The human eye cannot discern flicker with a frequency greater than roughly 30 Hz. A light turning on and off faster than this does not appear to be turning on and off. Instead it appears to have a constant intensity to the eye.

Human ears however are much faster, that is, better able to perceive a signal changing quickly in time. The human ear can detect frequencies from roughly 20 Hz to 20 000 Hz, provided those frequencies arrive as sound.

Now suppose there were an easy way to change light of time-varying intensity, into sound of time varying intensity. Then you could "hear" light that was flickering too fast for you to see it.

The humble silicon solar-cell can convert a time-varying light signal into a time-varying electrical signal. This small electrical signal can then be fed to an audio signal amplifier and converted into sound.

Step 1: Parts:

1 audio amplifier (Radio Shack Catalog #:277-1008)
1 1/8-inch (3mm) headphone plug, can be mono or stereo
1 small Silicon solar cell with approximately 1 cm2 in area
1 1/2-inch PVC pipe cap
some wire, solder, epoxy, black paint, etc.

Step 2: Assemble and Wire Up the Light Sensor.

The wiring diagram, first pic in the stack below, shows how to wire the solar cell to a mono audio plug, or how to do the same trick with a stereo plug. Basically for the case of a stereo plug, it is wired up so that electrically it's a mono plug.

In words the tip of a mono audio plug is named "tip", and the ring shaped conductor behind the tip is named "ring". I have connected the positive side of the solar panel to tip, and the negative side to ring. In the little picture in the wiring diagram, ring is labeled with a "1".

The second pic in the stack below shows the actual wiring itself.

You might be wondering what kind of dark magic was necessary to turn that messy box of solar panel shards, shown in step one, into the pretty little perfect circle which will fit neatly inside a 1/2-inch PVC pipe cap. The answer is shown in step 3, sort of, and step 3 is optional.

You see, working with little solar panel shards is kind of a pain in the ass, and I don't recommend you do this unless
(a) You're a masochist - that is, you like pain and frustration
(b) You don't have a pre-made solar cell that will neatly fit inside the light shield

By the way, the "light shield" is another name for the PVC pipe cap. It's intended to block out stray light. In step 4 I paint it black, for extra super-duper light blocking ability.

Step 3: (OPTIONAL STEP) Cutesy Recycled Circuit Board Circles!

Uh... lets see... This step involved: De-soldering parts from a junk circuit board. Cutting off a piece of depopulated board with a radial arm saw. Using a hole saw to cut out some little circles. Then attaching the circles to a bolt, and loading this bolt into the drill press, for purposes of slimming down the circles, decreasing their radii by placing a file against their edge, while the drill press is spinning them. Then sanding the paint off the little circles by rubbing them against a sheet of sandpaper held to a clipboard.

Then I carefully broke off a piece of solar panel shard. Then carefully soldered it to one of the little circle boards I made, and then I soldered some wires to the appropriate places on the panel.

Verily, this is the story of the genesis of the little circular solar-cell board seen in Step 2.

Step 4: Gluing and Painting

The purpose of the little PVC pipe cap enclosure is twofold.

One is that it offers some mechanical protection for the solar cell, which is kinda delicate. So the board and wiring are glued in place with epoxy.

The other purpose is to block out light coming from most directions. So I do like the old Stones song suggests, and paint it black.

Step 5: Test the Light Sensor (in DC Mode)

This is just a quick test to see if the light sensor is working, not wired up backwards, etc.

I switch my multimeter in small-dc-current mode, then connect the probes to the audio plug and see what happens.

It looks like the light from the fixtures above is causing the solar cell to produce about 20 microamps of current.

This is good.

Step 6: Plug It In. Turn It On.

The plug from the light sensor goes to jack on the audio-amplifier labeled "INPUT".

Then turn the turn the little gain/volume knob on the side to turn it on.

Also important: the amplifier should have a 9V battery installed in it, or a power source of some kind.
;-)

Step 7: Various Light Sources

Many artificial forms of light vary in intensity with frequencies in the audio region. Some examples include incandescent light bulbs, LED displays on microwave ovens and other kitchen appliances, computer monitors, TV-remotes, etc.

Provided you've put everything together correctly, you will hear some kind of sound coming out of the amplifier when the solar cell is receiving light from a time-varying source.

In case all you have are non-varying "DC" sources of light, like sunlight, try waving your hand back-and-forth in front of the solar cell, quickly blocking and unblocking the light reaching it. This should produce a soft clicking noise at each dark-to-light, light-to-dark, transition.

Step 8: LED "tea Light" Candles Play Music for Some Reason

Seriously.

Just place the light sensor near the LED candle and in many cases you'll hear cheap electronic music coming out the amplifier. The one in the picture below plays "Fur Elise".

It is also possible to tap into these little LED candles and listen to the music via wire, as shown in this instructable:
https://www.instructables.com/id/Listen-to-a-led-tea-light/

I suspect that the reason for this is that those mass-produced music chips also work well as a source of time varying "flicker". That is, this signal looks like flickering light when driving an LED. The same signal sounds like cheap electronic music when driving a speaker. The same chips are used for both, or at least that's what I suspect. If anyone can confirm this hypothesis via a source in the Chinese knick-knack industry, please comment.

Step 9: Some Captured Sound Samples

For those of you who asked for them, I captured some sound samples from the light-to-sound project. These are uncompressed .wav files, sampled at 44 KHz and 16 bits per sample. Each is roughly 10s in duration.

I realize downsampling to a lower sampling rate, and/or encoding to a compressed format (like mp3) would certainly save y'all some space, and download time. But this might also introduce artifacts into the sound that weren't there in the first place, and I'm trying to be scientific about this, sorta.

Update 20 Nov, 2009: This additional step was added on 20 Nov, 2009, about a 1+1/2 years after the bulk of this instructable was published. Sorry it took so long. I don't have a good excuse for this.

Update: 27 Feb, 2021: The LED candle melody, which I previously called, "unknown melody," is actually a Chinese pop song, titled 老鼠爱大米, which roughly translates to "mouse loves rice." Credit goes to member whc2001 for letting me know this in the comments.
Numerous versions of 老鼠爱大米 can be found in the usual places, e.g. Youtube (https://www.youtube.com/watch?v=EQJGzcy13ls), and other tubes(https://v.youku.com/v_show/id_XNDUyMzc3NzY5Ng==.html), or even MuseScore(https://musescore.com/sheetmusic?text=mouse%20loves%20rice), just by searching for it via the string "老鼠爱大米" or "mouse loves rice." This song has lyrics too, so you can sing along, if you want to. This is difficult for me, but probably easy for anyone who understands Chinese. Maybe too easy, if this is the kind of song with earworm potential (https://en.wikipedia.org/wiki/Earworm) ; i.e. the type of catchy, memorable song that get stuck in one's head for days at a time.