In the dark, the photocellís resistance is high, and the voltage dividerís output is sufficient to charge the capacitor C2 quickly, achieve breakover voltage of the diac, turn the triac on, and bring the lamp to full brightness.
No doubt there are other solutions. The floods are a good choice for aiming light where it is needed. For example, two are pointed towards the garage door area to illuminate that area of the garage, even though the lamps themselves are some distance from the door in order to be clear of the door when it is raised. Although I could have installed low-profile tubes directly over that area, they would have been covered by the raised overhead garage door, and I would have had a potential dark spot.
Makes me think of another idea. Maybe its already out there and I haven't noticed, but why not use gradual dimming and brightning for car headlights. Seem like a concept that would be a very welcome chance to the sudden flash seen when switching between high/low beam.
I'm a bit of a lamp snob; I'll only buy from the "big 3," GE, Philips and Sylvania for quality reasons. I too have been installing CFLs in my house since they firsat started becoming available and my utility offered rebates (no longer available) to try to make them economically viable. I haven't been able to put them yet in any fixture served by a dimmer switch and I discovered a long time ago that they fail quickly if they are switched on and off a lot so I don't have them in my walk-in closet. In my vanity light bars, I mix CFLs 50-50 with incandescents because of the warmup time. I have a 42W spiral CFL burning base up in my laundry room and my family complains about the time required to come to full lumens. Dick doesn't say how well insulated his garage is or where he lives but my garage might be too cold for 1/4 - 1/2 of the year for the CFLs (or the linears that somebody else mentioned) to come up to full lumens quickly.
It is my understanding that flourescent lights are life limited by two things: hours of use and number of starts. For that reason automatic light sensors should have a longer "hang time", leaving the lamps on for as much as 30 minutes once they are lit. Incadescent lamps are much less sensitive to starts (although they, too, typically fail at start) and so can be set for much shorter hang times to save on the electricity.
As I learned it, the circuit does not use a voltage divider to fire the Diac to trigger the Triac. Back in the 60's I was instructed that the resistors and capacitor formed a phase shift network to control the timing (phase) of firing the Diac to control the conduction angle of the Triac. Except, back then it was UJT's and SCR's...
....I'm pretty sure what you've done violates electrical code.
It's not a particularly good idea to take something you brewed up and hard-wire it to the electrical system in your house. Most likely, you won't be the only owner of this house ever. You might die tomorrow, and nobody would have any idea you'd stuck something 'custom' into an electrical box. You didn't even put a fuse on this.
If you burn down the house with this gadget, and your insurance company can prove it, they could easily deny the claim. You're also legally liable forever for this install, so if you forget to remove it when you move and somebody else dies in a fire, they could come after you. I think if you're an EE, you're a easier target, 'cuz "you should know better".
I know as Americans we think we own our homes so we can do whatever we want it them. But that's not true. You could get hit by a bus tomorrow and nobody would ever know you'd put this in an electrical box. We hear regularly of somebody who covered a dry well with wood 100 years ago, and some kid falls in today since the wood rotted out. Houses in Europe are hundreds of years old.
IMHO - Design News should NOT have posted this or any other gadget that is hard-wired into household power.
If you must do something like this, much better to at least put it in a project box and pigtail the cords out of it. At least that way it's obvious to anybody looking that you've kluged something together.
Some times it is just fun to buld it yourself to both learn how it functions and test yourself as to making improvements. I applaud people who have the expertise and patience to come up with a device like this, even if they could have bought one. Then again I really used to like customized cars and souped up engines. Anyone could buy either, but how many could do it themselves?
Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies.
You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived.
So if you can't attend live, attend at your convenience.