I built an electronic camera shutter release using a microcontroller and an optocoupler and a socket a stereo phono jack to accept the shutter release cable. Pretty simple stuff. To fire the shutter, the tip and ring of the jack have to be shorted to the sleeve of the jack. And that is what the optocoupler ostensibly does. But I am having a problem that I don't quite understand. The micro always does its thing as expected and the optocoupler does its. But the camera mostly isn't responding as I thought it would. It doesn't fire the shutter UNLESS I have my DMM leads across the socket pins to measure the resistance drop while the optocoupler is triggered. In that case it ALWAYS works. I am guessing that there is some internal resistance in the DMM that is resulting in my circuit working, but I don't know why that matters or how large a resistor I should add to my circuit to simulate the DMM. Any thoughts?