Sensor for Controlling Household Lamps

www.x10.com has a bunch of home control stuff

No need. Plug-in devices are available over-the-counter at any principal domestic hardware store.

Disclaimer: Contact a qualified electrician to do the job. Don't attempt this if you are not an electrician!

Apart from that, Do you mean, turn on when the power goes down? If yes, then connect it to your normal lighting circuit, not the switch wires, but the actual circuit that runs round to feed all the light circuits! If you want to get a bit more technical, you could have a dedicated circuit that has a test key before its own breaker! Emergency lights charge off the light circuit and automatically switch when there is a power out!

aviation red lights on any high towers, are controlled by such darkness sensing devices. for any flying objects, the presence of these towers are indicated by these red lights when it is dark. u may contact those tower erectors who r doing it for telecom towers. or else civil aviation deptt. concerned. hope u get an idea.

Common, cheap, night lights sold in hardware stores to grocery stores use that idea. Maybe they can be adapted to switch battery power instead of AC. Otherwise, purchasing a proper emergency light that is designed to a code is the quickest, easiest and ultimately the safest way to do it. Cheapest is a toss up. If you build yourself, you would still have to provide a battery, purchase components to build circuitry, construct some sort of pleasant looking (optional) enclosure. With LEDs and current battery designs, I'm sure there must be small, low profile, reliable and relatively cheap emergency lights available at big box hardware stores next to those smoke detectors (and CO detectors) that you should have installed years ago. And if you did, IT'S TIME TO CHANGE THEIR BATTERIES !

I installed four cheap night lights in strategic locations around the house in baseboard outlets. Now I don't stub my toes often any more. When the power goes out, which doesn't happen often, I'm on my own, but there's plenty of time between those occasions to recuperate.