Build Instructions

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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

• 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:

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

  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:

  Therefore we can estimate:

  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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