Analyzing a LED VU Meter circuit

Hi everyone, this is my first post around here. I hope to learn as much as possible and be helpful to everyone.

However, I need some help analyzing a circuit (maybe I should be posting in the Audio Electronics section, but I guess this is right for now.)

I'm building a LED VU Meter which I want to connect to my computer (or cellphone) and display the level of sound as it's being played, For that, I'm using the schematic found here: http://pira.cz/enbar.htm

I've gone ahead and copied the schematic into CL and used Fritzing to setup a breadboard with the circuit. The CL schematic is here:

What I need is to get maybe a step-by-step explanation of what the circuit is doing, based on the inputs and the various components. I'm hoping to be able to model the circuit using differential equations, but the number of components plus the use of ICs will probably make that a little difficult. (I will be keeping this circuit for personal use, but I also need the equations as this is part of an assignment for a course.)

Also, any corrections, suggestions or ideas are very much appreciated.

Thanks in advance to everyone for your help.

by kenshin23
October 06, 2013

@kenshin23,

Welcome to CL.

Describing how this circuit works using differential equations is a bit of a stretch because it has three sets of non-linear operations in it.

The first is D1, which with C5 and R3 forms a simple half wave rectifier.

The second is the linear to log (dB) conversion of the LM3915.

The third is that the LM3915 is effectively a form of digital to analogue converter and so is perhaps not best described by differential equations.

You might start off by having a good read of the datasheets for the two main devices:

http://www.ti.com/lit/ds/symlink/lm386.pdf

http://www.ti.com/lit/ds/symlink/lm3915.pdf

Very briefly, the LM386 amplifies the audio input signal. D1 peak detects it with an RC decay set by R3*C5. The peak detected signal is then fed into a resistor ladder with taps at exponentially increasingly spaced intervals.

Each tap is fed to the inverting input of an open collector output comparator. The non-inverting input of each comparator is fed to a reference voltage. The output of each comparator is fed to a LED through a series current limiting resistor.

As the input signal amplitude increases, the peak detected voltage increases and so more comparators in the ladder turn their outputs on (they go low) so more LEDs turn on.

The RC decay is to stop the output just registering the peak and then never dropping down again. The meter would then just dispaly the highest peak in any input sequence and not show the dynamic behaviour of the signal.

To get the hang of how it works, you can model the LM386 very simply in CL as a fixed gain block. Follow it by the existing rectiifer circuit and then model the LM3915 as a log scaled resistor ladder driving a set of comparators the outputs of which then drive the LEDs.

That might help give you enough insight to do the assignment without it turning into a PhD thesis ...

:)

by signality
October 06, 2013

Hi @signality, thank you very much for the thorough explanation and guidance. It helped me a lot to explain how the circuit works.

I tried testing the circuit on a breadboard (and checked, double-checked and even triple-checked the connections) but ended up with a burnt 12 V transformer and possibly the LM 386 as well

In any case, I rewired the circuit to use only the LM3915 with a 9V battery and it turned out ok.

Again, thank you so much for the help.

by kenshin23
October 10, 2013

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