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From: wsr@lmi-angel.UUCP
Newsgroups: sci.electronics
Subject: Re: Wireless data link
Message-ID: <152@lmi-angel.UUCP>
Date: Wed, 11-Mar-87 13:40:18 EST
Article-I.D.: lmi-ange.152
Posted: Wed Mar 11 13:40:18 1987
Date-Received: Sat, 14-Mar-87 15:03:53 EST
References: <214@pluto.UUCP> <250@pluto.UUCP> <175@tiger.Princeton.EDU>
Reply-To: wsr@lmi-angel.UUCP (Wolfgang Rupprecht)
Distribution: na
Organization: LISP Machine, Inc (Cambridge Engineering HQ)
Lines: 40

I briefly hacked around with sending analog signals optically about 15
years ago. High Power leds and laser-leds were prohibitively
expensive, and therefore out of the question for a hack project. I
decided to try a normal 1 watt flashlight bulb. It was dc biased to
its normal operation (6 volts in this case), and ac modulated roughly
+/- 50%. The whole circuit was a 741 opamp and a darlington
2n2222/2n3055 pair for the output. Feedback was around the whole
opamp-transistor system. 

The receiver was a normal photo-diode (this one had a guard-ring, so
it was a bit quieter, noise-wise), attached to a 1 meg pickup
resistor, and amplified by a u736 fet input op amp. The amp was run
gain 1 for dc, and essentially open loop for ac. I mounted this in a
soda can, with one end cut out, and a second soda can section with
both ends cut off holding a soda-can diameter lens with a 6" focus.
This system was focused by eye (you can watch the image forming around
the detector as you get close).

The ultra-hack receiver could easily pick up the hum from street
lights, and car headlights over a 1/2 mile range. What was interesting
was to listen to ripple from the car alternator in the car's light
output. You could tell what rpm the engine was doing. (It was
interesting to watch people shift...). 

I could clearly pick up the transmitter (which was attached to an FM
radio as a signal source) over a 1/4 mile range. Up-ing the lightbulb
power should in theory increase the useful distance (by roughly the
square root of the power increase. How about a car headlight? Hmm). 

The frequency response of the system was surprisingly good. As long as
the light bulb is in the white-hot range, black body radiation cools
it *quickly* giving an excellent frequency response. As the bulbs dc
bias is turned down, the bulb cools and it radiates a *lot* less. This
slows down the max dTemp/dt, and lowers the high end frequency
response. This was quite noticeable experimentally.

Its truly amazing at how much performance one can squeeze out of
normal flashlight bulbs. 
-- 
Wolfgang Rupprecht	{harvard|decvax!cca|mit-eddie}!lmi-angel!wsr