Why not use one of those backyard auto light fixtures? They are cheap n will do exaclty what you want.
You'll have to install a relay so when the light goes off in the morning it will power your trip circuit instead of going dead. On the other side of the relay you can now power using batteries instead of wall juice to activate the motor or solenoid.
To adjust the amount of light that will trigger, they do come with a sensativity adjustment. Otherwise you'll just have to positioning it for a darker part of the sky or ground so it won't turn off too early, by covering the sensor you can also limit the light hitting it to eactly when it shoudl take the picture.Total cost... you may already have everything you need right in your back yard.
That timer idea is excelent, didn' t see it till after I posted.
With a 3/4" dome, the area of the photocell is less than 3% of the area of the dome. Wouldn't a person want to measure all the light hitting the dome?
Great advice. I looked up the TSL230R, and it looks to be perfect for this application. Thanks much.
I had not looked into the temperature dependence. Good point. I think I'm going to use the TSL230R chip suggested above. It's temperature stable and can be configured for optimal sensitivity. Yes, I plan to have the shutter in B mode, since the exposure will be on the order of 8 sec. @ f22. Someone mentioned a servo for the cable release -- I agree that a stepper motor of some kind would work well. I like the idea of being able to move it incrementally, so as to minimize vibration. I'm thinking the flash sync would be a good way to detect when the shutter has actually opened; at that point stop pushing and start timing, and then release after 8 sec.
TSL230R has three different versions with 5, 10, and 20% precision depending on the price. Make sure you select the one which is sufficient for your application. The cheapest 20% still must be within 1/3 of a stop, or am I wrong?
I would think that +/- 20% would be close enough for this. I'll be shooting FP4, exposing it at EI 80. I quickly scanned the info sheet on the chip; do I understand correctly that the output is a pulse frequency proportional to the light level? If so, that should be fairly simple to implement ...
OH be sure to compensate for the difference in day light from day to day as it changes with the seasons. Sun rise if not exactly the same every day but maybe close enough.