Ray of Light - QSY to nanowave with OptoMod

There are lots of ways to communicate via lightwaves and this project gets started with a simple optical headend transceiver compatible with direct modulation or the more complex modes by using a PC to do the modulation and demodulation using DSP and SDR techniques.

My project is based on the eloquent technique of modifying a high power LED for both transmitting and receiving the light, as developed by Stuart G8CYW LED transceive, allowing the use of only one set of optics such as a Fresnel lens or telescope system to be used instead of requiring a duplicate array for the receiver side with corresponding bulk, expense and tracking alignment of dual instruments.
Stuart's information describes modification to the readily available Osram Dragon series LED, whereby the internally mounted reverse voltage protection diode must be removed from the circuit. The 1W and 3W class red LEDs are encapsulated in a gel, so this operation is possible using a surgical blade to cut the gold wire, best accomplished in circuit so that an effective cut is confirmed.
Modify the Golden Dragon
The 1W Golden Dragon LEDs in rx mode operate best with around 38v reverse bias, this is adjusted whilst aiming at a modulated light source such as a CFL or incandescent lamp. Sensitivity and bandwidth is a compromise. 3W Platinum Dragon LEDs need around 80v reverse bias. When used for reception, stray capacitance must be kept to a minimum therefore the high - Z preamp is best constructed by direct wires connection (standard VHF practices) to the LED. I have mounted the preamp, transmit modulator and -ve bias power supply inverter modules around the edges of the heatsink. Shielded tx audio, rx audio, +12v power and +12v tx power cables connect from the headend to microphone & monitor or PC and +12v power source respectively. Switchover to transmit mode is achieved by powering up the +12v tx line.
OptoMod Schematic
In my application, a general purpose telescope featuring Maksutov-Cassegrain optics was chosen and my least favourite 10mm eyepiece attached to the headend. The telescope is returned to optical or video use by swapping out the eyepiece in the usual manner. One problem was encountered: when attempting to align the telescope using the "red dot finder", the LED light was not visible - due to the red filters used in the supplied finderscope. A gunsight was attached in place of the original finder.
Headend on telescope
Now to the second feature used by Stuart G8CYW and the UK teams; complex modulation and IF subcarrier techniques.
In the UK it appears commonplace to use ssb transmission from a portable radio such as the FT817 by constructing a transverter mixing the RF signal down to the upper audio range of 10 or 20 kHz.
I thought it may be easier to use a standard PC laptop running one of the SDR soundard programs,

http://www.g4jnt.com/SDRTxSW.htm
and


http://sourceforge.net/apps/mediawiki/drm/index.php?title=Main_Page
with


http://www.rarewares.org/aac-decoders.php

Whilst these programs are limited in the frequency range available, it provides a compatible signal without the necessity to build a special transverter / transceiver combination


First Light

Headend on 127mm MCT