Read all your questions.

We currently have stations working each other over a 22 mile line of sight
path with LED transmitters.

Forget building a photodiode/op amp receiver. You can buy top of the line
Burr-Brown opt-301 ($16, comes in a shielded can package) or BurrBrown
opt-101's ( $6. same electronics, but unshielded DIP package). If you are
interested in the opt101 unit, it's being sold for $2.50 each at this
moment. These Burr Brown units are very very hot receivers and include the
op amp and photodiode in the same package. They draw 120 microamps from a 5
volt supply, so there's alot of sensitivity available for low $$!

For less than optimum sensitivity requirements, TAOS has a whole line of
optical receivers (photodiode/amp assy) in the same package-their prices
are $3 to $2.50 each. Taos sends free 'samples'.

Are you a member of the of the laser communications email list??? These
guys do many different modes and have good general knowledge of lightwave
transmitters and receivers. They are mostly ham radio operators, but there
is no licensing requirement on the lightwave bands, so the list is open to
all who have an interest in lightwave communication.

For serious distance, use an 8 by 10 fresnel lens (commonly called a page
magnifier), available for 6 bucks at you local office supply or stationary
store.

My interest is in extremely weak non line of sight troposcatter
communications (also known as over the horizon communications by the
military). For this, photomultiplier tubes, big light collectors and
optical passband filters are required and the rates are 1 to 3 bits per
hour using .001 Hz bandwidth receivers (DSP soundcard/PC with good
software). Others in the group do high speed digital modes, still others do
carrier modulated studio quality audio transmissions.

If I can help further, send me a message off list.

Art

At 09:37 AM 3/13/04, you wrote:
>Hi all,
>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).
>I intend to encode data by time-modulating these pulses, e.g. at a 1mS
>rate transmit both clock and data (one clock and one data bit/pulse, in
>sequence, continuosly.. clock pulse always present, data pulse present
>if "1" and absent if "0").
>The receiver would filter out DC, through a high-pass filter to detect
>almost only those very brief pulses, and nothing else.
>
>Does my design makes sense?
>
>I'm now on the component identification phase. I examined the datasheets
>of all the IR LEDs and photodioes/phototransistors that my favourite
>online shop sells (small shop but excellent service and prices), and I
>selected these devices:
>
>tx:
>TSHA6203 (875nM)
>TSIP5201 (950nM)
>
>rx:
>BPW83    (875nM and 950nM)
>
>The other devices I examined in detail were:
>BPW21, BPW34, BPW40, BPW41, BPW42, BPW77N, TSTS7103
>
>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.
>2) What is in general preferable on the receiver side: photodiode or
>    phototransistor? Why?
>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).
>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?
>5) At such high pulse currents, should I fear damage to my eyes if I
>    accidentally look into the (invisible) beam?
>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).
>    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?
>
>Thanks!
>Andrea
>
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