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 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 some 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.