This circuit is divided into two parts: part one is Preamplifier circuit, which consist a BC547 transistor, a Condenser Mic and some resistors & capacitors. Second part consist a 8 ohm speaker and a 555 timer IC, which is oscillating in Astable multivibrator mode, with approx. 66 KHz frequency.
555 Timer IC
8 Ohm Speaker
Resistors - 470 Ohm, 1k, 10k, 47k, 100k, 680k ohm
Capacitors - 10µF, 1nF, 100nF
The audio amplifier circuit is shown in above diagram. Control PIN 5 of 555, has been used which is generally kept grounded through .01uf capacitor. Control PIN 5 is the point of 2/3Vcc inside the 555 timer IC, so we can change this 2/3Vcc voltage through this PIN. And changing the voltage at this PIN change the width of output pulse, irrespective of value of RC components in 555 timer circuit. It follows the same principle of Pulse Width Modulation (PWM) to modulate the output wave. We have used this functionality of Control PIN in this circuit.
Speaker does not respond to high frequency, so when there is no voltage at control PIN 5, speaker doesn’t produce any sound. When we create some sound near Condenser Mic, that sound is converted into electric signal by the Transistor, and this electric signal is fed to the control PIN 5 of 555 IC. The output pulse at PIN 3 modulate due to this voltage at control PIN, and speaker detects this DC component of Output pulse and produce sound. Basically when there is voltage at PIN 5, width of the output PULSE increases for a moment and that is detected by Speaker.
Resistor R1 is used for biasing of condenser Mic and R2 and R3 is used to provide proper biasing to transistor. We can test this circuit by blowing some air from mouth towards the Mic, the speaker will generate sound accordingly