VU Meter

Project & Notes

Introduction

Being a product of the Eighties, I consider a VU meter as a definite must in any stereo system and therefore incorporated it into my audio amplifier project. All this little meter does is light some LED’s in sequence with the volume of the music by reading the voltage applied to the speakers. The scale is logarithmic, but not calibrated.

The design is based around the LM3915 from National Semiconductor.

Design and Schematics

There are many schematics on the Internet available for a VU meter, but I settled on the one in the application notes for the LM3915. Not surprisingly, I couldn’t get it to work at first (nor any of the numerous others I tried), but by rearranging the connections somewhat I got a satisfactory VU meter. (It works for me – your milage will vary.)

Schematic for the VU meter as built by me. (See text for values, explanations and warnings.)

© Robert I. Sadler, 2009

Calculating R1 and R2

The speakers connected to this system are 4Ω and rated at 3W. A few calculations later and it seems that 3,5V is the maximum voltage that can be applied to a speaker:

tex:$\eqalign{V &= \sqrt{P \times R} \cr &= \sqrt{3 \times 4} \cr &= 3,464}$

I have found that the sound from the speakers become distorted long before the volume is turned up full, so I've decided that only 3V will be required to turn on all the LED’s (VREF). I like the LED’s bright, so about 12mA of current (ILED) should do it. According to the technical notes, the calculation of R1 is:

tex:$\eqalign{R_1 &= 12,5 \div (I_{LED} - V_{REF} \div 2200) \cr &= 12,5 \div (12\!\times\!10^{-3} - 3 \div 2200) \cr &= 1175,214 \cr &\approx 1\rm{K}2\;\Omega}$

The calculation of R2 is:

tex:$\eqalign{R_2 &= (V_{REF} - 1,25) \div (1,25 \div R_1 + 8\!\times\!10^{-5}) \cr &= (3 - 1,25) \div (1.25 \div 1200 + 8\!\times\!10^{-5}) \cr &= 1560,178 \cr &\approx 1\rm{K}5\;\Omega}$