Hi

At that distance, unless your light is really bright, you would need to
collimate it properly. Buy an al-cheapo pair of pocket binocs, and split
them, one tx and the other rx.
If you put a led tx and rx next to each other, at both sides, you can get
duplex comms.

For simplex usage;
The way to collimate is to put the two binoc halves facing each other,
level, and aligned, say two metres apart. Set focus on both sides to infinity.
Look into the one, and put a powered led at the eye position on the other
one, and move/focus until brightest. fix in place.
Leave led on, and now put a reciever where your eye was, and position for
best signal recieved, then fix.
Once collimated, you can move them apart to test range using audio, whatever.
This is how theodalites are collimated, (and work)

They simply use powerful pulsed leds, and here the guy at the other side
holds a retroreflector. On good day, with about a big mirror array(~1m),
you can get about 5Km, that means the little led is being picked up over
10Km. Modern units get 3Km with a single prism(retroreflector ~70mm
diameter), and new laser devices get 7Km on a single prism. Obviously this
is all top quality optics, but I think you could get 100m range with a toy
binoc.

Roland


At 08:56 PM 04/05/04 +1200, you wrote:
>> I have to know when something crosses 30 metres long line. If the
>distance was less than 8m, then I would use Everlight's IR receiver,
>ELIRM8601 for example (which responds to 38khz modulated IR signal).
>But I have to cover 30 metres. Any ideas? Laserdiodes?
>>
>
>
>Some modest lensing would almost certainly allow this - easily tried.
>Failing that, visible laser pointers would do it with ease, but visibility
>may not be what you want. That said, if the transmitter is recessed in a
>wider blackened housing  and the receiver similarly, then it could be made
>that a laser dot would only be visible when you were being detected.  If you
>don't want the red dot to give the game away as they walk through then IR is
>the way.
>
>Glass has varying attenuation at IR depending on composition but it would be
>easy to try some cheap glass lenses and see what happens - possibly at
>transmitter end only. The aim is to produce a parallel (more or less) beam
>of diameter equal to the lense diameter. An ordinary red high efficiency LED
>makes an extremely bright night time beam at tens of metres - an IR remote
>LED and lense should do similar. .
>
>
>        RM
>
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Regards
Roland

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