I have a problem: We use some Raspberry Pi to manage some contactor with a simple relay. This relay can manage one or more three-phase switches, so basically it is linked at least to three contactors. The relay sends an impulse both to put up or down the contactors.

The problem is that our software needs an input to understand if the contactors are up or down because we cannot demand everything to the software. There can always happen something that will cause the system to go out of sync.

How can I read the status of the contactors? My knowledge tells me that I should put a transformer 230 > 5 under the contactors, then a resistance to bring the 5 V to 3.3 V which is the maximum voltage bearable for a Raspberry Pi's GPIO, but I don't know if this solution is enough, also because the contactors have different negatives... Is there another possible solution, even electronically, to read this information?

  • \$\begingroup\$ Do you have any spare auxiliary contacts on any of your contactors (or could you add a set on)? \$\endgroup\$
    – brhans
    Commented Nov 3, 2016 at 14:37
  • 2
    \$\begingroup\$ Virtually the same question as this one: electronics.stackexchange.com/questions/266734/… \$\endgroup\$
    – Andy aka
    Commented Nov 3, 2016 at 15:05
  • \$\begingroup\$ should this be asked on RPI.SE? \$\endgroup\$
    – dalearn
    Commented Nov 3, 2016 at 17:10
  • \$\begingroup\$ You might change the title to something like "Measuring 230V from Raspberry Pi" - I was thinking you wanted to power it from 230V. \$\endgroup\$ Commented Nov 3, 2016 at 23:44

2 Answers 2


The industry-standard way of doing this is to use an optically-isolated coupler. The proper industrial method is to use a modular system where you can select a module for mains power input and logic-level output. For example this AC input, 5V logic output module from Opto22:

Opto22 AC input, 5V logic output isolation module

If you were designing your own circuit, you could do an equivalent with an opto-coupler component such as a TIL111 or one of the MOC series. An example circuit:

Example AC mains input opto-coupler circuit

  • \$\begingroup\$ Thank, but we should use one of them for each contactor, not for each relais, right? \$\endgroup\$
    – mario
    Commented Nov 3, 2016 at 15:40
  • 2
    \$\begingroup\$ How many, and where, you use this circuit depends on your application, and trust of other parts of the controlled system. If you just want to see if the relays operate, then you use one on each relay output. If you're really paranoid, you put one on each phase after the contactors. \$\endgroup\$ Commented Nov 3, 2016 at 15:47

You can use a photo interrupter to look at the mechanical state of the contactor. A description of how they work can be found here; rohm optical interrupter


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.