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Audio VU Meters & Raspberry Pi

using VU Meters to monitor system activity on Raspberry Pi

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Webinterfaces to monitor your RPi are neat and useful, but what if you want to have an analog display that physically shows you what your RPi is doing?

There are many explanations out there that show how to use a small LCD Display to monitor your Raspberry Pi, but I haven't found any about using a VU meter.

This project is especially useful for people who salvage old hifi equipment to create a casing for a raspberry pi. Use the old fashioned VU meters to give your raspberry pi a more hipster look :)

This is a beginner friendly tutorial with lots of examples.
We will explore different methods of driving the VU Meters using PWM and DACs.

Story

I started building a NAS from my old hard drives and got a Raspberry Pi as server. Then all the wires and everything was just laying around randomly and I thought that I should make a nice case for it.

First, I wanted to reuse old hifi equipment for that, but I couldn't find any cheap and broken device to salvage.

So I decided to get a custom made laser cut case and thought how I can add some nice elements that are not only decorative, but also having some extended functionality.

I like analog devices. Big nobs, needles, and there like, so I came up with the thought of adding some VU Meters to the case to have my system activity monitored.

I got some cheap ones from Taobao, but I think you can also find them on Banggoodor Aliexpress. Taobao is even cheaper than other online portals, but everything is in Chinese :)

Beginner Friendly Tutorial

If you go to the instructions section, you can find a full step-by-step guide that explains the whole process.

When I was finally done with the whole project, I thought that I should document it. And because it touches on so many different subjects (electronics, python, interfacing, GPIOs, PWM, etc), I thought I can make it as beginner friendly as possible.

Further Information

I got the basic know how on reusing old meters from the Post Apocalyptic Inventor, you can watch his video here:

  • 1 × Raspberry Pi 3 Model B+ Should work on any Raspberry Pi
  • 2 × VU Meters Any analog VU Meter will do, ideally salvaged from old hifi equipment. Technically you can also use a voltmeter that is rated for 0 - 5V.
  • 3 × Trimpots You should be able to adjust resistance between 0 and 20k Ohms. Maybe use more trimpots to get a finer adjustment range
  • 1 × Multimeter
  • 1 × Bench Power Supply ideally can support low voltages of less than 3 V. Best: can step up in 10mv increments

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  • 1
    Analyze your VU Meters

    Firstly, we need to find out the range, sensitivity and specifications of our VU Meters. Those can vary greatly, depending on the manufacturer, type and previously intended usage.

    A VU Meter is basically a low current ammeter. For this application we will focus on the voltage required to gain a specific current flow through the VU Meter. Remember that Ohm's Law states that the Current is depending on the voltage and the resistance of the circuit. You can read all details about VU Meters on Wikipedia.

    VU Meters operate in a quite low voltage of around 100 - 1000 mV. Therefore, the better your bench power supply, the easier it gets.

    Let's get some basic measurements:

    1. Measure your VU Meter's internal resistance using a multimeter. My VU meter has an internal resistance of 655 Ω

    2. Attach DuPont Wires to your VU Meter

      I directly soldered female DuPont Wires to my meters

    3. Connect your VU Meter to your bench power supply and have at least 3 trimpots in series, but do not switch on your power supply yet. My bench power supply lowest reliable voltage is 1.24 V. If your's can go lower, it is even better.

    4. Set your trimpots to the highest resistance

    5. Attach your multimeter to the positive and negative terminals of your VU Meter

    6. Switch on your power supply

    7. Slowly decrease the resistance of your trimpots until your VU Meter shows maximum deflection (needle goes towards the maximum value).

    8. Measure the voltage across the VU Meter. In my case: 0.443 V or 443 mV

    9. Measure the resistance across all your trimpots. In my case: 1190 Ω

    VU Meter Gauging Circuit

    Schematic 1: VU Meter Gauging Circuit

  • 2
    Optional: Calculations for the Geeks

    Now that we have some basic specifications of the VU Meter, we can make some calculations.

    The current required for the needle to move is calculated using Ohm's Law:

    Ohms Law

    Where I is the current, V is the voltage, and R is the resistance:

    670uA

    This is useful to know if you have a different Voltage value from your power supply.

    Using a 5 V Power Supply

    Let's assume you have a 5 V power supply and want to get a ballpark estimation of the required resistance of your trimpots:

    r trimpot

    Therefore we can estimate:

    6796ohms

    That you need an resistance of about 7000 Ohms in order to drive your VU Meter properly with a 5V power supply.

  • 3
    Considerations for driving the VU Meter by the Raspberry Pi

    Now that we understand the VU Meters, let's have a look at how to connect and drive them from the Raspberry Pi.

    By default the RPi has a 3.3 V or 5 V supply rail. That would be total overkill for the sensitive VU Meters. A regular GPIO Pin delivers 3.3 V if it is set to "HIGH", so that would be also too much and not variable.

    So let's have a look at how we can get that 3.3 V to a much lower value and also change the output value:

    PWM (Pulse Width Modulation)

    The simplest way to control the voltage (and effective current) applied to the VU Meter is by using PWM (Pulse Width Modulation).

    There are two ways of creating a PWM Signal from the RPI:

    1. Software PWM (fast & simple, can use almost any GPIO)
    2. Hardware PWM (special python module, can use limited GPIOs)

    In the steps below, I will go into detail for both ways.

    DAC (Digital to Analog Converter)

    The RPi doesn't have any DAC integrated, so we need to hook one up.

    I will go into different DACs at a later point. E.g. how many VU Meters we want to control is another consideration for choosing a DAC.

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