Stompbox Real Sensors

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Lab 2: Controlling an Effect with Real Sensors
Designed by Edgar Berdahl

Due on Wednesday, July 20th at 9AM

Install the lab software

Power Connections

  • In order to run circuits on the breadboard, you need to get power to it. The Arduino gets 5V from the USB connection, so in this section you will make the 5V accessible to the breadboard.
  • Make sure that your Arduino is mounted at the very end of the breadboard with the higher-numbered rows, as shown in the following picture. In that case, the GND and 5V pins will end up in the 19th row. (Otherwise if your Arduino isn't mounted this way, you simply won't be able to rely on the row numbers, but you can still figure out how to wire things up!)
  • Recall first that that holes in the solderless bread board are wired together as shown:


  • Using short jumpers, connect the GND row to the blue "GND" bus on the breadboard, and connect the 5V row to the red "power" bus on the breadboard.
  • Connect the "GND" and "power" busses from both sides of the breadboard together as shown.


Viewed from further away, your breadboard should now look like the following:


First Circuit: Potentiometer

Now you will build the following voltage divider circuit, to connect one potentiometer to analog input A0.


For some help on placing the wires, please see the following picture. In order to ensure the correct orientation, ensure that the text on the potentiometer is facing away from the Arduino (see also oblique picture in next section). Circuit2.jpg

Add A Second Potentiometer

Now connect a second potentiometer to analog input A3 using the same voltage divider circuit.


Again, ensure that the text on the potentiometer is facing *away* from the Arduino (see the red boxes in the following figure):


Add A Button

Most stomp boxes have a button, so we include that as well according to the following circuit:


The corresponding picture of the bread board is shown below:


Add An LED

Finally, we add an LED for fun and to help with debugging.


Test The Tremolo Effect

Now you will test a very simple tremolo effect.

  • Connect headphones or some small loudspeakers to the audio out jack on the Beagle Board. (It is the one closer to the middle of the board.)
  • Connect an input audio source, such as a small MP3 player or the audio output of your laptop, to the audio in jack on the Beagle Board. (Maybe if we plug in the microphones half-way they will work and we could use them instead?!?)
  • In the terminal, change to the directory for this example by typing cd ~/stompbox/simple-stompbox
  • Then type qjackctl & and press the "Start" button to start the Jack audio server.
  • Open pd by typing pd &
  • Open the patch simple-stompbox.pd using the File|Open menu option.
  • Start audio signal computation by checking the compute audio button in the main pd window.
  • The oscillator's output should be displayed by the horizontal slider and LED in real time. (If not, please go back and make sure you carried out all of the previous instructions.)


  • Verify that turning the potentiometer connected to input pin A3 on the Arduino adjusts the value A3 shown in the main patch window. This should also change the output volume of the effect.
  • Verify that turning the potentiometer connected to input pin A0 on the Arduino adjusts the oscillator frequency.
  • Look inside the subpatch pd user-interface, which access the Arduino, and figure out how you could access data coming from the other analog inputs. Would you need to add an outlet to the subpatch?
  • Verify that when you press the button, the mode checkbox changes its state. Modify the effect to so that the mode checkbox implements a bypass switch. We will ask some volunteers in class the following morning to explain how to do this.

Build A New Effect

  • Build a new effect whose parameters are adjustable by the potentiometers and button. We will ask for volunteers to demonstrate this the following morning in class. You can build on what you did in lab 1 if you like.
  • Think about the result
    • do the controls allow the entire space of parameters to be explored?
    • is it easy to choose any desired set of parameters?
    • does the effect sound good?
    • does it have dynamics?
    • would you want to use it live?
      • If not, what is missing still?

Appendix: Install Stompbox Firmware onto your Arduino Nano

If this wasn't already completed for you before starting the lab, then you must do it yourself!

  • Change to the ~/stompbox directory by typing cd ~/stompbox at the console.
  • From the terminal, run the command arduino & to start up the Arduino software.
  • From the menu Tools|Board choose Arduino Duemilanove or Nano w/ 328
  • From the menu Tools|Serial port choose /dev/ttyUSB0
  • Open the sketch ~/stompbox/simple-stompbox/FirmataAllInputs_ExceptOnePWMOut/FirmataAllInputs_ExceptOnePWMOut.pde
  • Press the Verify button.
  • Check the Arduino messages window to make sure that you have successfully installed the firmware. (Usually, it will say something like Done compiling and Binary sketch size ...)

Important Note

Some small amount of the text and images here was taken from prior laboratory exercises for the course Music 250A. We regret that we do not know who contributed these elements, but these people are likely to include Wendy Ju, Bill Verplank, Michael Gurevich, and possibly more.

Stompbox 2011