In Reply to: Re: Car counter posted by Vorel on 05/30/00 at 1:06 PM:
Thanks for your info. Apparently, using a coil as opposed to an antenna bypasses the FCC concerns, as AMB (the world's largest manufacturer of timing devices - everything from the Olympics to NASCAR) does not have any FCC approvals. If they had, I would have requested a complete schematic as required by law. :-)
I think that having narrow bandwith transponders (+/- 2.5Khz) with a bandwidth seperation of 20Khz (ten channels from 200Khz to 300Khz) would work perfect.
I thought of using a DSP and some FFT math to do a spectrum analysis, but that would require a lot of work compared to having 10 tunable receiver circuits that simply go high/low depending on the presence of the tuned frequency.
I have searched for simple op-amp circuits, and apparently the bandwidth gain is a problem when the frequency gets too high, meaning that you only get an amplification factor of 2 or 3, which I don't think is enough to apply such a weak signal.
Transmission range of the AMB units is approximately 24 inches. I did get a couple of the transponders (that were dead), and they use 7 transistors in an LC circuit. My guess is some sort of flip-flop arrangement to generate the square-wave, which is pretty decent considering it is using only 2 volts. The magnetic coils that they use for transmission are really different. They look like speaker voil coils, but have a ferrite center. The wire gauge is extremely fine (human hair size or smaller). The Nicad (single 2.4v cell) lasts approximately 30 hours on a full charge.
I know very little about oscillators and even less about op-amps. Once I get it to a digital level, I can do anything. :-) So, if any has any schematics of a nice 200Khz temperature and voltage insensative square-wave oscillator, please let me know! I have done a few weeks worth of experimenting with colpitts and other oscillator designs, and either they will not work in the 1.8v-2.4v range or are extremely temperature and/or voltage dependant.
Thanks for any help!