At 15.44 13/03/2004 +0000, you wrote:
>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.

Yes, I checked that of course.. it's 2.5A (at least in one of the two
devices I mentioned).


>[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.

Oki, thanks.


>>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.

Did you try with an internally opaque tube, or IR reflective?


>>   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.

Well, it's intriguing for me at least.

>Good luck.

Thanks.

Andrea.

>
>Dave D.
>
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