You can turn anything into a speaker!
So, we all know that putting a coil and magnet together makes a speaker, and applying a voltage to a speaker causes the surface to move back and forth, causing the air to expand and contract and put sound into your ear canal. What we might not know, is that almost everything does this when we give it enough power!
Now, as a proud former student of ECE 310 and ECE 311, I will present to you a horrifying circuit that will make all the audiophiles fear for their lives:
It certainly looks amplifier-shaped, but uses none of the knowledge generously provided by the junior year circuit classes. While the “correct” amplifiers use the small signal models characteristic of semiconductor devices, my circuit simply uses a square wave input to toggle the transistors between being a short circuit to ground, or an open circuit to ground. This effectively allows an external power supply in the Pi Room to be toggled between powering and not powering a device plugged in.
Two things:
First, this circuit uses a darlington configuration for the transistors, where the emitter of one transistor feeds the base of the other. This allows a rather weak base current from a microcontroller to be able to fully transition the transistor from an open state to an almost short circuit. This affects switching speed, but this doesn’t matter for our application at hand.
Second, the input signal to the transistor is a square wave. That’s right, this amplifier is going to sound horrible! This is okay, as you are going to see we will be using some interesting objects to produce the sound. Some devices don’t respond too well to continuous fluctuations in their operating voltage, so this setup allows for them to operate “normally”, swinging between their operating voltage, and not powered. Additionally, our sound producers will sing as loud as they can!
All right, enough nerd talk, let’s quantize some samples into one bit (😱) and store them into the tiny 32KB ROM on an Arduino to play them. (About 10 seconds of music took up a quarter of this storage!)
Of course, I try it out on a speaker, and we can clearly hear the music. What I show later in the video, although very faint, is that shorting our output leads causes the music to become audible in the transistor alone! How’s that for “solid state”?
I was curious, and tried shorting the output across the tab of a soda can because I thought the acoustics of the can cavity would allow sound to propagate well, and it ended up being pretty loud!
Finally, I plugged the output into a giant spool of wire, to see if having an audio-modulated magnetic field would make anything interesting happen, but I think ultimately the wires themselves were vibrating, causing the sound instead. It was cool regardless!
