I am planning to make an experimental communications system.

I will use a high power LED (≈3watts) Amplitude Modulated with a sound signal and eventually Pulse modulated with digital data for the transmitter.

And a focused lens with some sort of photo detector (PhotoTransistor, PhotoDiode, PhotoResistor, Plain old LED etc.) for the receiver.

To amplify the small AM signal from the photo detector, I am looking at microphone preamp circuits. How much gain will I need to amplify this to around MP3 player output?

(A high impedance output is fine for I will be feeding it into another amplifier stage, I just want the approximate voltage of an MP3 player output)

  • \$\begingroup\$ A transmitted PWM signal, should be received as AM by a slow photo detector. \$\endgroup\$ Dec 23 '13 at 4:14
  • \$\begingroup\$ Are you aware that the light output signal will likely be a very non-linear replica of the sound signal inputted. This means high distortion on reception. Also, the low frequencies of audio means you won't be able to get rid of some of the effects of ambient light changes - you might even get a strong 50 or 60 Hz signal too. \$\endgroup\$
    – Andy aka
    Dec 23 '13 at 10:43
  • \$\begingroup\$ I don't care about perfection, HIFI audio, etc. I got it to work across my room with a flashlight and a solar panel. I think I can go farther \$\endgroup\$
    – skyler
    Dec 23 '13 at 14:11
  • \$\begingroup\$ Put the detector end in a long tube, point it at the transmitter, and most sunlight issues should go away - mechanical design to shade the detector. Use a laser diode for the transmitter and it gets even easier (and takes less power) \$\endgroup\$
    – Ecnerwal
    Dec 24 '13 at 15:42
  • \$\begingroup\$ Line level is usually 1V, which should be the level audio equipment is expecting. Mic preamps usually offer about 40db of gain; I'd expect you to need V/V gain between 20 and 100. At 100 it'll be noisy. \$\endgroup\$
    – pjc50
    Dec 24 '13 at 18:07

Make your preamp with a variable gain. Determine the desired gain experimentally. As you would imagine, the desired gain would depend on: distance, sensitivity of the detector, focusing lens on the transmitting LED.

You optical communication should work fairly well indoors and outdoors at night. Making it work in daylight may be a challenge. The Sun can saturate the detector.

  • \$\begingroup\$ I will achieve a variable gain with a POT. What is the maximum gain I should need? \$\endgroup\$
    – skyler
    Dec 23 '13 at 2:24
  • \$\begingroup\$ @skyler I would suggest reading through these slides. Then look up schematics for photoconductive amplifier and photovoltaic amplifier. \$\endgroup\$ Dec 23 '13 at 2:43

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