I'm trying to build a VU meter circuit, and followed this guide/diagram (link) vu meter circuit

I understand how the circuit should work in theory (mainly the OP-amps and the "logic"), except for the ground for the input signal. In the version I sketched up in CAD I hooked up what would be AudioIn+ to R1 and AudioIn- to GND. Would that cause any problems? If not, would it even work as intended?



1 Answer 1


If AudioIn+ AudioIn- are line outputs or headphone outputs then yes, you can connect them to this circuit as you described.

However, this is a very basic circuit with a lot of issues.

It won't handle negative voltages well, it will probably clamp them, so if you are listening on this same signal, it may be distorted.

The resistor ladder is linear, human hearing is logarithmic, you really want a logarithmic resistor ladder. You could change this to a logarithmic ladder, but obtaining the proper resistor values is a challenge.

A better circuit would have peak detectors with decay.

If a working circuit is more important than learning about opamps, then consider an LM3915 chip instead. It has all of this functionality in one chip.

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    \$\begingroup\$ The LM3915 is not made anymore. Digikey has zero in stock. Fake or defective ones are probably sold online. I agree that a peak detector is needed to feed it. \$\endgroup\$
    – Audioguru
    Commented Nov 18, 2023 at 0:50
  • \$\begingroup\$ The goal of the project is to learn about PCB fabrication and SMD soldering, so function is not a top priority. I found that this circuit would be a good enough balance between SMD complexity (ICs) and circuit simplicity. That's also why there is no LM3915 and just pure opamps. \$\endgroup\$ Commented Nov 18, 2023 at 5:54

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