematic shows. \$\endgroup\$ Commented Oct 22, 2019 at 19:32

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