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I have a 2 Relay module from Sainsmart. http://www.sainsmart.com/arduino-pro-mini.html/

The output has 3 connections instead of just 2. I want to know which two of these three connections I need to wire up. I want to find this out without actually having to connect my final 240V cable.

How can I do this? Is this possible with a multimeter? If yes, how?

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I didn't see the documentation, but they are likely complementary with one 'normally closed' and the other 'normally open' with a common. You can likely tell with an ohm meter. – kenny Jul 14 '12 at 18:40

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There is a schematic available as one of the product photos on the page you linked:

enter image description here

This shows that of the three contacts associated with each relay, the center one is the common connection, one of the others is a normally-open contact, and the third is normally-closed.

Which of the actual physical pins is NC and which is NO isn't obvious from the photos and schematic, but there may be some indication in the silkscreen that isn't visible in the photos. In any case, it would be really easy to check by, for example, just hooking up a desk lamp or something through the relay on one or the other side.

You could equally well (and more safely) use a multimeter to measure the resistance from either side to the center contact and determine which is NC and which is NO.

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Thank you. There was similar drawings on the circuit board, and using this drawing i actually understood what these mean. I was now able to figure out that middle is always connected. Right connector is nc and left is no. – Michael Frey Jul 14 '12 at 19:42

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