On Mon, 20 Sep 1999 11:11:35 +1000 Des Bromilow writes: > anyone know how to build such a beastie? Anyone ever built one? You can buy them in many forms. But it is within the realm of things that can be built at home if you really want to. > I envisage something along the lines of a pulse with a delay which > is then translated to length, but the detection of the echo, Generally how they work is to generate a pulse, then sample the voltage some time later. I think it was once common to use a "snap diode" (tunnel diode?) to make the pulse, though now a fast logic gate could do it. The sample delay is controlled by an analog one-shot. This allows nanosecond delay variations to be easily implemented. A simple device would do the measurement repeatedly just taking one sample each pulse. This avoids the need for rapid conversion and storage of the results. A slow A/D converter PIC is plenty of processor for it. You'd need an analog "front end" consisting of pulse generator, delay unit, and a fast sample and hold gate. ___________________________________________________________________ Get the Internet just the way you want it. Free software, free e-mail, and free Internet access for a month! Try Juno Web: http://dl.www.juno.com/dynoget/tagj.