220K (+) ___/\/\/\___ | | | | | |\_ | |_|/|__|___|- \_ | |\| | \___|__ P.D. __| _/ (out) | |+_/ | |/ | ___|__/\/\/\_ | | \ | / V \ gnd / | (+)
Something I always wondered regarding UFO sightings. Say its night, and
you see a light in the distant sky. Is it really an airplane? Or suppose
it's behaving oddly, performing maneuvers impossible for an aircraft, etc.
Could the optical emissions coming from that object be identical to an
ordinary 120V AC lightbulb? Specifically, is the LIGHT AMPLITUDE of that
object pure and smooth DC?
Or is it some kind of AC white noise or
signal? After all, nearly all manmade light sources are modulated as a
result of their AC power supplies, so their brightness is vibrating with
120Hz audio frequency. Connect a solar cell to an audio amplifier, hold
it under an incandescent bulb, and you'll hear MMMMMMMMMM at 120Hz (two
flashes per 60Hz cycle.) Therefore we should ask: what sort of vibration
might be imposed on those distant and mysterious 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 amplifier,
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 hadn't tried
viewing them. Recently I saw an article by Don Lancaster (or Forrest
Mims?) 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 types of headlights give different pitches of "bell" sounds.
(So when you see one of those single flickering headlights in the
distance, that's an offroad motorbike, and its light is audio-modulated
with extremely loud
Aircraft strobes are easy to detect as a loud clicking. Other aircraft
lights *may* have a standard 800Hz modulation (from their 400Hz supplies),
but I found that it wasn't loud enough to hear from distant aircraft
lights. Perhaps the thermal inertia of their filaments tends to filter
out all the high frequencies, whereas 60Hz is slow enough to be
"broadcast" by light bulb filaments. Maybe with a low-noise detector and
some bandpass filtering, the 800Hz of aircraft lights could be sensed.
Rather than using a telescope and a solar cell, I put together a
better viewer recently. Binoculars can provide a parallel "sighting
scope," even when one eyepiece is occupied by a photocell. A Seimens
BPW34 P.I.N. photodiode and a low-noise 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 onto a set of large-aperature nighttime binoculars. Any light
source seen by your eye through one side of the binocs will be heard as
optically demodulated audio picked up by the other side.
The following schematic depects a Current-to-Voltage converter circuit
photodiode: if a 10 microamp signal comes from the photodiode, then a
2.2v signal will appear at the opamp output pin. The input is DC coupled
for good lo-freq response. However, bright lights will overload the
circuit, so night-use only.
Use 9V battery as power supply +9V to opamp (+) pwr (pin 7) -9V to opamp (-) pwr (pin 4) probably needs a capacitor in series with the output (try 10uF) 220K (+)9V ___/\/\/\___ | | | | BPW34 | |\_ | |_|/|______|___|- \_ | |\| 2| \___|__ __| _/ 6 (out) | 3|+_/ | |/ | 47K ____|__/\/\/\_ | | \ | / 47K V \ gnd / | (+) 9V "I to V" LO-NOISE PHOTOCELL PREAMPLIFIER
Here's an AC-coupled version: 2.2 Meg (+)9V ___/\/\/\___ | | | | BPW34 .1uF | |\_ | |_|/|__.__| |___|___|- \_ | |\| | | | | \___|__ / __| _/ (out) \ | |+_/ 10K / | |/ \ | 47K | __|___/\/\/\_ | | | | \ | V / 47K V gnd \ gnd / | (+) 9V
I powered mine with a single 9V battery. Batt (-) goes to gnd and to
pin-4, while batt (+) goes to pin-6 and to the (+) points on the
schematic. The output signal will ride on
4.5VDC, so you might need to put a capacitor in series with the output to
block the DC from your audio amplifier (my audio amp already had an
internal capacitor in series with its input.) Either that, or use two 9V
batteries wired as a standard Op-amp bipolar supply, then ground the (+)
input-pin of the op amp directly to the batteries' common connection.
If you build this "UFO Scope," definitely make a point to use it quite
a bit before going hunting for "craft." You'll 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 $#!%!, the aliens modulate their
ship-lights for voice communications!"
Spa fon!         ; )
To greatly increase the sensitivity (by maybe 100x), replace the BPW34
with a phototransistor. I haven't tried this myself, so I can't recommend
any particular phototransistor to try. Find one with a large active area
if possible. Electronics Goldmine has a couple different kinds.
Another idea: build two sensors, install them in both eyepieces, and send
the signals to stereo headphones. Then put an IR filter over one lens,
and either no filter or an IR-cut filter over the other lens (or perhaps
magenta on one, and green on the other). With the view through both
eyepieces totally blocked, this would be harder to aim, but it would let
you HEAR THE COLORS of the lights as stereo audio inside your head.
Multiple colors should sound like various separate "instruments" located
in the widely spread "orchestra." If the light source was changing
colors, this might sound very interesting. At the very least, it would
give you more clues for recognizing mundane light sources. For example,
incandescent lamps would be loud in the IR earpiece only, while merc vapor
lights would not.
If one of these audio photosensor circuits was attached to the eyepiece of
a large telescope, might any interesting sounds be received? For
the flame of a candle *sounds like* the low rush of a burning candle. If
the nucleus of a comet contains wailing gas jets, occasional explosions,
vibrating plasma, etc., perhaps some of the comet's reflected light will
become modulated, and the original sounds in the comet's atmosphere could
be extracted by the telescope and photodetector. If the gain of my
circuits could be raised by orders of magnitude (or use a PMT rather than
photodiode,) it might become possible
to monitor the moon at new (dark) phase, and pick up tiny brief sounds of
lunar meteor strikes. Their brief flashes would sound like clicks.
Star-twinkle, if it contains moving interference fringes, might do more
than make rumbles and thumps, it might "ping" or "squeak." Lunar
occultation of stars also might create brief audio tones because of
interference patterns. And, if significant numbers of amateur astronomers
start listening to the sky as well as watching it, perhaps unexplainable
noises will lead to new discoveries for conventional (non-fringe!)
So far I've not encountered any mysterious lights. I've found that my
single opamp stage doesn't give enough gain to "hear" the dimmest of the
visible light sources without burying them in noise, so it's time to
modify the thing. Add some extra gain stages, bigger initial gain
resistor, a few tens of picofarads across the gain resistor to prevent
oscillation, maybe a phototransistor replacing the photodiode to give
higher front-end gain, and perhaps sacrifice low-freq response by making
it AC coupled, so any bright lights won't drive the opamp's output to the
rails. Or give up and start over by building a photomultiplier version
with high-volt power supply.