I am considering a project that involves setting up a 4x5 to take an (unattended) early morning photograph once the light reaches a certain level. This would be dawn, but pre-sunrise. I would appreciate any feedback/advice on the approach I am considering.

I have some experience building devices that use microcontrollers to monitor temperature, and so want to do the same with photoresistors. My thought was to create a diffuser (would half a white ping pong ball work?) that has a photoresistor placed under it to read the brighness. I would then do some tests on location -- wait for the desired light level, use my hand-held light meter to see what the exposure would be, and at the same time record the resistance value from my DIY setup. The idea is that the microprocessor would watch for the desired resistance, and when it reaches the threshold value take the photograph. Would a lens to focus the diffuser shell onto the photoresistor make it more accurate? Would it be necessary?

On a related front, any thoughts on devices to press the shutter release cable?