tower tech, there are many sensors available to detect light. The variety will depend on the accuracy, environment, etc.. I would do a search on light sensors using one of the many search engines. If your looking for something COTS(commercially off the shelf) then you might try a radio shack or something. What will be switched? This will determine what you can use for a switch (relay, transistor, solid state relay, etc..). I have been working on a project similar to yours only we are sensing flame instead. The environment demands a higher end sensor that costs ~$50 off the shelf. This is over kill for your application. I bet you could get a photodiode, transistor, or similar device for well under $5. Well I read the date on this thread and it is likely dead but I already typed this in so hopefully someone will find some use of it. Thanks Buzzp