Can you hear it?


Simple amplifiers which are available from Radio Shack and as kits can be combined with various transducers to convert different forms of energy such as light, and magnetism into sound.




Build the kit amplifier.

Solder leads to the mono-phonoplug or buy a commercial phono plug with wires -installed.

To Do and Notice

Solar Cell

Attach the solar cell to the phonoplug using clip leads (or solder it to the plug.)

Plug the phonoplug into the amplifier, and turn on the amplifier.

Point the solar cell at a fluorescent light and turn up the amplifier until you hear a sound.

What's Going On? Solar Cell.

The sound you hear is 120 Hertz (cycles per second).

Fluorescent lights turn on-and off 120 times per second.

The solar cell converts light into electric current, which then flows and stops 120 times a second.

The amplifier amplifies the current and feeds it to a speaker which converts the current into sound which you can hear.

Optional, look at the sound on an oscilloscope or compare it to a 120 Hertz tuning fork.

More Solar Cell activities

Other experiments with light:

Point the solar cell at the sun or an incandescent lamp or a flashlight and you will hear nothing. The Sun is a constant source of light which the solar cell turns into a constant current, which makes no sound. (Use a meter to measure constant, DC, currents.)

Mask off all but a small part of the solar cell. Cut a small hole 1/8" in diameter or so in black paper and then cover the solar cell with the paper. Spread the fingers of your hand apart and wave them back-and forth in front of the solar cell. Listen to the popping noises. Move a comb back and forth between the sun or an incandescent lamp and the hole. Listen to the sawing noise as the teeth of the comb block and transmit the sunlight.

Drill regularly spaced holes around a disk and spin the disk between the solar cell and a constant light source, such as a flashlight. Listen to the tones produced by the flickering light.

Stretch a balloon over the end of a toilet paper tube. Glue a small piece of mirror onto the balloon, e.g. use cool melt glue. Reflect constant light onto the solar cell off the mirror. Speak into the open end of the toilet paper tube while you do this. Your voice will be heard on the amplifier. Your voice wiggles the mirror which modulates the light, which is converted into sound&emdash;the sound of your voice.

What other changing lights can you hear?

e.g. Light reflecting from a wind rippled puddle.

To get rid of background hum, connect two identical solar cells opposite to each other (in anti-series) and shine a light onto only one of them. Light illuminating both cells is cancelled, light illuminating only one is amplified.

Pick-up coil

Attach the pick-up coil to a phonoplug with solder or clip leads and plug it into the amplifier.

Hold the pick-up coil next to a small DC motor and spin the motor by hand, listen to the sounds. Drive the motor with a battery and listen to the sounds. Vary the speed of the motor.

Hold the Pick-up coil next to a fluorescent lamp. A television. A computer. A refrigerator which is running,an electric drill. Listen to other appliances and tools.

Move a magnet close to and then away from the coil. Listen to the clicking noise. Notice the "frying bacon" noise. otice that the coil is attracted to the magnet.

Stretch a piece of music wire, piano wire, a hacksaw blade, or a piece of welding rod between a magnet and the pick-up coil. Pluck the metal. Notice the electric guitar sound. Most excellent dude.

Magnetize a hacksaw blade in alternating north south bands and then move it back and forth under the coil.

What's Going On? Pick-up coil

The pick-up coil is a coil of wire wrapped around a piece of iron. Changing magnetic fields through the coil create an EMF, electro-motive-force around the coil, which drives a current through the coil. The amplifier converts these changing currents into sounds allowing you to hear the electromagnetic fields produced by various motors, tools, and appliances. So when you move a magnet towards and away from the coil you change the magnetic field through the coil and produce a clicking sound. The "frying bacon" sound is more interesting, it is the sound of magnetic domains changing inside the iron core of the coil. When the wall of a magnetic domain in iron moves suddenly as a magnet is brought towards the iron then the magnetic field around the iron changes. This changing magnetic field is turned into sound by the coil and amplifier. Many randomly changing magnetic domain walls create the "frying bacon" sound known as the Barkhausen effect. A vibrating magnetizeable wire near a magnet also produces a changing magnetic field which is turned into sound by the coil.

Piezo-electric disk

Attach the Piezo-electric disk to a phono-plug using solder or clip leads.

Insert the plug into the amplifier.

Tap the piezo-electric disk and listen to the tones it produces. Blow on it. Tape it to tables, metal bowls, slinkies, cake racks, clothes hangars, and other things. Tap these objects and listen to the sounds they produce.

What's Going On? Piezo-electric disk.

When a piezo-electric material is deformed, such as by a sound wave, it moves charge and produces a voltage. This voltage can produce a small current which is amplified and turned into sound. The amplifier can make sounds which normally are too quiet to be heard by a human ear loud enough to be heard.


Wind your own coil.

Use a relay as a coil.

Advanced pick-up coils.

Attach one coil to a tape player or radio and then pick-up the signal it transmits with a second coil and amplifier. Can you shield the signal?

Advanced solar cells

Attach an LED to a tape player in series with a 9 volt battery and a 470 ohm resistor, the longer leg of the LEd should be next to the positive terminal of the battery.

(If the LED doesn't light add a coil in parallel with the tape player. For a coil, use a relay, a pick-up coil, or wind your own 100 turns around a bolt.)

Scientific Explorations with Paul Doherty

© 2006

10 October 2006