Talk:250a Microcontroller & Sensors Lab Pd

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Lab 2: Microcontroller and Sensors
Due on Wednesday, Oct. 12th at 5PM

For this lab you need your Satellite CCRMA kit. It is important that you have some time to focus on the final part of the lab "Putting It All Together." This is why some parts of the lab are optional.


Mount Arduino And Bread Board

  • Mount the bread board on your perfboard as shown in the picture below, so that the higher numbered rows are facing the USB and ethernet ports.
  • Insert your Arduino Nano into the bread board all the way at the end toward the higher numbered rows and seated roughly in the middle (see below).


Download Lab

  • Use the same procedure as described before to power up Satellite CCRMA and login as the user ccrma with the password temppwd.
  • Make an empty lab2 folder for this lab. You can do this at the terminal by typing

mkdir ~/lab2

Notice that in linux if you press the TAB key in the terminal, it will try to guess what path or filename you are typing. Use it to minimize your typing!

  • Download the lab to your kit by running the command


  • Extract the ZIP file by typing "unzip". Finally, move the result into your lab2 directory by typing "mv 250a_pduino-0.5 ~/lab2". (You can now delete the unnecessary ZIP file using rm

Prepare Arduino

  • Now see which devices are attached to Linux by running the command following command. They will appear the be files. Wow, there are so many!

ls /dev

  • To list only the serial interface devices type

ls /dev/tty*

  • Now use the retractable USB cable to plug the Arduino into one of the USB ports of the Beagle Board. As the Arduino powers up, you should see some lights come on.
  • Now run

ls /dev/tty*

again, and you should now see the new device /dev/ttyUSB0 appear also, which represents the Arduino Nano.

Troubleshooting: If you do not see /dev/ttyUSB0, then try rebooting using sudo reboot to see if that fixes this problem. (If you reboot, this will take about 45 seconds, and you will have to login again using ssh. If that doesn't work, come talk to us. If you are a Linux pro, you can try to debug the problem yourself by typing dmesg and looking at the result.)

  • The next step is to install some simple firmware onto the Arduino so that it knows what we want it to do. Start the Arduino software in the terminal by typing

arduino &

  • Open StandardFirmata from the Arduino software pull-down menus File|Examples|Firmata. Look at the program. This is what will control the Arduino.
  • Select Arduino Nano w/ ATMega328 under Tools->Board and /dev/ttyUSB0 under Tools->Serial Port. Then hit the Play button to verify and compile the program.
  • Upload the Firmata firmware to your Arduino Nano using upload button, the fourth square button from the left (the one with the sideways arrow). If you watch the RX and TX lights on the Arduino carefully, you will "see" the data flow over the serial USB link into the Arduino as the firmware is uploaded.
  • Close the Arduino program by closing all of the Arduino windows. (This is important because it frees up the USB serial port so that Pd can talk to the Arduino board later. Pd and the Arduino software will not share the Arduino Nano device at the same time.)

Routing Power On The Breadboard

  • When changing the circuit on the breadboard, it is a good idea to unplug the USB cable from the Arduino, so unplug it for now.
  • Remind yourself about how the sockets on the breadboard are wired together:


  • Now connect jumper wires as shown in the image below so that the upper and lower long columns can serve as a power supply for your future circuits.
  • Connect a short jumper from row 19 on the left (which connects to the GND pin of the Arduino) to the blue column.
  • Connect a short jumper from row 19 on the right (which connects to the 5V pin of the Arduino) to the red column.
  • Finally, use two longer jumpers to connect together the blue columns and the red columns as shown below:


  • Check your wires now to make sure that you did it correctly!
  • Now plug the USB cable back into the Arduino. You should see the blue light come back on on the Arduino. (If you don't, you have a short circuit! In other words, somehow you connected the wires together incorrectly, effectively connecting the 5V and the GND pins of the Arduino together. Unplug the USB cable again immediately and fix the circuit! Short circuits are big no-nos because they can (and often do!) destroy the Arduino. This is why we recommend unplugging the USB cable while you are changing your circuit.)

Buttons, Switches and LEDs

  • Build the circuit that is detailed in the following figure. Use components and jumpers to construct your circuits on the solderless bread-board. Here's how to wire a simple 2-resistor circuit on the solderless bread-board (for example R1 = 10K, R2 = 10K):


Use the voltage divider equation to calculate hypothetically what A0 should be.

Optional: Use the digital multimeter to check the voltage at A0. Set it to the direct current voltage "DVC" scale at 20V. On the multimeter, connect the black probe to COM and the red probe to V-Ohm-mA. Connect the other end of the black probe to ground and the red probe to the A0 wire. Also optional: Use the digital multimeter to find out what the "5V" supply actually is.

Build the Button and LED Circuit

We'll start our tutorial with three simple light circuits.

