I'm thinking that the backyard auto light fixtures I've seen are 120V; there will be no external power available where the photo will be taken. Also, I'm not at all sure what the specs are for the sensors -- how repeatable they are, or what it would take to modify them so that they switch at the light level I want.
As to the timer: the sun rises almost 3 hours earlier in June than it does in January. That schedule is pretty well known, but with weather variabilty affecting brightness, it's really not possible to know on any given day when the light level will first reach the point where the photo must be taken.
The difference between winter and summer sunrise extremes is 2 hours and 45 minutes. I need to take the picture when the light has reached a particular level, which on a clear morning would be about 15 minutes before sunrise. But changing weather conditions would affect the exact time. Measuring light levels, rather than time, seems the more practical approach.
An interesting thread; I can't quite visualize how you are going to plan the picture without having been there at the right time of day, which would mean that you could presumably return the right number of minutes later the next day......
One reason for using a lens would be that if you used an opaque barrel you could limit the angular field of view of the photosensor and do a Zone-System-type exposure for the key area of the scene!
10 RELEASE = 156
20 INPUT LIGHT
30 IF LIGHT < RELEASE THEN 20
40 PRESS SHUTTER
You don't need a diffuser or a lens. A soda straw will work fine to exclude extraneous light.
Measure the light level at the desired time of the day with a light meter.
I __assume__ you can get somewhere in the vicinity close enough to make the measurement, since sunrise covers a large area of sky.
Then set up the light meter in broad daylight, pointed at the sky (with a snoot to limit its field of view and exclude the sun) and read the light level. Put a combination of ND filters over the snoot until you get the same reading as sunrise.
Place that filter set over your sensor, pointed skyward under the same conditions, and use that level to calibrate the trip point.
You're right about the time of day ... but this isn't about the time of day, it's about the light level. I want to have a photograph at the point where the light first reaches a pre-determined level. I've gone out and metered it -- the exposure would be 1 sec. @f8, EI 100. I'm going to run some tests with a photocell before deciding on a condensor lens. It turns out that the light level I'm waiting for is pretty much at the low end of the photocell's practical measuring ability. A condensor moves the effective brightness (on the photocell) into a resistance range that allow for more accurate measurements.
If this were just one photograph, I'd not be building this thing. But it's for a project that entails a photograph every two weeks, for a year, to document seasonal changes on the prairie. I simply cannot expect to be available to take the photograph on every one of those mornings, but I can expect to be able to set up the camera sometime during the day before and have it capture the scene as planned.
Wouldn't it be less trouble to simply get the trip point value directly? (I took a meter reading with my Luna Pro the other morning -- I'm waiting for brightness that translates into 1 sec. @ f8 for my FP4). Knowing that, I will simply see what resistance I get from my photocell at that light level, and plan accordingly. A condensor may be necessary to amplify the energy delivered to the photocell, so that the value is more reliable; some testing should help me decide on that point.
Why not just wake up early...
I do (at least, 5 am seems early for me). But the photograph(s) will be taken 25 miles from where I live (but right next to where I work). I'm not usually at work before sunrise. And this will be a one year project -- a photograph every two weeks, at the point where the light reaches the pre-determined level. Setting up the day before seems the most practical approach.
The light fixture... the realy is hot, realy will be open when it is dark but once it gets light, the realy contacts shut (NC contacts) will switch your external circuit on, and the 8 sec timer starts it's count down. Using a large enough elecrtolytic cap bleeding off to ground via a resistor can adjust to 8sec easily enough without many components.
BTW sun rise is later by about 1 minute per day. It won't matter is you use the photo sensor but if a cloud is on the horizon n it clears a bit later your shot won't be what you planed it to be.