what are the additional components on a relay board for?

[Anduril]

Hey there,

I thought about building a node with relays to controll my light. I looked for the relay and noticed these boards with added components (ICs, resistors,...) in the shop. What are they good for? Can't I just buy a bare relay module and mount it with screw terminals on a custom PCB?
And what is meant with "Make sure to power the relay module separately if you use more than one relay." (http://www.mysensors.org/build/relay)? Does this mean to power VCC and GND seperatly or will the current on the data pin cause instabilities with several relays working?

Best regards,
Anduril

[AWI]

The extra components on the board are mostly an optocoupler and nescessary compenents to let that work. A relay consists of a coil (inductive resistance) which can ruin your arduino if directly attached. Thats the reason for the optocoupler. The same goes for powering the relay. An inductive component make a lot of "noise" on the power line when operated. Also it takes a substantial amount of power whicht is to much for the regulator on a standard arduino. I would allways recommend separate power for one to many relays.

[Anduril]

thanks for the clarification. I'm not going to use the regulator at all, as I'm powering the whole setup with a HLK-PM01 (when it finally arrives).