On Wed, Apr 08, 1998 at 12:47:47PM +1000, Clyde Smith-Stubbs wrote: > 2) I need a "light beam" sensor to detect vehicles or people moving down > a driveway - much like a door minder used in shops, but to operate outdoors > spanning a distance of about 20m. It should preferably use infra-red so > as not to be visible (this is not critical, but if it uses visible light > it should not be too bright). The electronics side of this is reasonably > straightforward - I'd modulate the beam at some frequency, recover > the modulation and differentiate it to detect sudden interruptions. There are ready-made commercial sensors (which use modulated IR beams) for applications like this. But the distance you require is quite long, I'm not sure if the typical sensors can handle it. You definitely need the type with separate IR transmitters and receivers, the ones using a mirror on the other side have much shorter distance limits. (I have successfully used Omron sensors for detecting vehicles, the distance was approx. 10 meters, which was the speficied maximum distance for the sensors I used) -- // Timo Rossi //