Quick question on relay pinout

Hi all

I have a small relay on a board. The markings on it are the current / voltage limits and "SRD-05VDC-SL-C" and also three adjacent pins labelled + - and S, with the other contacts being the usual NO/NC on screw terminals. Also has "Keyes SRly" silkscreened onto it.

It's rated at 250VAC/10A along with other quite high DC ratings for the contacts, so is quite a handy unit, it came packaged in one of those starter kits for Arduinos that I got a year ago and now have a use for, assuming I can get it to work :slight_smile: I don't want to play pass the current if I can help it, a bit of care and a question might stop me burning something out.

The pins would be for the coil, but I'm not quite sure how to wire them up: + and - to 5V and the coil should actuate the relay? But then what is the S pin for.. Coming in a kit like that I have to assume that I'm not expected to have a 12V source just handy, so maybe 5V also on the 'S' pin will actuate it?

Does this help?

That would appear to be exactly the same relay, the case labelling is the same. The schematic there doesn't show a third pin though. I put an image of my relay at IMG_1987s | Rob Murphy | Flickr to show what I mean.

Use a multimeter and see what IO pins/screw terminals are connected to which relay pins.

OK. I made another image of the back of the board and pretty roughly marked out the traces. Well, OK, very roughly.


The diodes are on the other side of the board, the one on the long edge is the one for back EMF so that's a good start. The other trace that just stops in mid track is connected to an LED but I can't see where the other end of that LED is. That top pin is the one marked 'S'. Maybe needs to run through the LED and the ohmmeter isn't supplying enough voltage for that.

Of the three large solder pads at the header (right ) end of the board, the outer 2 are connected according to ohmmeter. None of the pins themselves seem to be connected to anything though.

It seems I have the coil isolated at least, if I can't work it all out I'll just solder some wires onto what would appear to be the coil and hopefully get a little click rather than a puff of smoke.

Here is a schematic of how your relay board is likely wired if it helps.


Thanks, that works perfectly just running off a 5V breadboard supply. Mine is slightly different, looks like the S pin feeds the LED before connecting to the transistor but the result is the same.

Do I need to put a resistor in the signal path ? Probably not I'd guess as it's already connected to the LED and its resistor.

Thanks groundfungus, thanks CrossRoads.

The - + S pins are "Ground-Voltage-Signal" often called "GVS". This is the 3-pin semi-standard used by servos and Sensor SHields and Arduino derivatives like the YourDuinoRobo

It's easy to connect many different devices with 3-pin cables like that. See:

There are many relay boards that are similar to the one you have; many have optical isolation like These

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