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 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.
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!
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
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)
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.
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
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
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:
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
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.
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.