Using a Solar Panel to See Sound and Hear Light

Normally we use speakers to hear music that comes from an electronic audio source, like a stereo or a TV. Those devices send an electrical signal to the speakers: the higher the voltage the louder the sounds, and the faster the changes in voltage the higher the tones we hear coming from the speaker.

Now instead of using a stereo to produce an audio signal, we can also connect a solar panel to the speaker. That way we can hear changes in light intensity. That way... we can hear light!

With just a solar panel and an amplified speaker, we can discover that a hidden world of light and sounds is surrounding us.

And it gets even better: once we have that working, we can also do the opposite. We can modulate a light source to flicker at the same frequency as music. Then we can see sound and not only that, but make it audible again by holding the solar panel in front of the light!

There are three parts to this Instructable: first making light audible, then making sound visible (also called light modulation) and then as a bonus transmitting sound via laser.

I did this project with groups of first-year art school students as an introduction to the electronics workshop. We spent about 1.5 hours making this. Most students told me they thought it was amazing :-)

Most instructions on light modulation I found were using parts that are not easily available (to me). The main problem was the audio source, which needs to be powerful enough (most audio outputs on consumer products can only drive headphone speakers).

I used a separate amplifier, which allows me to use any audio source while also making sure it will not be connected directly to the 9 Volts battery. That's a big plus if you want to avoid liability for damaging people's expensive phones during a workshop :-)

I needed the project to be easy to do, with as few parts as possible. This is also why I used 9V batteries and a small amplified speaker with a battery instead of wall power adapters and stereo sets. This is what we used:

• 12V G4 LED lamp (the two metal pins of this type fit nicely into the breadboard)
• Amplified speaker (like this one, with an 3.5mm audio cable coming out of it)
• 2x alligator cables
• 4x alligator to female jumper cables
• 2x alligator to male jumper cables
• 2x male-male jumper wires
• Half-sized breadboard
• 2.5 Watt solar panel (you may want to twist/cut off the cable connector if it has one on the back)
• 18 Watt TDA2030 mono amplifier (here or here)
• 2x 9V batteries
• Audio cable with 3.5mm mini jacks on both ends. (If you want to use your phone as an audio source you might need an audio adapter for your brand of phone.)

You will also need a precision screwdriver.

If you want to try transmitting sound via laser, you will also need:

• a KY-008 laser module
• 2x male-female jumper wires
• 270 Ohm resistor
• tape
• translucent (not transparent!) tape or foil.

Start by connecting the speaker to the solar panel.

Note that the audio plug coming from the speaker has three parts. Take the two cables with alligator clamps on both ends. Clamp their alligator clips on the top and bottom part like in the photos. Make sure they’re not touching the middle part of the plug.

Take the cable that is now connected to the tip, and connect it to the positive side of the solar panel. You do that by putting the alligator clamp over the metal dot on the side of the plus sign.

Connect the other cable coming from the plug to the minus side of the solar panel.

Turn on the speaker. Set the speaker to the highest volume by turning the volume knob all the way to the right.

Put the solar panel on the table at arm's length. Try to position it against something so it stays upright.

First a warning: 1 in 3.000 people suffer from photosensitive epilepsy. This small lamp and the frequencies we'll use are probably not going to be a trigger if you have that condition, but you might want to avoid looking directly and closely into the light anyway.

Now you are going to connect a bright LED lamp to a 9-Volt battery, via a breadboard.

Breadboards work as follows. The holes in the middle of the breadboard are connected in columns of five (as highlighted green in the image). The horizontal rows, with the red and blue lines next to them, are used to connect components to a power supply. Red is plus and blue (or black) is minus, just like batteries have a plus and a minus.

Now pick up the LED lamp. If you hold it a bit at an angle in relation to the rows of the breadboard, you can push it in with one leg in the red row and one leg in the blue row.

Get two cables with alligator clamps on one side and a male connector of the other side. (Alligator clamps look like little metal alligator jaws. Hopefully the difference between male and female cables is obvious from the photo.)

Clamp one alligator clip to the plus side of the battery (so where it says ‘+’ on the side). Put the other side of that cable into a hole on the red row of the breadboard (on the same side the lamp is on).

Do the same for the minus side. Push that cable into the blue row. The lamp should light up. If not, double-check the connections.

Aim the solar panel at the lamp. If you disconnect one alligator clip from the battery and then use it to tap the battery contact, you should hear the lamp turning on and off through the speaker.

Can you also hear what happens when you scratch the contact?

If you hear the speaker popping and scratching, you are already hearing light and the changes in its brightness!

