The Audio Crayon - Making LEDs Dance to Sound

Introduction -

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Flashing LEDs in time to music is nothing new. But this is a new twist in the packaging to make it more interesting. The best thing is the cost - under $10 for all of the parts.

A simple electronics circuit takes an audio input and modulates the brightness of a group of LEDs in time with the music. While you could just use a group of LEDs why not mount them inside a semi-transparent plastic container?

I found a large crayon-shaped plastic bank in a discount store and immediately saw the potential for putting a light source inside the bank for some kind of decoration or novelty item. The bank I found measures approximately 22 inches (56 cm) long and 2.5 inches (6 cm) in diameter. It retails for about $7 but I was lucky enough to find it for a dollar.



Parts List
1 multi-LED flashlight
1 plastic crayon bank
1 LM386 amplifier
1 .1 mf capacitor
1 10 mf capacitor
1 250 mf capacitor
1 100 ohm resistor
1 10K potentiometer
1 3.5 mm mono plug
9V battery clip
hook up wire, PC board, etc.
9V battery

I welcome constructive criticism and inputs from those with more electronics expertise. If anybody can suggest a better circuit, or better values for the components in the circuit to improve the performance it would be welcome.

Step 1: Preparing the LED Flashlight and Crayon Bank

Most LED flashlights use three batteries for a total of 4.5 volts. They have anywhere from three to a couple of dozen LEDs. Any flashlight where you can remove the battery compartment and connect to the light assembly's contacts that fit inside the crayon bank will work.

The bank has a removable plug on its bottom. If you're lucky the flashlight will fit into the opening reasonably well. My LED flashlight was slightly larger than the opening. The crayon is made from fairly thin plastic and any knife sharper than a butter knife can be used to increase the size of the opening

Step 2: The Circuit

The circuit

The LM386 amplifier chip has been around for decades because it's reliable, simple to use, and very durable. It should cost about 50 cents or so. It's available from Amazon.com and standard electronics part suppliers.

The wiring is almost a direct copy from the application notes, replacing the speaker with the flashlight assembly.

Nothing's especially critical about the circuit. The potentiometer adjusts the gain of the input which indirectly controls the brightness level.

A normal line-in audio source (computer sound card, MP3 player, stereo output, etc.) is connected to the mini plug.

Step 3: Possible Enhancements

Ultimately I hope to mount the circuit board and a rechargeable battery into something that fits in the bottom of the crayon.

A built-in microphone will make this a self-contained audio flasher, great for attention in a nightclub.

Two of these audio crayons could be mounted on the sides of a computer monitor and hooked up to the computer's stereo output for visual effects to go along with listening to music or playing computer games.

Audio crayons can be mounted on the speakers of a stereo or home theater system.