I picked up one of those Radio Shack “300 projects in one!” Electronic Hobby kits. After messing around and making the first project (“the chirping bird”), I realized that I don’t quite understand where the sound is coming from.
It’s a very basic setup - there’s a light-sensing cell that is wired along with two resistors, a transistor, and three capacitors, to a speaker. When light hits the photo cell, it goes through those things that it’s wired with and to the speaker, creating a repetitive chirping sound. The amount of light falling onto the cell varies the speed of chirping.
But what’s actually making the sound? I thought that you needed a basic oscillator or an IC or something to create the sound - what in this incredibly minimal setup is producing the tone?
Without seeing the circuit I can’t say, but it doesn’t take that much to make an oscillator. There are some fairly simple circuits you can use to make oscillators, many of them with just one or two transistors.
A “chirping” sound can also be made by taking advantage of the properties of RLC circuits. The speaker is an inductor and you’ve also got capacitors, so you’ve got all of the components for an RLC circuit. An inductor and a capacitor are both storage devices. If you select the components very carefully, what can happen is that the energy discharging in the capacitor can charge the inductor, then the inductor discharges and charges the capacitor, etc. etc. over and over producing a “ringing” that decays fairly quickly (like a chirp).