11-05-2020, 02:02 PM
Hi Amie. Thank you for the kind words. There's further background on the project here: http://www.radiocraft.co.uk/opto.htm The original reason for this project was because it looked like it could be a way to get 405-line television back on the air - legally - since unlike radio the light band (apparently) needed no licensing. Things have since moved on in this regard since and licensed analogue television in VHF Band I now looks definitely possible, but that's another story.
The beauty of this optical method was its simplicity. In essence, all you needed to do was place a photodiode in the modulated light beam and out would pop a ready made television channel (vision and sound) that you could connect straight to the aerial socket of a set! Well, actually a few more components were needed but not many.
Two methods were used to modulate the beam and development notes are attached. The first was to use an RF amplifier module and connect it straight to a visible red LED, in parallel with some DC bias. The second used more LEDs, infra-red this time, and a MOSFET transistor driver circuit. I started by using a special 'tuned cavity' LED to reach frequencies of 40 MHz and above. Later, I found that more standard LEDs would sometimes work, but at reduced modulation depth.
Steve
The beauty of this optical method was its simplicity. In essence, all you needed to do was place a photodiode in the modulated light beam and out would pop a ready made television channel (vision and sound) that you could connect straight to the aerial socket of a set! Well, actually a few more components were needed but not many.
Two methods were used to modulate the beam and development notes are attached. The first was to use an RF amplifier module and connect it straight to a visible red LED, in parallel with some DC bias. The second used more LEDs, infra-red this time, and a MOSFET transistor driver circuit. I started by using a special 'tuned cavity' LED to reach frequencies of 40 MHz and above. Later, I found that more standard LEDs would sometimes work, but at reduced modulation depth.
Steve
