Odd useful bits of circuitry.

Op-to couplers are a great way to interface micros to odd bits of equipment that would normally need mechanical switches to operate them. Their big advantage is that the provide electrical isolation (normally a few thousand volts) between the micro and the device. This means that device can be wired totally independantly of its circuit design - for example allowing it to be run off the same battery as the micro even if the device switch circuitry does not share a common contact with the supply rails. The other advantage is that the isolation provides protection against electro-static discharge.

Opto-couplers consist of a LED (usually infra red) and a opto transistor in a single package. If current is passed through the LED its light switches the transistor on. Opto-isolators need to be wired into the device correctly in order to work. Using a meter measure the voltage across the device's switch in order to work out the polarity - and then wire as per the diagram :-

A dual opto-coupler and resistors in use to interface a camer requiring both a power on switch and a shutter trigger switch.