In a previous Instructable, we constructed audio speakers using plastic cups, coils of wire and magnets. Here we reverse what's going on with those speakers to see if we can make a plastic cup microphone!

Materials used:

Plastic cup

42 gauge magnet wire

Neodymium magnets - We used a large 1" x 3/4" in our setup

Various electronics (see pictures and schematic in last step)

Step 1: How Do Speakers Work?

Here are the original speakers we made with some 30 gauge magnet wire, plastic cups, and magnets. They made for some decent speakers (considering what they were made of).

You can read about our previous speaker adventure here, but here’s a quick recap: The cone of a speaker moves quickly back and forth, producing sound. In our plastic cup speakers, a coil of wire is taped to the bottom of the cup, while a strong, stationary magnet sits nearby. When current flows through that coil of wire, it moves, because it’s acting like a little electromagnet. It attracts to or is repelled by the nearby magnet. This motion wiggles the back of the cup to make sound.

In the presence of a magnetic field (provided by the magnet), a coil of wire with a current flowing through it will feel a force. That force is what moves the speaker.

Back in the 1800s, scientist Michael Faraday figured out how this relationship between magnetism and electric current works both ways. Just as a changing electric current can induce magnetism in the coil, if you move the coil back and forth manually you’ll create a current in the wire. Theoretically, it should work like a microphone!

Step 2: More Turns!

Using our original speakers as a microphone didn't work. There was barely any signal there...so we tried more turns of wire! More turns usually equals more voltage! We switched to using 42 gauge magnet wire and 600 turns...we got a stronger signal!

We 3D printed a small spindle and wound 1500 turns of the 42 gauge wire and glued it to the back of the cup. A second 3D printed part, a bracket, held a powerful 1" x 3/4" neodymium magnet a short distance away from the coil.

For those with a 3D Printer, here are the STL files for the spindle and the bracket.

This worked better, but we still needed to amplify the sound...

Step 3: Ampify the Sound!

See above for a detailed schematic of the amplifier circuit. It's not the greatest audio amplifier, but it sure did increase the signal strength! As you can see/hear in the video, the signal was greatly increased.

There's a lot of buzzing coming from the circuit, but it definitely created a microphone (even though we might sound like monsters :) ).

Stay tuned (he he), we might try to make a ribbon microphone soon!