LEDs also behave like photodiodes, albeit weak ones,
so you would have to add an amp/comparator to get
the recovered signal back to TTL levels.

I measure about 300mV from a clear red LED under a 12W
tungsten desk lamp.

Using a LED as detector would allow you to use
ONE device for both functions so have no issues with
sharing the tunnel.

Robert

Tobias Wieler wrote:

> 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 phototransisto=
r=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 h=
ave=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 =
the=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.
>=20
> Thanks,
> Tobias Wieler
>=20
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