At 04:22 AM 09/14/1999 -0400, you wrote:
>I need to transmit small bursts of repetetive digital data via RF (or
>otherwise wireless)  in an automotive type environment and receive same via
>a separate receiver approximately 6 feet away.  The method must be very
>reliable and made so as not to cause or receive interference from sources
>external  to the circuit.
>Can anybody provide some feedback as  to how one might (as simply as
>possible) design such a connection?
>Any and all help is greatly appreciated.


For short range (less than 10 or 12 feet), you can use poor mans fiber
optic cable. It sounds bad at first, but it DOES WORK, and the best part is
that you don't need special RF chips, antennas, it's interference free and
can be contained in the existing enclosures. It's also RUGGED as Hell,
easily concealed and cheap!

Transmitters are simple LED's and receivers are photodiodes. The fiber
optic cable is 20 to 40 pound test fish line, make sure you get the clear
kind and make the ends with a razor sharp blade in a singe cut.

I am currently using this for communication between my keyer and my ham
rig. This Fall, I hope to convert my wall mounted t-stat to a similar
system to avoid an rfi problem when we transmit on the ham radio at high
power.

If you need more range or a higher data rate, use a laser diode, find the
focal point of the collimating optics and put your fiber optic cable there.
I don't know the range of this setup, but would guess it to be 1km for
narrowband and 100 feet or more for medium bandwidth. I HAVE NOT tried the
laser transmitter myself, but the sub mini surface mount (RED) LED's work
like a champ.

Snag an LED out of a late model piece of consumer electronic scrap gear and
look for the sub mini smt type led's (the kind with the flat leads coming
out of the sides of the package 180 degrees apart). While it's still
mounted on the pcb, file down till you almost hit the die--making a flat
face on the front of the led package. Now, use some Comet or abrasive
containing toothpaste and polish the front surface on a piece of glass.
When the LED has a flat and smooth face, you can attach the fiber at point
blank range. I like to attach the cable to a small chunk of plastic with a
pin hole in it, the plastic needs to be .25 inches thick and snug when the
line is forced into the hole. The plastic is the only thing that keeps the
cable at right angles to the LED's active area. Glue the led so that it
shines through the pin hole. After the glue sets, you can slide the fiber
optic cable into the hole until it butts up against the flat surface of the
LED. After the assy is glued and tested, be sure to coat the entire assy
with something that doesn't let stray light in (any stray light that gets
into either end of the cable is interference and degrages the signal to
noise ratio).

Before hanging the messenger or having him committed, try sighting down a
piece of fish line with the the far end close to a light and you will
easily observe the light transmission capability of the poor mans fiber
optic cable.

OK, got my shields up-

Regards,

Art



PS: Note that if you only need 6 feet, and IF you can arrange for line of
sight between the RX and TX, you can use an IR LED and transmit through the
air.