dc42:
If you are driving the eBay relay module from the Arduino's 5V supply or from the same supply that is powering the Arduino, then the optical isolation is doing nothing useful. It's only helpful if the relay board has an independent power supply.On your custom relay board, did you remember to include a diode across the relay coil?
Yes, there is a diode across the relay coil. Attached is the schematic of my 8-relay module. The diodes across the relay coil are D2, D4, D6, D8, D10, D12, D14 and D16. There are two inputs to each relay because I use a matrix configuration to control 256 relays. Please let me know if you see anything fishy. Many thanks.
Protel Schematic.pdf (23.8 KB)