|Re: HP48 IR to RS232|
Message #3 Posted by Reinhard Hawel on 11 May 1999, 5:43 p.m.,
in response to message #2 by Steve
I can dig my old schematics out, if I find them. I did this some years before, but abandoned it, because the transmission width was to small (abt 3m). The 48 uses no modulation and if you see the signal on a scope you will note, that the bits are built of very narrow pulses compared to the time between the bits. (I remember 0=pulse, 1=no pulse, just because you must detect the startbit somehow.)
To kill the noise from surrounding light you must build very steep lowpass or bandpass filters, which means, that you'll get into trouble with the damped oscillations resulting from feeding a single pulse through such a filter. That was the main reason for abandoning the project. Now, with some cheap microprocessors like the small pic series (12C508, ...) this project would be considerable again.
I don't think you'll find any ICs for amplifying such slow signals, so you'll have to build your discrete circuit. (I think, you'd be lucky to find any single IR receiver IC without the electronics for a complete TV set. If you know one, it would be nice, if you'd drop me a mail.)
I recommend that you test your circuit against other sources than light bulbs, like sunlight, light tubes, ... which have very different noise characteristics.
As I said, my circuit was completely analogue and it could be possible doing this with the help of a small microprocessor, just for suppressing the damped oscillations. Your processor should know the timing, this would be a real help. Maybe it could build a simple digital filter to enhance signal quality.
Shocked ? If you simply want to send the signal over abt 50 centimeters to 1 meter, I think there's no problem with the noise, bus as you'll know signal strengh decreases non-linear, so that it could be rather complicated to build a circuit for, let's say 6 meters.
Just my 2 cents