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Optical amateur "radio"

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LAN_403

Conundrum

Sun Sept 23 2012, 01:25PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Anyone tried this yet?

Also, cheap solar wafer cells can be used in series to make an RX module by adding AC bypassing capacitors on each cell so the signals add rather than cancelling out.

A relevant idea is to use a surplus VCSEL from a broken laser mouse as a "Poor man's APD".
This involves running it in negative bias with about 10V, limited to 1uA maximum through a high value resistor.
I have yet to attempt this but the monolithic block sort can be harvested easily and the unused section hacksawed off.

ShawnLG

Mon Sept 24 2012, 05:56AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

A few years ago, I had did an experiment using tone modulated code over five 5mm IR leds setup downstairs and a few IR pin diodes as the reciever setup upstairs with preamp. Spectran was used to pull the signal out of the noize. There was no line of sight. The signal had to reflect off of several walls to reach it's destination. I wonder if it is possible to make a long distance contact by relying on cloud scatter...

Conundrum

Mon Sept 24 2012, 06:08AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Should be, I will have to try it

-A

Hon1nbo

Mon Sept 24 2012, 09:04PM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: North Texas
Posts: 1040

When I was in middle school I used a simple circuit to modulate a laser beam's intensity to an audio source, and transmit the audio to my stereo on the shelf.

Maybe I should look into it again. Might give me something to do with my currently idling hands.

-Jimmy

Conundrum

Fri Sept 28 2012, 10:04AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Extra credit for using a Pi for the signal processing

-A

Shrad

Fri Sept 28 2012, 10:52AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

if you want high bandwidth, you have to modulate the upper 40% amplitude of the optical power, with optional photodiode feedback

easiest way would be to use a simple constant current source and AC couple the output of an AGC signal amplifier to the current delivered to the laser diode

Sulaiman

Sat Sept 29 2012, 06:32AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If you have a few old FM CB rigs (UK limit) you can use one for transmit
(e.g. 100 Ohm in series with led with anti-parallel 1N4148
and a second 100 Ohm across the output,
if I had a higher power led then just one series 47 Ohm resistor would do.
. still 'playing')
e.g. SFH4501

and one for receive
(photo diode across 50 Ohm resistor (47R, 100R etc.)) as the Rx 'antenna'
e.g. BP104

If you have a colaborator use 4x CB rig for full-duplex using a different channel for each direction

Old CB rigs are so cheap that building even a simple Rx/Tx system is more expensive!

Since at work we frequently throw away damaged magnifiers like this

I use the lenses from them,
not perfect but good enough.

The German military used filament lamps audio modulated by a mirror
and Selenium detectors for front-line short range communications in WWII.

Steve Conner

Sat Sept 29 2012, 01:27PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Good idea, but you'd probably want to add some DC bias to the LED/laser so it always operates above threshold. This can be done from a separate supply using a "bias tee".

Proud Mary

Sat Sept 29 2012, 02:31PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

1348928535 543 FT144843 Mercury Lamp Valve Modulator

And this one for the faint-hearted.

1348928597 543 FT144843 Mercury Lamp Mod Solid State

The transistor acts as a shunt, and wastes power, rather than letting the negative resistance of the arc get out of hand.

In either circuit, if 100% modulation is exceeded, the arc will go out, and not re-ignite without going through the whole procedure of allowing to cool down, re-igniting it and allowing it to reach its plateau performance which takes about quarter of an hour.

The polarity of the arc should be changed frequently to reduce electrode erosion. Maximum modulation frequency is about 10 kHz.

Sulaiman

Sat Sept 29 2012, 02:34PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Steve,
I did it that way for simplicity and it was only a quick test,
the Tx is more like 'pulses' than a true sinewave but it seems to work as Tx/Rx is FM,

since this thread I'm going to revive my experimenting a bit
as I now have a potential 'collaborator' within range of my 2m 5W handheld so we can co-ordinate 11m optical comms over 2m ham band .. just for a bit of 'fun'
I may even 'invest' in a couple of higher power leds/laser diodes.
Possibly dc reverse bias on the photodiode also to see if the reduced capacitance helps etc.

A mercury arc is a little too much for me !

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