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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?

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  • \$\begingroup\$ Do you have any spare auxiliary contacts on any of your contactors (or could you add a set on)? \$\endgroup\$ – brhans Nov 3 '16 at 14:37
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    \$\begingroup\$ Virtually the same question as this one: electronics.stackexchange.com/questions/266734/… \$\endgroup\$ – Andy aka Nov 3 '16 at 15:05
  • \$\begingroup\$ should this be asked on RPI.SE? \$\endgroup\$ – dalearn Nov 3 '16 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\$ – immibis Nov 3 '16 at 23:44
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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

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  • \$\begingroup\$ Thank, but we should use one of them for each contactor, not for each relais, right? \$\endgroup\$ – mario Nov 3 '16 at 15:40
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    \$\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\$ – Peter Bennett Nov 3 '16 at 15:47
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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

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