Module 5. Arduino for Data Collection

Introduction

Now, we are ready to start working with microcontrollers. We used the Arduino Nano which is a small board based on the ATmega328P microcontroller, but any Arduino Unos work in pretty much the same way.

To directly program the Arduino Nano, you will have to use C++. Learning two programming languages at once is not easy. What we have done instead, is upload code to the boards so that you can control it by sending commands from your computer through a serialized interface called UART (see definition below). In Python, you will write code that runs on your computer, and that will send messages to the Arduino to control it.

We will use the Arduino-Python3 library to achieve this.

This module relies on a lot of concepts and technologies, some of which you may not have heard of before. There are glossary and theory sections at the end of the lab that you can reference.

Task 1: Getting Familiar with the Breadboard

To understand how the breadboard is connected, use a multimeter to probe the pins. Some questions to consider:

• Which setting on the multimeter should you use to determine which pins are connected and which are not?
• How do you reliably obtain contact with the metal under the plastic cover of the breadboard with the thick multimeter probes?

Draw a picture of the breadboard and its internal connections in your lab notebook.

Task 2: Controlling an LED

We start by using the microcontroller to drive an LED.

Circuit

• Place a 100 to 500-ohm resistor in series with the LED. The resistor limits the current moving through the LED so that it does not overheat and get damaged.
• Make sure to connect the cathode of the diode to ground. The cathode is the side with the shortest leg and with a small flat notch on the diode housing.
• Notice that we are connecting the LED to pin D3. This will be important in the next step when we want to control this pin.

After you are sure you have connected everything correctly, you can connect the Arduino Nano to your computer using the USB cable. You will see a red status LED on the Arduino board turn on to indicate the Arduino is powered on and ready to execute your commands.

Programming

Now, you can control the Arduino by typing the following lines of code into a new file in Spyder. The program will establish a connection with the Arduino over UART and set the external LED to be on.

A variable PORT_NAME is used here, whose value you must update to correspond to the correct port on your computer. You can find the port by following the checking the Device Settings (Windows) or the line of code given below for a Mac. The Arduino library may be able to find the port name automatically. You can try this by using the default constructor without any arguments board = Arduino(). If you get an error message “Could not find port”, you will need to be explicit about the port name and/or data rate with the form board = Arduino(‘9600’,port=PORT_NAME) – see example below.

# import libraries
from Arduino import Arduino 
import time
PORT_NAME = 'COM3'                   # example of Windows port name 
#PORT_NAME = '/dev/tty.usbserial-1420' # example of Mac port name
board = Arduino('9600',port=PORT_NAME)               # connect microcontroller 
#board = Arduino()              # may be able to find the port without being told
print('Connected')              # confirms the microcontroller has been found
board.pinMode(3, 'OUTPUT')      # configure pin D3 to be an output pin 
board.digitalWrite(3, 'HIGH')   # make LED light up
time.sleep(30)                  # lights LED for 30 seconds
board.close()                   # close the connection to the board

Note:

• You must type ‘OUTPUT’ , ‘HIGH’ and ‘LOW’ with the capital letters.
• If you have problems, try disconnecting and reconnecting the Arduino or restarting the kernel in Spyder.
• If the LED does not turn on, you can try turning it around to see if it was inserted the wrong way.

Flashing LED

You can now expand your program to do something more interesting. What about making the LED flash on and off continuously? To achieve this, add the following to the code you just wrote:

# enter infinite loop 
     while True:
     board.digitalWrite(3, 'LOW')  # set pin LOW (0V) 
     time.sleep(1)                 # wait 1 second 
     board.digitalWrite(3, 'HIGH') # set pin HIGH (5V) 
     time.sleep(1)                 # wait 1 second

This is what is called an infinite loop – a loop that will never stop executing. To terminate the program, you have to click the red square stop button located in the top right corner of the Spyder Console.

Controlling LED brightness

Imagine that the above code was executed with a very small delay between each time the LED was turned on and off. It is going to appear as though the LED is half as bright, as it is only turned on half the time. By altering the ratio of time the LED is on to how long it is off, we can further control the brightness of the LED.

The function analogWrite continuously turns the output on and off, with a duty cycle specified by the second argument to the function. D = 255 corresponds to a duty cycle of 100% (fully on), and D = 0 corresponds to a duty cycle of 0% (fully off).

Alter your code to use the analog print function. It could look something like this:

 

from Arduino import Arduino
PIN = 3             # pin that the LED is connected to 
PORT_NAME = 'COM3'                   # example of Windows port name 
#PORT_NAME = '/dev/tty.usbserial-1420' # example of Mac port name
board = Arduino('9600',port=PORT_NAME)   # find and connect microcontroller
print('Connected')  # confirms the microcontroller has been found
board.pinMode(PIN, 'OUTPUT')    # configure pin to be an output 
pin board.analogWrite(PIN, 200) # set LED to half brightness
board.close()       # close serial connection

 

Task 3: Controlling an RGB LED

We are now ready to look at an RGB LED. This is a device that contains a red, green and blue LED in one package. Controlling the relative brightness of the different color-channels will enable us to display just about any color.

To be able to set the brightness of each pin, we will have to use the PWM enabled pins of the Arduino Nano. You can use pins D3, D5, and D6 as shown in the circuit below.

Remember to include current limiting resistors in your circuit.

The following code will continuously flash the three primary colors for one second each.

Then try: except block is part of Python’s error handling functionality. Placing your loop inside this statement enables you to use the shortcut ctrl + c to terminate the program (the console must be active for this to work). This shortcut is very commonly used to terminate program execution.

The except block also closes the serial connection to the board properly whenever the user terminates program execution.

