[SOLVED] The photophone challenge

[freemind]

Hi everybody, (reply: "Hi, Dr. Nick" even though my name is not Nick)

I saw this design contest near where I live where participants had to design and build an apparatus that would transfer an audible vocal message over a distance of @ least 20m, by using visbile light. Of course, I'm talking about a photophone. Now, my group (me and 3 others) attached a magnet to a diaphragm that would vibrate in accordance with the sound, and since the moving magnet was in a solenoid, it would generate a current. This current would power an array of LEDs. As for the recieving end, we used the traditional selenium solution. Why so convoluted? For one thing, we weren't allowed to use transistors. Only the basic electrical components.
I would really like to hear how other people would have approached this. After all, this is not very original, and originality was stressed for the contest (which would explain why we didn't win the grand prize). How would you have done it?

[gschjetne]

What about using a photocell for receiving? Would this violate the rules?

[hitssquad]

For the transmission end, I would have used a diaphram vibrating a light-weight mirror reflecting a bright light shone by a car headlamp.

[freemind]

Actually, we did consider using a vibrating mirror, but quickly shot down the idea - too quickly i must say - since this was essentially how Bell did his photophone. We thought it wouldn't take us far in terms of originality, and sure enough, there were at least three groups that used the vibrating mirror, only one of them making it to the top 5.
We also considered a photcell initially, but soon realised that its generated current was too feeble to power the speaker without massive amplification. So we ended up using an array of photo-resistors. That way, our speaker was reliant upon the significant variations in current.

[hitssquad]

Well, if originality is stressed I guess the problem is therefore very tricky (I did not know Bell had made a photophone - or maybe I forgot). How about a pair of cellophane or Saran Wrap squares suspended vertically ~1mm apart from each other and the gap in-between filled with smoke? Speaking into the cellophane vibrates it and therefore changes the distance-between layers and further-therefore the smoke thickness which finally affects the total opaqueness in real-time according to vocal-wave modulation. Shine a light through it while speaking and you have your voice encoded in light.

[freemind]

Wow, that's an amazing idea! Although, I'm not sure if the fluctuations in pressure/density of the smoke will be significant enough to create marked variations in the amplitude/frequency of the light (perhaps I'm worrying too much ). Nevertheless, it is definitely quite original! :biggrin: