The figure below shows a schematic diagram of a audio level or VU meter circuit. A VU (Volume Unit) meter is a very useful device to show the audio level in audio devices from which a user can maintain a certain audio level and get rid of overloading the output.
Working of the circuit is
simple; the input audio signal is first amplified by the 2N3904 NPN transistor and then rectified by the 1N4148 diodes and fed to the 1mA meter. The 1M potentiometer is used to adjust the sensitivity of the circuit. The circuit is using a 1mA DC meter. You can also experiment with other higher or lower value milliampere meters.

After completing the circuit you have to calibrate it by adjusting the 1M potentiometer. The operating voltage of the circuit is 9 to 12 volt DC. You can also try low voltage like 3, 5 or 6 volt. 

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VU Meter Circuit Diagram