On Wed, 8 Apr 1998 00:33:18 -0600 Gus Calabrese writes: >I need some suggestions on a couple of sensors; > >1) I need to measure water level (and thus contents) in a concrete >tank. > A pressure sensor >connected to copper or other metal pipe should hold correct reading >for >a long time. The method using pressure of air trapped in a pipe to the bottom of the tank is used to control the filling of a clothes washer. It works rather well, though the air only has to stay trapped for a few minutes during the filling process. Even if the pipe were perfectly airtight, there would still be a long-term problem of air being lost by dissolving in the water, which would cause the water level in the pipe to rise and the indicated level to fall. It should be possible to use a small air pump to clear all water and debris out of the pipe and also offset any small leaks. Once air starts to bubble (slowly) out of the bottom of the pipe, the pressure inside is a correct representation of the water depth. The air pump would only need to run for a short time before taking a reading. The pipe should end an inch or so above the bottom to allow room for debris. A vertical float in the form of a long tube could be used. It wouldn't have to move far; attach it to some sort of force sensor at the top. The higher the water rises, the greater the upward force from it. Of course at some level the force would become downward due to the weight of the float. If it's really important to detect overflow or underflow, back up the continuous sensor with a float switch. The types used to control sump pumps appear to be widely available and duarable. >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. Radio Shack used to sell such a unit, maybe they still do. Infrared LEDs and photodiodes should work but likely would need lenses. The Radio Shack unit has two lenses on it. One unit functions as transmitter and receiver and only a simple plastic reflector of the type used on bicycles is needed at the other end. > 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. This should make it immune to external >lighting, >fog etc. I've had good results using the 567 tone decoder IC in a closed circuit like this. The oscillator signal modulates the source. The input to the chip comes from the detector (somewhere, there needs to be a 180 degree phase inversion from OSC R to IN). The phase detect output is shorted to Vcc to force the oscillator frequency to stay constant. Since the same oscillator is used for transmitting and receiving, it doesn't matter if the oscillator frequency drifts. The 567 includes the oscillator, synchronous detector, and level gate functions all in one chip. It uses more power than some other possible implementations but in this case quite a bit of power is needed for the IR LED anyway. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]