A really useful relay module, triggered by 3.3V or 5V with a built in optocoupler.
The relay on the board is a 5 volt version, but the board has been configured in such a way that it can be powered and reliably triggered by 3.3 volt micro controllers.
I realise that these are quite expensive, compared to the 5V versions, but I needed one in a hurry a few weeks ago and discovered that there weren't any on eBay (at least in the UK). So I imported some for me and some for eBay... They were actually quite hard you find ;)
Connections:
VCC: 5v or 3.3v positive
GND: Ground
CH1: 5v or 3.3v data
RGND: not needed, connected to GND
Data sheets for the main components are readily available from the web:
Optocoupler (marked EL817 C845): https://www.everlight.com/file/ProductFile/EL817.pdf
Relay(marked SRD-05VDC-SL-C): https://www.circuitbasics.com/wp-content/uploads/2015/11/SRD-05VDC-SL-C-Datasheet.pdf
Protection Diode (D3, marked T4): http://www.s-manuals.com/pdf/datasheet/1/n/1n4148w_galaxy.pdf
Power transistor(marked J3Y): https://pdf1.alldatasheet.com/datasheet-pdf/view/226239/BILIN/S8050.html
Thanks to @TigerRob for the reverse engineering.