If you are just holding the solar panel in front of the bright light, you won't hear anything. This makes sense (lightbulb moment!) -- sound is changes in air pressure. If there are no changes in light intensity in front of the solar panel, then the voltage coming out of the solar panel will also stay the same. In that case the speaker will also not move air and no sound will be audible.

Continue on to one or more of the following experiments. Disconnect the battery first.

• Use the flashlight of your mobile phone. Turn it off and on quickly while aiming it at the solar panel. (On the iPhone you can vary the light intensity quickly by holding the flash light icon for a second and then sliding up and down the indicator.)
• If you turn the screen of your phone off, and then hold it above the solar panel (front face down), you may hear popping noises. (It works with most smartphones.) Any idea why? (Answers below.)
• Take a remote control like from a TV and point it at the solar panel. Push different buttons. Why does it work?
• Aim your solar panel and speaker at lights that are wall-powered. (You may need to hold the speaker to your ear.)
• There are apps that will turn the flashlight of your phone into strobe lights. For Android there is a particularly good one which is also free: Strobily.
• Toys with blinking LED lights in them.
• Try a TV or other screens. They need to be turned on, but you should hear a noise even when there are no moving images shown.

What do you hear in each of these cases?

• The noises coming from a phone are its infrared sensors that are there to sense if you are holding it to your face.
• Remote controls make sounds because they also use infrared light to transmit signals that are recognized by the TV.
• The hum you hear from wall-powered lights is the alternating current powering these devices. The 50 or 60 Hertz frequency of the wall power is ‘visible’ in the light they produce.
• What you hear when holding the solar panel in front of the TV is the refresh rate of the screen.

You may realise by now that there are a million things making sound (by generating flickering light) without you realizing it! The solar panel is an antenna that helps you discover this hidden world around you.

When I was writing these instructions, the wind-powered ventilator on my neighbor's roof was reflecting sunlight into my room. I found the pulsating light quite annoying, until I realized I could point my solar panel at it as well. Click on the video above to hear the result (turns out the thing sounds like a purring cat :-D). If you listen closely, you can hear the frequency change because of the wind speeds.

I read that you can put the solar panel and speaker in front of a headlight of a parked truck with the engine idling and that you can hear noises from the engine. They say they were "able to hear an occasional buzz as a bugs would fly through the headlight beam. Some flashlights will make a ‘ding’ sound if they are tapped while shining on the photocell."

We heard differences in light intensity, so let us now try the reverse: making sound visible through our lamp. That is called light modulation. It means changing the intensity of a light to transmit a signal (in this case, sound frequencies).

We should be able to pick up that modulated light with our solar panels, so we will be transmitting sound via visible light.

To do that, we will expand the setup we have so far with an audio source, an amplifier and a battery.

The amplifier is needed to amplify the electrical signal coming from the audio output of your phone or laptop. The audio ports of these devices normally only have to send out enough power to work with the tiny speakers in headphones. In this case however the audio needs to compete with a powerful 9V battery.

The small amplifier has three pairs of connections that are labelled at the bottom: two for battery power, two for audio input and two for audio output. We'll connect all of them up in the following steps.

Insert an audio cable into your audio source of choice (phone, laptop or another media player).

Take two cables with alligator clamps on one side and a female connector of the other side.

Connect the alligator clamps to the free plug of the audio cable, just like you did before with the speaker (see the photo). Make sure the clamps are not touching the middle part.

Get the other end of the alligator cable that is clamped on the tip of the plug and push it on the pin on the amplifier that is labelled ‘IN’ (the label is on the underside of the board).

Take the other cable and push it on the pin next to ‘IN’ labelled ‘GND’.

Get a second 9V battery, wire it up as you did the other one but this time use alligator clip cables with female jumpers on the other end.

Now you can't really break anything with any of this stuff, as long as you keep + and - separated.

So MAKE SURE YOU GET THIS RIGHT or the amplifier will break:

• Connect the plus side of the battery to ‘VCC’ on the amplifier.
• Connect the minus side of the battery to the pin labelled ‘GND’.

A small red light on the amplifier should now indicate that it is powered. If there isn't, then check your connections.

Normally a speaker would be connected to the blue screw terminals labelled ‘OUT’ and ‘GND’. We are going to send the audio signal to the lamp instead.

Screw male-male jumper wires tightly into each of the screw terminals.

Connect the wires to the rows where the LED lamp is. Again, make really sure you connect the right wire to the right row: ‘OUT’ is going to plus (red) and ‘GND’ is going to minus (blue).

Re-connect the other battery to the lamp so the light stays on all the time.

Now turn on music on your device. Set the volume to near maximum.

Hold up your solar panel to the light. Enjoy!

