We've used oscillators in many Projects so far ... but we've never sat down and tried to figure out what an oscillator is and, how it works.
This Project will let us do just that. When you finish wiring this Project, press the Key. You'll hear a sound from the Speaker.
Now rotate the Control Knob back and forth. What do you now hear from the Speaker?
Oscillators make use of feedback. That means part of the output of the circuit is sent back into the input. Look carefully at the schematic for this Project, can you see where some of the output goes back into the input?
Let's look how the current flows. When you press the Key, current flows from the terminal of the Batteries through the Transistor's emitter, then to its base and back to the Batteries through the 22K and 50K resistances. Since we now have current flowing from emitter to base, current can flow from the collector to emitter (remember what we said about transistor operation back in Projects Meet the Transistor and Transistors as Switches? in Transistor section).
Current will also flow in the OPT Transformer the Speaker is connected to. Think that electromagnetism might come into play here?
You're right-an electric current is set up from the top of the Transformer coil through the 3.3 uF capacitor, the Transistor's base and emitter, the Key, Batteries and back to the Transformer by the connection at the middle of its coil (that's called a tap).
As you might expect, charging and discharging of the 3,3 uF capacitor is the key to this Project's operation. When the 3.3 uF capacitor discharges, it does so into the base of the Transistor. It just happens that this current is opposite from the emitter to base current (it's + instead of —).
You guessed it, it shuts off the emitter to base current. But when the Capacitor discharges low enough, emitter-to-base current again flows, the 3.3 uF Capacitor recharges, and the whole cycle starts again. That's how feedback works. And feedback is necessary to make an oscillator go!