LM324 Voice level meter
Here's an LED level meter that can tell you the loudness of your voice. Use the earphone as a microphone and speak into it, or input any other audio signal.
When you finish wiring, turn power on and adjust the control volume so the LEDs 1 to 4 light up at the suitable voice level.
Now speak into the earphone in a low voice..only LED 1 lights up. See what happens when you yell into the earphone.
In this circuit, the AC output from the power amplifier is converted to a DC voltage by half-wave rectification using the Si diode. Then it is input to the comparator IC. The comparator has four reference voltages, so the four LEDs light up one after another when the voltage produced by half-wave rectifier goes beyond each reference voltage.
The ON/OFF speed of the LED flashing can be changed by changing the R2 and C6. Try it!