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Project Notes

#653 USB Audio Level Indicator Kit

An electret-input audio level kit based on cascading LM358 stages with LED indicators.


Here’s a quick demo..


PS: Samba do Cajón - Abellán Oficial


This 20-LED audio volume indicator kit is widely available from sellers on aliexpress - mine came from WAVGAT Official Store.

It is USB powered, and uses an electret microphone for sound input.

It produces a very nice symmetrical display, pumping out to right and left as the volume increases.

Kit Specifications

Voltage: 5-12V LED: 12 green, 8 red Board size: 160mm x 18.5mm

kit-parts kit-pcb kit-manual

Parts List

Item Qty Ref
2kΩ 1 R28
1kΩ 1 R24
510Ω 11 R26,27,22,21,18,17,14,13,9,10,5
5.1kΩ 3 R6,4,2
100Ω 4 R11,12,7,8
1N5819 1 D23
1MΩ 1 R3
500kΩ trimmer 1 RP1
1N4148 2 D1,22
430Ω 2 R23,22
200Ω 3 R19,16,15
10kΩ 2 R25,1
104, 100nF 2 C1,3
5mm Red LED 8 D14-21
5mm Green LED 12 D2-13
LM358 6 U1-6
DIP8 socket 6 U1-6
electret mic 1 MIC1
100µF electrolytic 1 C2
4.7µF electrolytic 1 C4
micro USB socket 1 K1

Circuit Design

Here’s my redrawing of the circuit using Fritzing, so any errors are mine. Layed out this way, the circuit behaviour is easily apparent. A quick summary:

  • the electret mic is based with a conventional input circuit wit the AC signal offset and fed to LM358 (U1a) op-amp, configured as a non-inverting amplifier with the amplification (sensitivity) set with RP1
  • LM358 (U1b) buffers the signal and outputs the “audio reference signal”
  • the audio reference signal is fed to 10 op-amp units used as comparators against 10 discrete voltage levels
    • the 10 discrete voltage levels are set with a multi-stage voltage divider
    • each op-amp output drives two LEDs (left and right)


The circuit is actually the basis of the LM3915 that I covered in #065 LevelIndicatorLM3915 and #202 Audio Level Indicator Kit. Note that the resistors are not identical on the LM3915, but form a logarithmic scale. If a linear scale is needed, there is the LM3914 that is otherwise identical. Linear scale is usually better for voltage control of batteries, and logarithmic one for audio level.

Construction complete:


Next Step

The unit works very well, even on 4.5V (3xAA). I’m thinking of making a case for it next with option of battery or USB power.

Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.