Canon DSLR Intervalometer

A lot of Canon cameras have a really simple cable release interface. 

Connecting the shutter tip to the base is the same as pushing the shutter release button on the camera.  Connecting the center ring to the base is the same as pushing the shutter release button half way, to trigger autofocus.  This means if you just wanted to make a cable release, the only parts you would need would be a 2.5 mm stereo plug and some wires, touch the wires together and the camera snaps a photo.

The camera accomplishes this by running 3 V to the tip and center through a high resistance.  When one of them is connected to ground, the voltage drops to 0, and this triggers the shutter.

We could attach the pins directly to arduino pins, but since the arduino runs at 5 V, we would be risking damaging the expensive camera.  I wanted to minimize that risk, so I'm connecting the arduino pins to the base of two 2N4401 NPN transistors.  When we set the arduino pin high this will allow current to pass to ground.  We'll connect the base to the arduino ground though, since there is no risk in that.  Another (better, but slightly more expensive) way to accomplish this would be to use optoisolators.