Timo, you can easily use standard IR LEDs and a lens for the distance that Clyde requires. We did this in the early 70's... Our products were required to operate in harsh environments with steam, direct sunlight, etc. Now days you could easily do this for around $10 in parts not counting packaging. The main problem I have with the use of reflectors in Clyde's application, is that IR is going to reflect off of other surfaces such as vehicles. He really needs a separate transmitter and receiver to ensure the beam is blocked. As far as the rain water tank level, this has turned into a `Rube Goldberg' contest... After Clyde posted his actual requirement, that of measuring when the upper tank is getting low, a simple float switch could be used. Heck, you could use a toilet float and a mecury switch... In anycase, no power would be required for the sensor, no maintenance, the electronics (even a simple relay) could be located at a good distance from the tanks, and it would last longer than the human occupants... - Tom At 04:33 PM 4/13/98 +0300, Timo Rossi wrote: >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 // > >