Relay board with RGND... what is RGND?


I recently bought a few very inexpensive single relay boards. I’m used to the boards having a VCC, GND, and pins to energize each relay. This board has an additional pin, RGND. What is RGND and when should I be using it?

Thanks for any insight!

relay top.JPG

relay bottom.JPG

I suspect GND is for the connection to the Arduino in conjunction with CH1 and RGND is for connection to the coil power supply in conjunction with VCC.

But don’t blame me if the smoke escapes.


Your relay board features optoisolation. The RGND is probably the ground for the relays. See for more explanation.

If you need more help then a better picture of the relay module's top side would be needed.

Thanks Chagrin and Robin2. I’m still a bit perplexed even after thinking about what you said and looking at your links. My relay is a little different from these. I’ve included a better photo of the top of the relay as requested.

If I connect CH1 and GND to my Arduino(powered by USB), and VCC and RGND to a separate 5V supply (another Arduino powered by a 9V), the relay works. This makes me think that CH1 and GND work to energize the LED for the opto-isolator, and VCC and RGND energize the relay coil. I know that isn’t what the wiki-spaces article called for, but I don’t have a JD-VCC pin, so I thought this would be a good experiment to test the idea that Robin2 presented. BUT…

When I test continuity between GND and RGND, there is continuity. It would seem that this means that they aren’t isolated, so what’s the point of having two connectors?

I’m definitely new, as I’m having trouble seeing how the simple LED in the opto-isolated circuit schematic(from Chagrin’s link) would work (how the current would flow), if there is no ground. Is there a potential difference between the VCC and the IN1 pin? Does the IN1 pin basically become a sort of ground when it’s not energized?

Anyhow, If I only connect VCC to Arduino +5v, and CH 1 to my digital output pin(set to HIGH), nothing seems to happen and I measure no current flow. The two pins that are left over are GND and RGND. If I connect either of these pins to ground, the relay of course works. Intuitively, I don’t think I should connect these (GND and RGND) pins to anything other than ground for experimentation / test purposes.

Also, I think that I have a relay with an active high, because when I execute digitalWrite (relayPin, HIGH); the relay turns on.

I definitely see something that looks like an opto-isolator chip but the relay board does not have any jumpers, so I wonder if it uses some different convention for opto-isolation than the boards presented in the wiki-spaces article.

Also, from what I can see (and deduce) from the schematic in Chagrin’s link, there is a photosensitive transistor, so when it’s base is energized by light, the regular transistor’s base is energized allowing current to flow through and energize the relay coil. But… what the heck is the purpose of Diode D1?

It appears that the Arduino side of the optoisolator is CH1 (positive) to GND. The relay side would be 5V to RGND. Of course I note that you mention there is continuity between GND and RGND so that doesn't make sense at all; it's not properly isolated.

On the bottom of the board there's a white rectangle next to the RGND pin; do you see a trace there? It may be that you would need to cut that trace to eliminate the continuity between GND and RGND. The pads that are there suggest that you could use a zero ohm resistor to re-jumper it if necessary.

D1 appears to be an LED. Just an indicator to show you that the high signal from the Arduino is present.

Other than that it appears you have the right understanding of how the module works.