Rajkumar published another project on Electronics Lab, an 8 channel relay board:
8 Channel Relay Board is a simple and convenient way to interface 8 relays for switching application in your project. Input voltage level support TTL as well as CMOS. Easy interface with Microcontrollers based projects and analog circuits.
It seems the board above doesn’t have any isolation between the digital controls and the relay outputs which makes me a bit nervous using it with AC line voltage.
The eBay item below (and many other similar, all selling for $6 or slightly more) has an optocoupler for each output relay.
http://www.ebay.com/itm/5V-Eight-8-Channel-Relay-Module-With-optocoupler-For-PIC-AVR-DSP-ARM-Arduino-/310574415181
The optocouplers in that ebay relay board are not meant as isolation against relay outputs / mains voltage (the relay itself should provide appropriate isolation, assuming proper PCB layout). You might also note that the optocouplers on that PCB do not “bridge” any kind of isolation gaps, which also speaks against that theory.
Ofcourse the optocouplers provide some kind of isolation. Explanations about the purpose(s) of these optocouplers can be found here: electronics.stackexchange.com/questions/59277/why-are-relays-so-frequently-driven-by-optocouplers.
@udi
The relay itself is an insulator between the charge and the control circuit. The optocoupler, in your example, is to protect the control against the spikes that might generate the switching relay.
Coil VCC trace very close to COM contact. I’d be afraid to use this board in a project. Even cheap chinese boards look safer than this one.
Yes. The PCB layout as shown on the project page is almost criminally dangerous (no pun intended)…