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I have the relay you see here:

Picture of relay contacts

How do I read that diagram - or specifically, which contact is the common, normally open, and normally closed contact?

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  • \$\begingroup\$ this should explain it all glolab.com/relays/relays.html \$\endgroup\$ Commented Nov 10, 2016 at 19:46
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    \$\begingroup\$ I would read that drawing on the PCB as indicating the center as Common, left is NO and right is NC. \$\endgroup\$
    – brhans
    Commented Nov 10, 2016 at 19:49
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    \$\begingroup\$ And to avoid any confusion, get your multimeter out, switch it to continuity test and measure it. \$\endgroup\$ Commented Nov 10, 2016 at 19:51
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    \$\begingroup\$ The middle pin is always connected to the pin on the right [NORMALLY CLOSED], whenever the relay is turned on this middle pin is connected to the pin on the left [NORMALLY OPENED]. You should definitely test this using a multi-meter. I remember buying a relay module from a Chinese vendor and this drawing was wrong ! \$\endgroup\$
    – Elbehery
    Commented Nov 10, 2016 at 19:51
  • \$\begingroup\$ Relays are always shown with the power off, so NO/Com/NC left -to-right As FakeM & @Elbehery say, confirm with your multimeter. \$\endgroup\$ Commented Nov 10, 2016 at 19:59

2 Answers 2

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  1. Left terminal: Normally Open
  2. Center terminal: Common
  3. Right terminal: Normally Closed

Confirm by measurement.

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  • \$\begingroup\$ That is how I would read it too, and it makes sense knowing that the common pinout of a cheap relay has the common pin in the center (between the coil pins) but there is a chance it could have been routed by a fool (or because of a strange pinout of the relay, which that kind isn't, so it would have to be the "fool" option) so that the right-hand terminal is common. So yes, the most important thing is to test it by measurement. See which terminals are connected by "buzzing" it with a DMM with the relay switched in both positions. \$\endgroup\$
    – Majenko
    Commented Nov 10, 2016 at 20:37
  • \$\begingroup\$ You could also confirm it by looking at the data sheet for the relay and follow the (likely very simple) traces on the bottom of the PC board. \$\endgroup\$ Commented Nov 10, 2016 at 20:40
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I recently laid a board out with the exact same relay + header arrangement. Due to the pinout of the relay the simplest way of routing the traces results in the pinout given by @RichardCrowley.

Here is a render of the portion of the board with the relay header:

enter image description here

And this is the PCB traces of that section:

enter image description here

You can probably find the same trace layout on your board, or at least confirm which pin is connected to which connection on the terminal block using a DMM in continuity (aka "Diode") test mode (the mode that goes beep when you touch the probes together).

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  • \$\begingroup\$ Nice (I guess the clearance to the pour could be greater) Did you stitch traces both sides with vias or are the white dots just silk? \$\endgroup\$ Commented Nov 10, 2016 at 21:59
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    \$\begingroup\$ Stitched. The clearance is well within tolerance, though I did consider clearing right to the edge of the board. \$\endgroup\$
    – Majenko
    Commented Nov 10, 2016 at 22:00

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