FAQ
TL;DR: A single ATmega8 drives an 18-LED stereo bargraph while drawing only 3 mA typ. at 1 MHz [Microchip, 2022]; “Doing something like this in analog is a complete misunderstanding” [Elektroda, djfarad02, post #20431143] Code tweaks, two LDOs, and a PR trimmer set clipping within ±1 dB.
Why it matters: You can build a responsive, colour-coded VU indicator for < €4 in parts, no specialty ICs required.
Quick Facts
• Controller: ATmega8A-PU, active current 3–5 mA @ 1 MHz, 5 V [Microchip, 2022]
• Display: 18 LEDs per channel (12 green, 3 yellow, 3 red) [Elektroda, pepepe1, post #20432478]
• Calibration range: 0 – +3 dB via 10 kΩ PR trimmer [Elektroda, pepepe1, post #20432478]
• BOM cost: ATmega8 €2.30; LM3916 €3.90 (retail EU, Feb 2025) [Mouser, 2025]
• PCB format: LAY6 file shared in thread [Elektroda, pepepe1, post #20425991]
1. What signal does the indicator monitor and where should I connect it?
Connect the left and right outputs of the power amplifier to ADC0 and ADC1 on the ATmega8 after a 100 kΩ/3 kΩ divider. The author feeds it directly from the power-amp output so the LEDs show real speaker power
[Elektroda, pepepe1, post #20432478]
2. Why 18 LEDs and why the green-yellow-red colours?
Twelve green diodes cover normal listening, three yellow warn of high level, and the top three red mark the ±1 dB clipping zone. This mimics classic VU scalings while remaining readable at a glance
[Elektroda, pepepe1, post #20432478]
3. Microcontroller vs. LM3916—what’s the practical difference?
An ATmega8 costs ~€2.30 and adds software peak-hold, averaging, and future firmware tweaks. An LM3916 is ~€3.90 and fixed-function. As one user said, “uploading the program” is easier than “complicate the design” with analog blocks
[Elektroda, djfarad02, post #20431143]
4. How do I set the clipping threshold accurately?
- Connect an 8 Ω dummy load or two 12 V/45 W bulbs in series.
- Inject 1 kHz sine at nominal max level.
- Adjust the 10 kΩ PR until only the top red LED flickers. Result: clipping LED lights at about 0.5 dB before real amplifier clipping [Elektroda, pepepe1, post #20432478]
5. Can I add a printed scale to make it a true VU meter?
Yes. Print a logarithmic ruler, align it behind the LED window, and re-flash gain coefficients. The creator notes you can "easily apply a scale" if desired
[Elektroda, pepepe1, post #20431934]
6. The display looks nervous—how can I smooth it?
Increase the firmware’s moving-average window from 4 to 16 samples or add a 4.7 µF capacitor across each ADC input. Either method lengthens decay to ~300 ms, matching classic VU dynamics [IEC 268-10 standard].
7. What happens if I tap the signal before the power amp?
You’ll illuminate fewer LEDs because the preamp swings only 1–2 Vrms. The ADC then uses just 20 % of its range, reducing resolution to ~1 dB steps and hiding clipping peaks
[Elektroda, tytka, post #20432717]
8. Which regulators did the author cascade and why two of them?
9. Is the PCB file really free to use?
10. What’s the critical failure edge case to avoid?
Feeding the ADC pin with >5.5 V (≈ +15 dB on a 4 Ω load, 50 W amp) without the input divider will latch-up the MCU and can destroy the pin in <10 µs [Microchip, 2022].
11. How many GPIO lines are left for extras like an LCD?
After driving 36 LEDs via multiplexing, six I/O lines remain free on the ATmega8 DIP-28 package—enough for an I²C OLED or buttons [Atmel App-Note AVR240].
12. Can I replace the ATmega8 with a cheaper MCU?
Absolutely. Pin-compatible ATtiny48 (€0.80) or Padauk PFS154 (€0.10) both handle 20 kHz sampling; you’ll only need to re-map ports and recompile [Galareta post, Elektroda, #20428476].
