# Single LED VU Meter

Hi, I am building a project where a single LED(or several LED in parallel) is expected to behave like a VU meter. I would like the brightness of the LED to represent the loudness. Accuracy is not important but at least I need logarithmic indication. I would like to know what is the most elegant way to achieve this.

First, the famous LED bar driver LM3915 came to my mind. I think maybe I can combine all the output pins and connect them to the cathode of the LED, then limit the current of each pin to around 1mA. Maybe this would allow the LED to have 10 steps of brightness from 1mA to 10mA. But here's the problem:

I observed the Vmax of my audio signal is around 300mV. According to the datasheet's example configuration, the IC's high reference voltage(when all LEDs light up) can be adjusted down to 1.25V. Must I feed the IC with the amplified signal? It requires extra components and space. Or maybe I can set Vref down to 0.3V, but would it cause unbearable bad accuracy?

I don't know if using an IC is overkill, but I haven't found a discrete solution with logarithmic support. I would be grateful if you can give me some direction.

The LED is already logarithmic if the driving current is defied by your audio signal.

So you only need a voltage-to-current converter (with an op-amp).

Are you sure it has to be logarithmic? A regular-old electro-mechanical VU meter has a linear movement with a dB scale. Decibels are logarithmic so just the markings are logarithmic.

I’d try a [u]peak detector[/u] (AKA envelope follower) preceded by a gain stage to see if you get a “useful” result. You can adjust the peak detector’s RC timing depending if you want it to flicker or “fade”.

Or, if you wanted to use an Arduino you could use software to both adjust the sensitivity and do the log conversion.

…If this is just a “fun effect”, and not really a meter intended to measure loudness, take a look at [u]The World’s Simplest Lighting Effect[/u].

Or maybe I can set Vref down to 0.3V, but would it cause unbearable bad accuracy?

No the data sheet for the processor says that 1V is the lowest you can set the Vref.

For brightness control of a single (or parallel) LED typically PWM is used. No further chips required, only a current limiting resistor for each LED, and a transistor for switching everything. In this case the logarithmic scale has to be implemented in software.

Given that the eye’s response is logarithmic and the ear’s response is logarithmic then I would expect a simple linear driver to work without any extra messing about.

Grumpy_Mike:
Given that the eye’s response is logarithmic and the ear’s response is logarithmic then I would expect a simple linear driver to work without any extra messing about.

Which is what I pointed out in #1.

But you need a voltage-to-current driver.