I want to do a bidirectional half duplex communication  over a distance=20
of about 10cm (4") which seems quite trivial at first sight. The=20
difficulty is the fact that I have just narrow tunnel with 2,5mm=20
diameter (1/10") for the 4" distance so the LED (TX) and phototransistor=20
(RX) for each end need to be very close together to be able to 'see'=20
each other thru this tunnel.
I found a IR-LED and Phototransistor in 0603 packages which could be=20
arranged close enough - just the beam pattern is about 120=B0 so the=20
efficiency is about the worst you can get.
Using an optical fiber unfortunately not possible - the tunnel needs to=20
be necessarily empty.
I was thinking of using a 45=B0 semi-reflective mirror on each end to hav=
e=20
a beam splitter so the LED could look thru straight and the=20
phototransistor sit at 90=B0 for example - but the mechanical assembly=20
probably is a mess and how do you get the 45=B0 mirror well aligned on th=
e=20
pcb? Does anybody know of a device with LED and phototransistor in a=20
single package looking thru the same lens? So far I couldn't dig up=20
anything truely useful on Google.
The wavelength is not bound to IR - visible light would be acceptable=20
also. For the receiver both phototransistor or photodiode is ok - but=20
just the plain part without any preamplifier or other fancy stuff.

Thanks,
Tobias Wieler







--=20
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