On Tue, 11 Nov 1997 10:29:42 -0800 Rick Dickinson writes: >At 01:33 PM 11/11/97 +0100, Alessandro Zummo wrote: >>Il 11-Nov-97, Rick Dickinson scrisse: >> >[snip proposal for SSU] >What sort of granularity would be needed, The speed of sound in air varies with pressure, temperature, and humidity. Unless these factors are compensated for, granularity in the measurement finer than the uncertainty of knowing the speed of sound is not very useful. In most applications, an absolute result is desired. > and what is a reasonable >maximum > range? The maximum range is whatever the transducers, transmitter, and receiver allow. For the Polaroid system it's about 30 feet. This was well matched for the original application of focusing a camera. The parts in the Polaroid circuit are rather high-performance (200V pulse to transducer, stepped gain amplifier). I don't know if the Murata transducers could do better. I want to avoid having to do any sort of AGC, so the range >would have > to be known in advance to set the amp gain on the receiving >transducer. This would really detract from the general-purpose utility of the device. It shouldn't be hard to do AGC by using tri-state PIC pins to switch resistors from various points in the receiver amplifier to ground. Another way would be to vary the transmitter power. First try low power assuming a short range, if no echo returns use high power and blank out the spurious early returns. >>> - Baud rate/clocking (async 9600/4800/2400/1200? Autobaud? Sync?) I think async with a choice of 1200 or 9600 baud. 1200 could work better if the device is at the end of long wires. 9600 would allow continuous rapid measurements, or multiplexing the outputs of several units onto a bus. I like just sending ASCII text, since it allows making a quick start with dumb-terminal software (or an actual dumb terminal). If the unit does the conversion from time to distance, have a way to input the local speed of sound. There could also be a mode which just outputs the time. > >I have been doing a little bit of design brainstorming, and have an >idea or two > about implementing the interface portion. Really the form of the interface is the least of your worries. If you release the source code for the sonar core, then custom interfaces could be developed as needed.