Open air visable light communication optics - Page 1
Forum section: electronics / projects

author: skyler
author_rank: Regular Contributor
timestamp: December 18, 2013, 04:29:34 am
content: I want to communicate from a peak on a mountain 2.7 miles. I will be using a very bright LED and some type of photo element (photo multiplier tube, avalanche photo diode etc.) to receive the AM modulated signal.One problem I have is "what optics will I use to receive the signal" to "zoom in" to the light?I would like to build a system like this youTube linkSo in short... Where can I get the optics which the people thereused. I do not want to buy a telescope because of the expense and then my project won't be considered D.I.Y. Anymore and it will take out the fun, just putting a sensor on the end of the telescopeI am limited on money, I'm just a poor 15 year old experimenter. Keeping the lens part under 50 bucks would be great if possible.Someone suggested fresnel lenses. If so, where could I get some of those?

author: IanB
author_rank: Super Contributor
timestamp: December 18, 2013, 04:44:13 am
content: In the video they used optics at both ends of the link. The sender generated a focused beam much like a flashlight that was pointed towards the receiver. The receiver then focused the light down onto the light sensor.So for sending you might actually consider the reflector from a cheap flashlight. Maybe re-purpose an existing one or find one in a thift store to avoid spending money.For receiving you want to collect as much light as possible, and for the best signal to noise ratio you want to exclude as much surrounding light as possible. Which means you want a big collecting optic with good focus. Then you want to put the receiver right at the focal point.I can see a fresnel lens working, but I'm not really sure where to get one.Once upon a time I got great mileage out of scrap car headlight reflectors, especially the old round ones. If you take the glass beam shaping lens off the front of them, the reflector itself makes a really good straight beam. Anything that makes a beam will also work in reverse to collect and focus a beam.

author: calexanian
author_rank: Super Contributor
timestamp: December 18, 2013, 04:51:07 am
content: Receiver sensitivity will be your issue. I might consider using a digital camera and looking at a couple specific pixels in the frame. As far as modulating the signal, what frequency response do you want?

author: pmbrunelle
author_rank: Regular Contributor
timestamp: December 18, 2013, 02:13:25 pm
content: Maybe use the lens from the table of an overhead projector:http://en.wikipedia.org/wiki/File:OHP-sch.JPGYou may find a free one!

author: Rufus
author_rank: Super Contributor
timestamp: December 18, 2013, 02:24:04 pm
content: Quote from: skyler on December 18, 2013, 04:29:34 amhttp://www.ebay.com/itm/FRESNEL-LENS-FOR-DIY-TV-PROJECTION-SOLAR-OVEN-LOT-OF-5-/320419729128?pt=LH_DefaultDomain_0&hash=item4a9a810ee8

author: calexanian
author_rank: Super Contributor
timestamp: December 18, 2013, 02:57:59 pm
content: It has to be optical? You cant use microwave?

author: mark03
author_rank: Frequent Contributor
timestamp: December 18, 2013, 04:12:58 pm
content: Here's a good resource not yet mentioned:http://modulatedlight.com/optical_comms/optical_index.htmlYou need to worry about scintillation which could disturb your AM scheme. (This is what makes distant headlights flicker.) IIRC white LEDs are better than red, etc., because the spectral diversity will smooth that out to some extent. The challenge in collimating a power LED into a nice beam is that virtually all of them have 120-degree-wide patterns, so to capture all of the light you need ridiculously fast optics (faster than f/1). That's one factor in favor of fresnel lenses, besides the big aperture for cheap. The link given above has a page on evaluating fresnel lenses for this purpose.

author: Terabyte2007
author_rank: Frequent Contributor
timestamp: December 18, 2013, 04:18:08 pm
content: What about using Laser communications? You can use a relatively low power laser to get great distances and the parts for Mod/Demod are pretty cheap. I have done some experiments with transmission with lasers and it seems to work well.

