Does your amplifier have a level indicator? I always envied the fancy amps with LED level bar graph that you find on most stereo systems or amplifiers. This is your chance to build your own LED VU meter that you can link to your amp and watch it in action. The application is based on LM3915 chip. This is a monolithic Dot/Bar Display Driver IC made by National Semiconductor. It takes an analog voltage input on pin 5 then drives 10 LED’s providing a logarithmic 3dB/step analog display.

When measuring power, a 3dB increase means that the power input has doubled. As the power doubles, an additional LED will be lit until the maximum is reached. The display can be bar or moving dot depending on the connection of pin 9 to the positive supply. The LED drive current is regulated which eliminates the need for current limiting resistors. The supply voltage can be between 3V to 25V. Full specifications can be downloaded from LM3915 LED VU Meter National Semiconductor site.

LED VU Meter schematics

Parts List:

  • R2: 18 K, 5% , ¼ W:
  • R3: 2k7, 5%, ¼ W
  • R1: 10k, 5%, ¼ W
  • C1,C2: 1uF 50v
  • L1-L7: Green LED
  • L8-L10: Red LED
  • IC1: LM3915 18 Pin IC socket