One of the nice features of the CPC1218 and CPC1219 relays that I mentioned before is that they have a built-in current-limiting resistor. This is because they are designed to be a drop-in replacement for a certain popular type of reed switch, and the reed switch didn't need a current-limiting resistor. Anyway, this is nice because I cannot tell you how many times I have accidentally blown up an optoisolator by accidentally overloading the control side pins because I forgot to install a current-limiting resistor, or I accidentally shorted the pins with a test lead, or I accidentally plugged in power to the wrong place in the circuit. With a self-contained relay, none of that would have happened.