Simple Sound-Over-Laser Rig (and EMG/Faraday update)

Very simple project that provides a difficult (but not impossible) to intercept communication channel over short to medium distances. An audio output transformer lets a low-power headphone jack modulate the DC power supply to a laser pointer, while a photoresistor modulates a DC power source on the receiving end.

As a former laser geek, I’d point out that you’d want to find a diode module with minimum divergence in order to maximize the difficulty of interception. (And avoid foggy nights.)

EMG/Faraday cage update: The other night saw a kind of interference I haven’t seen in a long time, which I thought the Faraday cage was shielding. I’ve attached a screencap of the waveform and spectrogram. (It turns out there’s a reason professional/research electromyography makes heavy use of Faraday cages and shielded rooms.)

The signal occured very near to a point when I woke up about an hour earlier than anticipated, and was followed by a corresponding increase in EMG activity. This suggests, but does not prove, that the signal caused enough of a biological disruption for me to wake up. (For the record, both channels were recording from electrodes near my left ankle, and the common/reference electrode was on the backside of my left shoulder. The signal occurred in both channels with no visible difference between them.)

Upon inspection I discovered that there may have been a solid electrical connection between my body and the cage during the night. I fixed this, and the signal has not reappeared. (The fix may have nothing to do with the signal’s non-appearance, if it’s intermittent interference.)

At the same time as I implemented the Faraday cage fix, I also added the electromagnetic pink-noise noise generator from a few emails ago. Instead of an extension cord, I used about 25 meters of speaker cable spliced together, looped about three and a half times around where I was sleeping. The far end of the cable was twisted together and it was driven by a small computer-speaker amplifier and pink-noise source as usual.

Since the signal didn’t return, it’s not possible to say if the pink-noise generator would have prevented the interference from waking me up, if that’s what caused me to wake up. (The idea being to raise the background noise level to the point the nervous system doesn’t react to the interference.)

Anyway, the security link…


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