Variable Audio Oscillator
This project is a pulse-type audio oscillator that can be changed in frequency as the tuning capacitance is changed. A low tone is produced when the capacitance is at its maximum and a high tone is obtained with minimum capacitance.
The reason for this change is that the capacitance charges to a voltage greater that the 3V of the Battery, due to the induced voltage of the bottom half of the center-tapped Transformer winding. This voltage must then be discharged through the total resistance of 790K ohms (100K + 220K + 470K) to about 3V, before the Transistor can conduct again to produce another pulse of output (and another charge greater than 3V on the Capacitor).
You can change the repetition rate (frequency) of the operation by changing either the capacitance or the resistance value, or both. Experiment with different values of resistance, but do not use resistors less than 100K or the Earphone may be damaged.