Nikon D80 wired remote
I'm working on a "gigapan" like system that will take a panorama. For this my board will have to tell the camera to take a picture. A hack would be to use a servo and push the shutter button, but a wired remote would give more reliable results.
Somehow, I had come to think that instead of pulling pins to ground, my camera had a pin connected to VCC and that we had to pull things up to that voltage level. However things were not working properly.
This seems NOT to be the case!
The situation is as follows.
My (cheap) wired remote cable has three wires, white, yellow and red. white is GND, red is FOCUS and yellow is SHUTTER.
In rest you can measure about 5V on the FOCUS pin (I've observed 4.3V). To activate the focus, pull the FOCUS pin low. At this moment, the camera will enable the pullup on the SHUTTER button. Before that it won't be able to see you connecting it to ground because it already IS at ground. To trigger a snapshot, pull SHUTTER low.
Luckily, instead of tying one pin directly to VCC, I tied all three camera pins to an IO. When I thought that one of them was tied to VCC in the camera, I simply programmed that pin as an output-high. Now that things seem to be the other way around, it's just a matter of changing the firmware. The ground pin will have to be programmed output-low.