  • In the first one, the LED is permanently on.
  • In the second, the LED only lights up when a button is pressed and a circuit is completed.
  • In the third example, we'll replace the manual switch with an Arduino pin (set to output mode), so we can control the LED from our program.


Power a LED (always on)


Build the following circuit on your breadboard. Use a 220 Ohm resistor (red red brown gold -- see this page about resistor color codes).

Because the LED is a diode, it has a set voltage drop across the leads; exceeding this causes heat to build up and the LED to fail prematurely. So! It is always important to have a resistor in series with the LED.

Also, another consequence of the LED being a diode is that it has directionality. The longer lead, the anode, should be connected towards power; the shorter, cathode, should be connected towards ground. (In the photo, the longer lead has a bent "knee.")

Make a light switch

Next, we'll insert a switch into the circuit. The momentary switches in your kit are "normal open", meaning that the circuit is interrupted in the idle state, when the switch is not pressed. Pressing the switch closes the circuit until you let go again.


Use a multimeter to see what happens to the voltage on either side of the LED when you press the switch.

Toggling LED with Pd


In the third example, we'll replace the manual switch with an Arduino pin (set to output mode), so we can control the LED from our program. The safe way to do this is to let the Arduino pin sink current - if we toggle the pin low, it acts as ground and current flows through the resistor and the LED as it did in the previous examples. When we take the pin high, to 5V, there is no potential difference and no current flows - the LED stays off.

  • Open the arduinoLab.pd patch, which should be in the directory ~/lab2/250a_pduino-0.5.
  • First look in the pd message window. Probably you will observe some errors.
  • To get pd to connect properly to the Arduino Nano, you need to correctly select the serial port # (see the upper left-hand portion of the patch). This functionality allows you to address multiple Arduino Nano's connected at the same time. Right now we only have one. Try selecting different serial port #'s until you see a message like "[comport] opened serial line device 4 (/dev/ttyUSB0)."
  • Verify now that communication with the Arduino Nano is working by turning on and off digital output 13. This pin is wired directly to an orange LED on the Arduino, so you can easily "see" the state of the pin.
  • Go to the "digital pin data direction" pane and set pin 2 to output.
  • Then go back to the "digital outputs" pane. When you click on the checkbox for digital pin 2, the state of the LED should change (on vs. off).

Optional: Try changing your patch so the light stays on when you press the mouse button, and stays off when you press it again. After that, change your patch so the light blinks on/off. Then, have your patch button switch the light between on and blinking.

Sensing buttons in software

We've used code to trigger output - what about the other direction, sensing physical input in code? Just as easy. Here is a simple switch circuit:

Image: switch.png Image:ButtonCircuit.jpg

When the switch is open, the Arduino pin (set to input mode) is pulled to 5V - in software, we'll read Arduino.HIGH. When the switch is closed, the voltage at the Arduino pin falls to 0V - in software, we'll read Arduino.LOW. The pull-up resistor is used to limit the current going through the circuit. In software, we can check the value of the pin and switch between graphics accordingly.

  • Setup the circuit and connect it to digital pin 4 (D4).
  • Although the "digital pin data direction" in the pd patch shows that D4 is set to input by default, actually it is not! To change this, first set D4 to output and then change it back to input.
  • Then look at the "digital input readings" pane while you press the button. But you will notice that the pin 4 reading does not change!
  • This is because the corresponding digital input port needs to be enabled. In other words, even if you set a pin to input, you need to also enable the inputs for that port. Each port is mapped to eight pins. Because pin D4 is part of the first eight pins, you should enable port zero in the "enable digital input ports" pane.
  • Now when you press the button, you should see the digital pin 4 reading change in the "digital input readings" pane.

Fading LED (Optional)

What about those "breathing" LEDs on Mac Powerbooks? The fading from bright to dim and back is done using pulse-width modulation (PWM). In essence, the LED is toggled on and off rapidly, say 1000 times a second, faster than your eye can follow. The percentage of time the LED is on (the duty) controls the perceived brightness. To control an LED using PWM, you'll have to connect it to one of the pins that support PWM output - 9, 10 or 11 on the Arduino. Then write a patch that cycles the PWM values.

For instance, for a single LED connected to pin D9:

  • Connect the LED to pin D9 of the Arduino using the usual circuit.
  • In ArduinoLab.pd, set pin D9 to pwm in the "digital pin data direction" pane.
  • In the pane for "pwm/servo outputs," choose pin 9.
  • Pull the "analog_output" horizontal slider back and forth and watch the LED get brighter and dimmer.
  • Fun extension: Get an RGB (red/green/blue) LED, connect it to digital pins 9, 10, and 11, and adjust the brightness of the three color components to cycle the LED through a rainbow of colors!

