The brilliancy of this ideea will be unfortunately superseded by the
technical problems of receiving IR using a LED. 12W is a little to
much power isn't it ? Don't forget it must be modulated on 38KHz or so
and the response time of a LED used as a photodiode is very small.
That's why where designed some much more efficient devices, like IRDA
pairs, with or without internal modulators and amplifiers. One of
those:

TFDS4500 from Infineon
=20
Also there are many chinese pairs only RX and TX running in IR, smd or
through hole pairs.

best regards,
Vasile



On 5/30/05, Robert Rolf <Robert.Rolf@ualberta.ca> wrote:
> 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.
>=20
> I measure about 300mV from a clear red LED under a 12W
> tungsten desk lamp.
>=20
> Using a LED as detector would allow you to use
> ONE device for both functions so have no issues with
> sharing the tunnel.
>=20
> Robert
>=20
> Tobias Wieler wrote:
>=20
> > I want to do a bidirectional half duplex communication  over a distan=
ce=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 phototransis=
tor=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=
 have=20
> > a beam splitter so the LED could look thru straight and the=20
> > phototransistor sit at 90=B0 for example - but the mechanical assembl=
y=20
> > probably is a mess and how do you get the 45=B0 mirror well aligned o=
n 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
> >=20
> >=20
> >=20
> >=20
> >=20
> >=20
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