Once you hear music through the speaker, experiment a little.

• From how far away is sound still audible (maybe with the speaker to your ear)?
• If you feel the sound you hear is distorted, even when you hold the solar panel a little farther from the light, then you might need to lower the volume on the amplifier. This is done by turning the small screw on the amplifier board with a small screwdriver.
• Can you hear sound from reflected light as well? Hold the back of the solar panel to the lamp and try different types of materials. From how far away does it work? What about your hand or the ceiling?
• Try to make the sound louder by putting a small reflector screen around the lamp.
• Does blocking other light falling on the solar panel make a difference?
• What happens to the sound when you add another bright light like the torch from your phone?

Isn't it amazing that this works?? You're listening to sound via light! If you hook up a really good stereo instead of the small speaker, you can make the sound quality so good that you wouldn't even know that it comes from a lamp.

Sometimes people are a bit confused about what it is that they just made. So here's a short recap. Audio signals travel through wires (of headphones and speakers) as electricity. The electrical signal that would normally go to the loudspeaker to make it go up and down is now going into our LED lamp. The lamp gets brighter and dimmer as the electric current varies, so it changes with the exact same frequencies of the music.

These changes in light intensity are converted back into electricity by the solar panel. That electricity we use as an input for our speaker, which allows us to hear it as sound.

Changing a signal like this is called amplitude modulation. Communication via light waves is not really different than radio waves. Light and radio waves are both part of the larger spectrum of electromagnetic waves, just with different wavelengths. Radio waves and Bluetooth signals we can pick up with antennas, light we can pick up with solar panels or light sensors. Optic fiber cables that form the backbone of the internet also send information via light.

One other thing this shows you, is that our ears can hear much higher frequencies than our eyes can differentiate as light. With a solar panel and a speaker you can hear AC light and the refresh rate of a television, even when our eyes only see constant light. Similarly, modulating the light with the frequency of sound makes the light flicker faster than our eyes can see --it flickers a bit with bass tones if at all-- but the solar panel and speaker will pick it up!

As a bonus, you can try to use a laser for light modulation instead of the lamp. The advantage of this is that you should be able to hear sounds from (much) farther away. Also, lasers are cool.

The disadvantage is: lasers are dangerous. You won't set anything on fire with these small modules, but you could hurt your eyes. So don’t look into the laser! And never point lasers at people!

To try it, disconnect one cable to the lamp’s battery and take the lamp out of the breadboard. Leave everything else you made so far connected.

The laser needs a lower voltage than the lamp, and we get that lower voltage by using a resistor. Push in a 270 Ohm resistor into the breadboard, with one leg in the red row and the other in a hole of the nearest 5-hole column.

Get two wires with male-female connectors. Push the female end of one wire to the pin labelled ‘-' on the laser module.

Push the other end of that wire into the blue row where the lamp was before.

Push the female connector of the other wire on the pin labelled ‘S’. Push the other end of that wire into the 5-hole column where you put in the resistor (so NOT the red row).

Tape the laser to the table or something else that is stable, to make sure it won't be aimed at your eyes or someone else's.

Put non-shiny tape or foil on the solar panel before holding it in front of the laser, so that the panel can’t act as a mirror.

Reconnect the battery. Do you see where the beam goes? Now make sure the music is playing, and hold the solar panel in front of the beam.

From how far away can you still hear the sound through the solar panel? Some say 800 meters distance is possible, but I assume they're using a laser more powerful than we are.

Can you make the sound stop completely by blocking the laser light with one finger? And with your whole hand? (Try this especially at a bigger distance and check with the speaker close to your ear.)

Interestingly, the laser works better at a bit of a distance, as opposed to the lamp which shines less light on the panel the further away you get from it (as described by the inverse-square law). The sound from the laser light falling onto the solar panel however gets louder, because the tightly-focussed beam widens out a tiny but over distance. So with laser light, more light falls onto the panel with more distance, and hence, more loudness.

Lastly, it's easier to manipulate this focussed beam with for instance a comb or a small ventilator to make sounds.

I hope you had fun trying out light modulation! There are a couple of interesting ways to take it further. During the light modulation workshops we tried making a modulated laser light circuit for instance, where a couple of solar panels would send the signal they picked up directly into another amplifier and laser, to serve as signal repeaters. We found that after a few hops the signal got too degraded, but I think with some more experimenting it should be possible to go around the building :-)

Some students found out that they could combine two light bulbs to mix different audio sources. It should also be possible to experiment with stereo sound and maybe even have different audio signals with different colors of light (or different polarization)! That technique is also used to send more data over single optic fiber cables, so it is possible.

Happy experimenting!