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I am putting together a PIR system and need a relay with 240V 13A, UK mains, input and a 3.3V output to go to a Raspberry Pi. The Raspberry Pi needs to listen for the presence of the mains PIR device. What type of relay do I need?

Just to be clear, I am not asking for any specific product recommendations, I just know nothing about relays so would appreciate a point in the right direction.

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    \$\begingroup\$ @Andyaka No, it's the other way around, OP needs to detect mains presence via GPIO. \$\endgroup\$
    – Justme
    Jul 29, 2021 at 12:18
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    \$\begingroup\$ Yes, it's a follow up of that question : electronics.stackexchange.com/questions/579364/… \$\endgroup\$
    – Blup1980
    Jul 29, 2021 at 12:19
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    \$\begingroup\$ If the PIR device uses a triac with RC snubber you may find that leakage through the snubber may tend to hold the relay on when the triac turns off. \$\endgroup\$
    – Transistor
    Jul 29, 2021 at 12:21
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    \$\begingroup\$ You'd not use a relay but an opto-isolator in that case. \$\endgroup\$
    – Andy aka
    Jul 29, 2021 at 12:21
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    \$\begingroup\$ Is this the type of thing I need? ebay.co.uk/itm/272619019157 \$\endgroup\$
    – James Geddes
    Jul 29, 2021 at 12:36

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You could get a relay with a 240V AC coil, and contacts that will work at a low DC voltage. Some relays may be unreliable when switching very low voltages and currents, because of the metals used on their contacts.

Alternatively, as mentioned in the comments, consider whether an opto-isolator would do the job better. You would need a small amount of circuitry to drive it from the mains.

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  • \$\begingroup\$ I agree that an opto is a good technical choice. But given the technical knowledge of the OP. I strongly suggest that he doesn't design anything that will be in contact with the main. Instead he should use standard, approved and certified, out of the box equipment \$\endgroup\$
    – Blup1980
    Jul 30, 2021 at 7:56

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