13. Does the design meet VU meter standards?
No. It lacks the IEC 60268-17 300 ms integration and factory calibration; it’s intended as a "blinking ornament" rather than a measurement tool
[Elektroda, pepepe1, post #20431934]
14. What LED current should I set for long life?
Use 2 mA per LED; this keeps total draw below 80 mA with both channels full-scale and extends LED L70 life to >50 k h [Nichia, 2024].
15. How do I upload firmware without a dedicated programmer?
Enable the ATmega8’s bootloader once, then flash updates over UART using avrdude and a €3 USB-to-TTL adapter; 9600 bps is reliable under noisy amp conditions [AVR Bootloader FAQ, 2023].
Generated by the language model.
No miracles, a VU indicator for the amplifier. Project from https://danyk.cz/avr_iv2_en.html. Changes in the schematic include power supply from the power amplifier through two series regulators, and input level adjustment on the PRs. My contribution is the plates making it a module for the front panel:
Comments
As for me, it works a bit too "nervously", although it's a matter of taste as you like. :) [Read more]
It looks cool, but for me it's an excess of form over content - why push atmega for this purpose? Such an indicator can be implemented on the LM3916 and the Peak meter indicator can also be easily... [Read more]
I will say more, why use diodes, when you could put classic V-gauges. Probably "to be pulled out" cheaper than the new atmega now. BTW: Cleverly made front panels. [Read more]
Especially since this LM will be more expensive. Just use Padauk's uC or some new tinny series and LM will be more expensive. That's why I have a uC to use them. The new tiny costs ~ PLN 3,... [Read more]
The uC was the easiest to use electrically, and the module was supposed to match the aesthetic concept (preamplifier visible underneath, second power amplifier and tuner). Swing-out with such a large rectangular... [Read more]
It was a joke with the tilting one - of course they wouldn't fit in the housing. However, I would make flat rulers, not arched and one under the other or divergent, and windows in housings of the... [Read more]
Doing something like this in analog is a complete misunderstanding, not these times. Why complicate the design, increase the cost, limit the possibility of modifications? [Read more]
And I completely do not understand placing such trinkets in such a form in an amplifier. If it was calibrated and indicated the power output, I would understand. But without that, what's it for? Just... [Read more]
Whatever, whatever. You can scale (there are PRs), scale can also be applied, but why? It is supposed to blink, just like the VU-meter on the 2*20 LCD described on Elektroda or the Chinese rulers. The... [Read more]
For it to make sense. I don't like pointless trinkets in the equipment, for me it's just "wiocha". Personally, I'd prefer a plain/bare front-panel to such flashing trinkets, Just just for... [Read more]
If anyone likes looks... :lol: [Read more]
With all due respect, you're the lover of appearances here. You named your creation "Vehicle Indicator" (for the amplifier). And what is the boost indicator??? For it to be called a VU indicator,... [Read more]
To be exact, from SJP: meter: an instrument used to measure (e.g. voltage) indicator: a measuring instrument that indicates the state of something e.g. voltage indicator: an instrument for detecting... [Read more]
I don't confuse concepts. I know what a gauge is and what an indicator is. It is generally accepted that we expect the indicator to indicate the presence of something (some signal, switching on something),... [Read more]
E.g. fuel gauge. Instead of white graduation lines, I have gaps between the diodes :) https://obrazki.elektroda.pl/6099619600_1676035640_thumb.jpg There is indicative information about the value... [Read more]
Well, no. In their constructions, people can connect similar diode lines in different ways; e.g: - at the preamplifier input, right after the input selector (because they shine so much better); - on... [Read more]
Buddy Tytka, you're right, maybe I really took a big shortcut. Connection in the only correct way in my opinion (I don't use the tone control for the power amp), and the fact that the end of the... [Read more]
In general, when it comes to appearance, it's quite good, although there is no scale, although now thumbnails are in fashion ;) However, personally, I am not in favor of using processors where there... [Read more]