[Laser] Another TX+RX system coming to life

[Art]

Hi Paulo,

Good luck with your experiments.

If you add a second LED, you will double your ERP (effective radiated 
power) as long as the series current remains unchanged.

Fresnel lenses are commonly used. They are inexpensive and available 
in office supply stores (page magnifier). While a large plastic or 
glass lens focuses the light better (to a smaller point), getting a 
lens much past 4 or 5 inches is diameter is expensive. Since small 
Fresnel's have 8 times (or more) light gathering ability, they are 
preferred by those looking to work medium and long range paths. 
Fresnels are available in 30 inch by 30 inch-or even larger, which is 
another reason to use a Fresnel.

The downside is that the Fresnel does not focus the light down to a 
very small point. So, some signal is lost unless a very large active 
area photodiode is used. The larger the photodiode, the more internal 
noise it produces.......so the use of a fresnel or a conventional 
lens is a balancing act.

Hope this helps.

GL.

Art


>Hello,
>more than a year after I've made my first post on this reflector I 
>have finally managed to assemble an MCW TX, RX is on the way.
>
>Nothing hi-tech, but hopefully a starting point for some fun.
>
>My TX currently uses a 10mm clear case red LED driven at 10mA 
>average, 488 Hz MCW carrier.
>
>The RX is a OPT202 with additional 3M3 series resistor to reduce the 
>device bandwidth, soon to be followed by a bandpass active filter.
>
>I have achieved a copy of the signal through ceiling reflection; 
>feeding the OPT202 output to the MIC input of a laptop running 
>Spectran (but it was audible too). No focusing lens was used. Total 
>distance: about 6m ("Personal Best", HI).
>
>A few of questions.
>
>If I add a second LED in series rather than dissipating heat in the 
>controlling resistor, I double the output power, don't I? Provided 
>the current flowing is unchanged.
>
>Using several LEDs rather than one will not affect the radiation 
>pattern, as occurs with Yagi antennas at RF, right? (I'm pretty sure 
>about this one.)
>
>Why do many people seem to use Fresnel lenses? Can't I use a el 
>cheapo magnifying lens from the china shop around the corner? It's 
>just a weight matter?
>
>What are your DX experiences /without/ a lens in front of the 
>detector? Probably with a laser rather than a LED...
>
>I will soon need a LOS wireless voice intercom that should work over 
>a distance of about 300m. Would this work with a LED as TX in a 
>urban environment? Fewer accurate beaming requirements and no risk 
>to beam a laser into someone's eyes. Any circuit to recommend?
>
>Cloud bounce. I read about K3PGP's experiments and others: were they 
>carried in rural areas or are there some chances in big cities as well?
>
>
>Thank you for your attention and your time,
>Paolo IK1ZYW
>
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