Image: Lab2-7.jpg

Using Analog Sensors

Now we will work with the continuous input values provided by analog sensors - potentiometers, accelerometers, distance rangers, etc.

Make a Light Dimmer

In this example we'll build a light dimmer: a knob connected to a light so that when you turn the knob, the light increases or decreases in brightness. We'll use a potentiometer. The potentiometer has three terminals - the resistance between the first and the third terminal is constant (10k Ohms in our case). The resistance between terminals 1 and 2 (and between 2 and 3) varies as you turn the shaft of the potentiometer. If you apply 5V to terminal 1, connect terminal 3 to Ground, you will get a continuously varying voltage at terminal 2 as you turn the shaft (Why that is the case will become clearer once you've learned about voltage dividers further below).

  • Connect the middle pin of the potentiometer to analog input 0, the other two to +5V and ground.
  • Through a 220Ohm resistor, connect an LED to pin 9(anode or long side to resistor, cathode to pin 9)
  • In the ArduinoLab patch, press the "Light Dimmer" button to preset the outputs properly. The patch expects you to connect the LED to digital pin 9 (D9) and the potentiometer to analog pin 0 (A0). Use the PWM controls under output controls in the ArduinoLab patch to control the lightness and dimness of the LED.

Image: Lab2-8.jpg

  • Optional: hook up two LEDs, one red one green. as you turn in one direction, red gets brighter; in the other, green gets brighter. In the middle, both are off.

Voltage Dividers

In your kit, the potentiometer, IR distance ranger, and accelerometer are especially easy to work with since they directly output a changing voltage that can be read by one of Arduino's analog input pins.

Other sensors don't give you a varying output voltage per se, but instead change their resistance. Examples in your kit are the force sensitive resistor (FSR) and the bend or flex sensor. It is easy to get a changing voltage based on a changing resistance through a voltage divider circuit (Wikipedia page). The idea is that you put two resistors in series between power and ground: one that changes resistance (your sensor), and one of a known, fixed resistance. At the point in between the two resistors, you can measure how much the voltage has dropped through the first resistor. This value changes as the ratio of resistances between variable and fixed resistors change. More formally:


  • potentiometer:


  • force-sensitive resistor (FSR):


Try both circuits. Test the resistance range of your sensor. If you want 2.5 volts to be the middle, make the comparison resistor (33k in the diagram) the "average" value of the FSR's resistance. Test this with a multimeter.

  • Bend Sensor


Thresholding with a Range Sensor (Optional)

Thresholding is the process of turning continuous data into a discrete yes/no decision.

To learn about thresholding, we'll connect the IR range sensor. The circuit is trivial: just connect red to 5V, black to ground, and yellow to analog input A0.

Take a look at the data the sensor returns with sensor_graph_02 - when the field of view of the sensor is clear (no obstacle - point it at the ceiling), it returns a low voltage. Move your hand high over the sensor, then start lowering it - you should see the output voltage rise, until you are about 4" away. The sensor has a range of operation of 4"-30".

Let's do something useful with that data. Imagine a smart cookie jar that reminds you not to snack in between meals. We could put an IR ranger into the lid. Whenever a hand comes too close, our program could play a warning sound or flash a warning light. Write a patch to manage this!

Tilt control with an Accelerometer (Optional)

In this example, we'll simulate the motion of a ball on a tilting plane in software and control the tilt through a sensor. Think of it as a first step to build your own electronic game of Labyrinth. The right sensor to use is an accelerometer. Accelerometers can report on both static and dynamic acceleration -- think of static acceleration as the angle the accelerometer is held with respect to the ground (the acceleration measured here is due to gravity). Dynamic acceleration occurs when you shake the sensor.


The accelerometer in your kit is a 3-axis, +/- 1.5-6g accelerometer sensor (1g is the acceleration due to gravity). You will have to solder on 0.1" header pins to fit this into the breadboard. The connections you need to make are Vin to 5V power from the Arduino , G to ground, and Xo, Yo & Zo to the first three analog input pins on the Arduino board. My default, the acceleration range is set to +/- 1.5g; you could solder on the gsel jumpers if you want less sensitive/greater range with +/- 6g.

Now, get a feel for the data the accelerometer provides. Use the Light Dimmer presets, which will track analog values on A0 and A1. Then pick up the Arduino+accelerometer board and tilt it in various directions. Start by holding it so that the accelerometer board is parallel to the ground. Find in which direction the X reading increases and decreases; do the same for the Y reading.

Next, change the PD patch so that you can also read the Z axis of the accelerometer.

Putting It All Together

  • Create a patch to make sounds based on button and sensor values from the Arduino. You can try to adapt your patches from Lab 1, or come up with a new patch.
  • Try to make a simple musical interaction. Think about music -
    • does it have dynamics?
    • can you turn the sound off?
    • can it be expressive?
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