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 18, 2013, 08:15:39 pm
content: http://ronja.twibright.com/about.phphas info on how to build one. It claims 1.4 km range, however that is calculated to work on almost any weather, except in the heaviest fog. You can increase range by increasing the size of fresnel lenses.http://ronja.twibright.com/drawings/dist.pdfrange projections. There are many factors when it comes to wireless communications. If you scavenge and do everything yourself you should be able to squeeze system under 50 bucks, however that's not going to be easy. Getting things cheap requires you to dive in (carbage bins).If I would actually have to have something similar up and running I would use wifi. Just get the biggest parabolic satellite tv dishes you can find (I've gotten 90cm satellite tv dish from next to carbage bin, I don't have use for it yet but it was free), use them as your wifi antennas, local legislation may prevent you from using it since it would shatter all legal EIRP limits.Quote from: skyler on December 18, 2013, 04:29:34 amUse lenses from cheap magnifying glasses. See the link above.

author: ConKbot
author_rank: Super Contributor
timestamp: December 18, 2013, 10:00:52 pm
content: Rather than just straight AM, I'd consider FSK on top of AM. Your LED would be modulated between two power levels (even on and fully off) at two different frequencies to represent binary 0 and 1. If you did pure AM, then setting up an AGC to keep the gain of the receiver correct could be more than a little hassle, along with other noise and pulsing light sources bleeding in etc. With FMK/AM you'd have a constant carrier amplitude, so your front end would just filter the incoming signal to your bands of interest, and apply the AGC to that to get your signal to send to your detector circuitry.As for lenses,http://www.surplusshed.com/A simple single lens collimator with the LED/photodiode placed at its focal point would get you reasonable collimation, since its not a single mode laser diode your source wont replicate a point source, and divergence will be quite a lot, but with good receive end technique it may not be as big of hurdle as it youd expect.

author: madires
author_rank: Super Contributor
timestamp: December 18, 2013, 10:12:40 pm
content: For such a distance you'll have to use a laser. A LED won't work. Sorry!BTW, the term for that kind of data transmission is FSO (Free Space Optics).

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 18, 2013, 10:22:23 pm
content: Quote from: madires on December 18, 2013, 10:12:40 pmReally?Quote from:http://modulatedlight.com/optical_comms/optical_index.htmlGood write about the subject(already linked above):http://modulatedlight.com/optical_comms/optical_about.html

author: komet
author_rank: Unknown
timestamp: December 18, 2013, 10:23:35 pm
content: I'm pretty sure you can indeed use an LED over that distance, especially in clear weather at night. There are commercial LED based systems claiming that order of magnitude.You could try pointing a telescope at it and putting a photodiode where your eye would go. Perhaps get an old telescope off ebay. It won't be very good, but I assume you're looking for a nice experiment and not trying to sell commercial data haulage.

author: komet
author_rank: Unknown
timestamp: December 18, 2013, 10:33:00 pm
content: Quote from: komet on December 18, 2013, 10:23:35 pmsorry, I overlooked your no telescope requirement. If you really want to DIY it, you could follow in Galilei's footsteps and grind your own lenses!http://www.newportglass.com/share.htm

author: Legit-Design
author_rank: Frequent Contributor
timestamp: December 18, 2013, 11:06:21 pm
content: Quote from: komet on December 18, 2013, 10:33:00 pmOr just get lenses from cheap magnifying glass.http://www.instructables.com/id/How-to-make-a-Telescope/not sure how this would work for this purpose. Ronja system would probably work better. One lens for receiving and one for sending.

author: peter.mitchell
author_rank: Super Contributor
timestamp: December 19, 2013, 08:33:07 am
content: have you considered running a small, flexible, transparent, extruded string of glass or plastic between the two locations? I have heard you can shine the light through it and it will come out at the other end quite spectacularly.

author: IanB
author_rank: Super Contributor
timestamp: December 19, 2013, 09:06:30 am
content: Quote from: peter.mitchell on December 19, 2013, 08:33:07 amFor a mile or two through the open air?

author: madires
author_rank: Super Contributor
timestamp: December 19, 2013, 12:06:02 pm
content: Quote from: Legit-Design on December 18, 2013, 10:22:23 pm"reduced energy density" for FSO?!? That's exactly what you don't want! A FSO link has to deal with wind, light, rain, snow, fog or whatever your local climate offers. A class 1 laser (low power, no eye damages, no paperwork) works well for Fast Ethernet or STM1 up to about 2 km in central Europe. I highly doubt that you can build a LED (current technology) based FSO link with the same performance and reliability.Update: I've searched for commercial LED based FSOs. The best I've found offers up to STM1 in the range of 1-3.5km in clear weather conditions. The laser based FSO links (class 1) I used so far fail at dense fog, strong snowfall or really heavy rain.

author: djococaud
author_rank: Regular Contributor
timestamp: December 20, 2013, 11:30:46 am
content: I would use a big bunch of power IR leds, and do FSK at the emitting side...I would olso use an IR filter to the reception side (just like a very big TV remote, wich could have a great range).You should do this to avoid sunlight or cars headlights interference.