Audible Light Sources

The following file deals with intercepting modulated light sources in an attempt to extract audio or other useful information.


(c)1996 William Beaty

Check out Bill's Amateur Science page.

Requires - Binoculars - Small solar cell - Audio amp - Headphones

Something I always wondered about regarding UFO sightings. Say its night, and you see a light in the distance behaving oddly, performing manouvers impossible for an aircraft, etc. Is the light amplitude of that object pure and smooth DC?

After all, nearly all manmade light sources are modulated as a result of their AC power supplies, so their brightness is vibrating with audio frequency.

Connect a solar cell to an audio amplifier, hold it under an incandescent bulb, and you'll hear MMMMMMMMMMMM at 120Hz. So, what sort of vibration might be imposed on those distant lights in the sky, hmm?

As a kid with an electronics hobby, I once taped a selenium solar cell to the eyepiece of a small 50X telescope, routed it to an audio amp, then pointed it at distant light sources at night while listening to the signal.

Incandescent streetlights give a deep hum, their AC light output is a pure 120hz sine wave. Mercury and sodium vapor bulbs are nonlinear, they give a complex 120hz waveform that sounds like WHAANNNNNNNNN. Neon signs sound different, with a squealy high frequency buzz component to their 120hz fundamental. Automobile headlights are DC, so I never tried viewing them.

Recently, I saw an article by (I think) Don Lancaster which mentioned that headlights are modulated by car vibrations, so I checked it out and yes, car headlights give off a continuous soft gonging sound even on smooth highways. Their filaments vibrate, and different cars give different pitches of "bell" sounds.

I put together a better viewer recently. Binoculars provide a "sighting scope" even when one eyepiece is occupied by a photocell. A Seimens BPW34 PIN photodiode an opamp front end gives a bit more gain than my selenium cell.

Headphones give much better low frequency response than a speaker. And the whole thing can be battery powered and duct-taped to a set of large-aperture nighttime binoculars. Any light source appearing in the field of view of one side will be heard as optically demodulated audio picked up by the other.

Click here to see the opamp schematic.

If you build a "UFO Scope," definitely make it a point to use it quite a bit before going hunting for "craft." You want to become familiar with the sounds of all conventional light sources, including lamps, headlights, aircraft, fires, and if you manage to crank the gain high enough, the twinkle patterns of various stars.

That way you'll be able to point the device at the local version of "Marfa Lights" and either say "yeah, sure, it's just headlights," or possibly "holy - ---, aliens modulate their ship lights for voice comm!" So far I've not encountered any mysterious lights.

I have found that my single opamp stage doesn't give enough gain to "hear" the dimmer light sources without burying them in noise, so it's time to modify the thing.

Vanguard Comments - this should apply to any light that can be focussed or imaged, including microscopes or telescopes. What if you hooked it up to a scope to see the modulations as a visible image? Or, use a frequency divider network to see if there are ultrasonic or higher freqs that could be down converted to audio? Lots of apps here, thanks Bill!