Andrea wrote...

>I'd like to build myself a Long-Range, Low-Power IR Transceiver.
>I know that the "long range" and "low power" may seem contraddictory,
>but I thought, if I produce a very intense but very brief IR "flash",
>the flash may be visible at a long distance, yet the average power
>will be low. Rather than emitting 30mW for 5mS, I could emit 3000mW
>for 50uS, with the same average power, but the intensity would be 100
>times higher (and the range, IIRC, 10 times longer).

There's a limit to how far you can go with this; look on the IR
emitter's datasheet and you'll probably see an absolute maximum
spec on the instantaneous current and it's likely to be a LOT
lower than you're hoping for.  Exceed it, and you fry the LED
even with very short pulses.

[some stuff snipped]

>But I'd have some questions for you:
>
>1) All the rest being equal, what wavelength is preferable, 875nM or
>   950nM? I know that 875 would be better if there're glasses inbetween,
>   but my application will be mostly free air.

I don't think it'll matter much.

>2) What is in general preferable on the receiver side: photodiode or
>   phototransistor? Why?

A photodiode, along with the very best low-noise amplifier you
can make or find.  A phototransistor is going to be far too
noisy, and at the low currents it's going to be operating at, it
won't have very much gain anyway.

>3) Do you have any better LED and photodiode/phototransistor to advice
>   than the ones I selected? I'd have no problems to buy those from
>   another shop.. if it's worth the extra shipping charge, etc.. of
>   course (i.e. I'd have to make two orders instead of one).

Sorry, can't help there.

>4) How does the world look like, at those wavelengths? Is it generally
>   "dark", or at 875-950nM reflections and absorbtions from objects is
>   more or less like visible light?

At near-infrared, things look pretty "bright".  Green foliage,
especially, has high infrared reflectivity.

>5) At such high pulse currents, should I fear damage to my eyes if I
>   accidentally look into the (invisible) beam?

Not unless it's a laser diode.

>6) To increase further the range, I'm thinking about making a 2m long
>   tube, internally reflective (aluminium foil), diameter slightly more
>   than the IR LED. This should decrease the angle of emission, but also
>   make it more powerful and concentrated, right? (i.e. a bit like a
>   poors' wannabe laser).

This doesn't work;  I've tried it.  Rather than narrow the angle
of emission it increases the angle.  You need a lens system of
some sort.

>   For the receiver I'm thinking about using a small telescope, pointing
>   it to the transmitter (hopefully 1+ Km away.. ehm). Will the lens be
>   transparent for the incoming 875nM or 950nM light?

There will be some attenuation by the glass lenses at those
wavelengths, but my guess is it won't be enough to negate the
amplification the lenses provide.

In all, you've got a REAL challenge here, if you're talking about
transmitting to a thousand meters and beyond.  Good luck.

Dave D.

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