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• Piezo To ADC Voltage Mapping, The Easy Way

  Jeremy • 05/25/2018 at 05:42 • 1 comment

  In order to read the piezos voltages in real time, we are going to use a RaspIO Analog Zero board. The board uses an MCP3008 analog to digital converter (ADC) that has an input voltage range of 0 to 3.3V.

  As a consequence, we need to map the output voltage of each piezo sensor to the 0-3.3V range. There are several ways to do that, but we're wondering here what's the easiest solution. And by the easiest, we mean the solution that requires the less components.

  First of all, we need to remember how a piezo sensor works. In short, we could say that a piezo sensor converts a mechanical strain into a voltage. But what happens when this voltage source is connected to a load? 

  Like any voltage source, a piezo sensor has its limits, and can only deliver so much current when connected to a resistor. When the load is too small, the current going through it becomes too high for the piezo to maintain a high voltage, thus the voltage decreases.

  Does that means that connecting a small resistor between the legs of a piezo decreases its output voltage? Yes, but not only: it also modifies its response, but in our case, we will assume that this effect is not a limitation as we only need to detect voltage peaks.

  Because the piezo's output voltage undergoes positive and negative peaks, we need to set a resting point of 1.65V (3.3/2 V) to be connected to our ADC. That can be done using a potentiometer, like so: 

  Let's consider a 10k potentiometer. In that case, if we use a 5V power supply, the potentiometer acts as a voltage divider:  

  Since we know that R1 + R2 = P = 10kΩ, if we want Vout to be 1.65V, we need R2 to be 3.3kΩ:

  So by using a potentiometer, not only we've managed to set a 1.65VDC input voltage at the input of the ADC, but we've also added a load (R2) in parallel with the piezo. The sensor is thus seeing a resistive load of 3.3kΩ (if we neglect the input impedance of the ADC), which means that its output voltage will be a lot weaker than if it was directly connected to the ADC.
  

  My first experiments show that, under normal conditions, no damping of the piezo is required. But in order to prevent the voltage from going above 3.3V or below 0V, I'm probably going to dampen it a little bit.
  

  Sounds that I'm lucky (or maybe it was all planned), as there is a potentiometer board that has eight 10k potentiometers. I've already bought one, which I found on Mouser (reference 932-MIKROE-316).
  

  Next time we'll see how to use this board to connect our 8 piezos to the MCP3008.
  

• Here are the sensors!

  Jeremy • 05/01/2018 at 14:15 • 0 comments

  I ordered 20 of those piezos: http://www.puiaudio.com/product-detail.aspx?categoryId=5&partnumber=AB1541B-LW100-R. I got those particular ones for two reasons: PUI Audio is usually pretty good quality, and they are small (15mm diameter).

  Here's a picture of most of them:
  

  They are exactly 15mm wide, which is perfect. Another consequence of their small size is that their output voltage is lower than bigger sensors. That's going to be useful in the future, but I'll explain why in another post. They are also pretty cheap, as I got them on Mouser (ref: 665-AB1541B-LW100-R) for $22 ($1.10 a piece).
  

  Well let's get to work now!
  

• Initial Project Plan

  Jeremy • 03/18/2018 at 16:48 • 0 comments

  Here's a list of things that need to be done for the project:
  

  • Add build instructions.
  • Develop the software and put it on Github.
  • Make the device's fabrication easy: make it out of cardboard.
  • Choose the sensors (piezo).
  • Design the sensors interface circuit, and make it.
  • Design the graphical user interface.
  • List of features (non exhaustive): 
    • Visualize and scroll notes in real-time (rhythm games style).
    • Progressive training mode which adapts itself to the player's level.
    • Import and export music sheets.
    • Possibility to manually change the tempo, and add a click sound.
    • Training mode that evaluates and rate the user's skills: jitter, velocity, etc.
    • Manage chords, and complex melodies.
    • Save playing stats, and show progression, parts to improve, etc.
    • Advanced training exercises to improve specific skills: velocity, rhythm, etc.
    • Rating system that is saved every time you play.
    • Complete stats logs.
    • Compatible with all the eXa software suite to allow music composition.
    • Probably some other things to add...

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