“This is an “Audio VU Meter” or “Sound Level Meter”, it is a general-purpose bar-graph Audio VU meter designed for fun projects. All you need is to hook up one wire to the output of the audio amplifier’s speaker pin along with GND and see the magic. The response of the circuit is very fast and it provides beautiful visual representation from audio input signal.
A simplified schematic is provided to give the general idea of the operation. The signal is applied to a series of 20 comparators, each of them is biased to a different comparison level by the resistor string. In the circuit diagram, the resistor string is connected to the 100K potentiometer which provides reference voltage 1.9V to 12V.
As the input voltage varies from 0 to 1.9V, the comparator’s outputs are driven low one by one, switching on the LED indicators. This circuit will work with the audio signal level from 1.9V to 12V, it will not work with the audio line signal. String resistor values calculated to use this project with audio amplifier of 1W to 10W. PR2 provided to adjust the input audio signal level. Testing the board is simple, keep both potentiometer PR1 and PR2 at the center, hook-up 2 wires GND and Input signal to Audio amplifier speaker out, adjust the PR1 so all LEDs are in ON condition at the full audio signal level.”