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Project Notes

#202 Audio Level Indicator Kit

Test an audio level indicator kit based on the LM3915

The Build

Notes

This is a build of an LM3915 Audio Level Indicator Kit. I picked it up for a few dollars from a seller on aliexpress. If nothing else, a cheap way to get an LM3915 and bunch of square LEDs.

Kit Specifications from the product page:

  • Power supply voltage: DC9V - 12V
  • PCB Size: 62mm * wide 27mm
  • comprehensive PCB silk-screen

The circuit is a pretty minimal implementation of a standard LM3915 circuit with no frills, but it does vary from the reference circuit in a couple of ways.

Upper Reference Voltage

The upper reference voltage on the LM3915 is set with pin 6 REF HI.

In this circuit, it is set independently of REF OUT/REF ADJ with a simple R6/R9 voltage divider:

  • REF HI = 245mV for 9V supply

This is below even −10 dBV, so the input signal requires further attenuation before it can drive the LM3915 effectively

Input Signal Processing

Input signal attenuation is achieved with R3/R7 10kΩ on each connection and the R4 50kΩ variable pot. Since there’s no coupling capacitor, that makes it possible to handle AC and DC signals. Although, with DC you might need to consider ground levels and isolation, as the input circuit puts 10kΩ between the external and internal grounds.

In fact here’s a test where I’m measuring the input voltage sensitivity with a variable DC supply:

AudioLevelIndicatorKit_dc_test

There is no external rectification or peak detection in the circuit. According to the LM3915 datasheet, it is designed to handle AC signals direct on pin 5, and respond to positive half-cycles only but will not be damaged by signals up to ±35V.

For a better audio level display, I’d probably use a circuit like that described in this Sound Level Meter post by Aaron Cake.

LED Current

LED current is determined by the current on pin 7 REF OUT. REF OUT is nominally 1.25V (I measured it at 1.236V) and goes to ground via R5, so LED current is (only) 1.2mA.

This is quite low for LEDs, and far from the recommendation in the datasheet for handling AC signals: “It’s recommended to use dot mode and to run the LEDs at 30 mA for high enough average intensity.”

R2 is included as recommended for LED supply voltages above 7V, as it limits the voltage drop across the LM3915 and therefore the power it needs to dissipate.

Mode Select

Jumper pin terminals are provided to select either the dot or bar display mode:

  • open: “dot” mode is where a single element lights to display the peak value
  • closed: “bar” mode is where all elements light up in bar-graph fashion

Closing the jumper straps the LM3915 mode-select pin 9 to V+, putting it into “bar” mode.

Construction

Breadboard

The Schematic

The Build

Credits and References

About
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.