# import libraries
from Arduino import Arduino
import time

PORT_NAME = 'COM3'    # example of Windows port name 
#PORT_NAME = '/dev/tty.usbserial-1420' # example of Mac port name 
board = Arduino('9600',port=PORT_NAME) # find and connect microcontroller
print('Connected') # confirms the microcontroller has been found

# give pins names, so they are easy to reference 
RED      = 3
GREEN = 5
BLUE  = 6
# configure the pins as outputs 
board.pinMode(RED, "OUTPUT") 
board.pinMode(GREEN, "OUTPUT") 
board.pinMode(BLUE, "OUTPUT")
# turn all LEDs off 
board.analogWrite(RED, 0)
board.analogWrite(GREEN, 0)
board.analogWrite(BLUE, 0)

Then try:

 

while True:
     board.analogWrite(RED, 255)     # set RED to full brightness 
     time.sleep(1)  # wait 1 second

     board.analogWrite(RED, 0)       # turn RED off 
     board.analogWrite(GREEN, 255)      # set GREEN to full brightness 
     time.sleep(1)                                # wait 1 second

     board.analogWrite(GREEN, 0)     # turn RED off 
     board.analogWrite(BLUE, 255)       # set GREEN to full brightness 
     time.sleep(1)                                # wait 1 second

     board.analogWrite(BLUE, 0)      # turn GREEN off

# press ctrl+c while the console is active to terminate the program

except:
     board.close() # close the serial connection

Exercises

You can now modify the code.

• Can you make the LED be orange?
• What about making the color of the LED change gradually?

Theory

DMM

When we start to work with hardware, it is important that we have the tools to visualize what is happening. This enables us to understand what is going on at all steps, and it gives us the ability to debug the system systematically if something is not working as we expect it to.

• The DMM (digital multimeter) is a great tool for measuring any parameter that does not change with time, such as resistance and current and voltage at DC (direct current).
• The oscilloscope is designed for measuring voltages that vary over time. This enables you to observe voltages at AC (alternating current). The oscilloscope also makes it possible to capture what is commonly referred to as transient behavior – that is, events that happen for a limited period of time as an external trigger moves a system form one equilibrium position to another.

Breadboard

A breadboard is a handy tool for prototyping. It allows us to connect components into circuits reliably without the need to solder. The internals of a breadboard is connected as shown in the image below. It is usual to use the horizontal traces on the side of the board as power (red) and ground (blue).

Note: For large breadboards, the power and ground rails may be split in the middle.

LED

Light emitting diodes output light while drawing very little current. Being diodes, they have a polarity. They only admit current to pass one way, from the anode to the cathode. You can see which side is the cathode by finding the side with a flat notch and shorter leg.

It is also important to ensure that the voltage across the diode does not exceed what it can withstand without overheating. LEDs typically have a forward voltage drop of between 1.8V and 3.3V and consume about 20mA during normal operation. Thus, as we want to drive them from 5V pins, we need to use a current limiting resistor in series with the diode. This resistor should be around 100 to 500 ohms.

Digital logic

All computers (with the possible exception of quantum computers) are based on binary digital logic. That is, there are only two states that signals can assume in the system. These two signals are referred to by many names: 0 and 1, OFF and ON, LOW and HIGH and so on. This system allows computers to make complicated decisions based on simple yes/no questions.

For the Arduino Nano, 5V is HIGH, and 0V is LOW.

Arduino Pins

The Arduino Nano has many pins that each can be configured to perform some function. Not all pins are equal. The abilities of each are written on the board for convenience. The most important ones are the following:

• GND means the pin is connected to the ground voltage level of the microcontroller.
• Vin means 5V, as that is the voltage we are powering the microcontroller with.
• D followed by a number (such as D13) signifies a digital pin. These pins can be configured to output LOW (0V) or HIGH (5V). They can also be used to read if the voltage applied to the pin is HIGH or LOW.
• A followed by a number (such as A2) are analog pins. In addition to being used as digital pins, they are capable of reading any voltage between 0V and 5V.
• The PWM capable pins of the ATMega328P are the digital pins 3, 5, 6, 9, 10, and 11. These are the only pins where you can use analogWrite() .

AnalogWrite

Because the Arduino is a digital device, it does not know how to produce analog voltages. That is, voltages other than 0V and 5V. To produce an intermediate voltage, we can, however, use a trick called PWM (pulse width modulation). By turning the pin to HIGH and LOW very rapidly and varying the amount of time spent in the HIGH state relative to the time spent in the LOW state, it is possible to manipulate the average voltage so that it becomes what we want. If we want to produce 2.5V for example, we just have to spend as much time in the HIGH state as in the LOW state.

The amount of time spent in the HIGH state vs. the LOW state is characterized by a parameter called the duty cycle D.

Note that this is not a true analog voltage. For motors, diodes and other devices, it is OK to use PWM to achieve an analog voltage. It is not possible to use this trick for many other applications, such as powering digital circuits.

The Arduino-Python Library

The source for the Arduino-Python3 library can be found here. This repository includes a description of features and the source code that you can have a look at to figure out what other functions you can use with the Arduino board. In the next module, we will use the pulseIn_set function to measure the distance to objects using an ultrasonic range finder.

Terms

The following list gives a brief explanation of each of the main concepts, with a link you can follow to find more information.

Microcontrollers are small computers where all the primary components have been placed in a single integrated circuit.

Arduino is the name of an Italian company delivering open source microcontroller hardware and supporting software. They have acquired a large community of users and developers, and the name Arduino is now synonymous with their development boards.

C++ is an important programing language. Arduino boards are programmed using a flavor of C++.

UART (Universal asynchronous receiver-transmitter) is a computer hardware device used for asynchronous serial communication.

Serial communication is a method of communicating information. It encodes the information you want to send into a sequence of bits (zeros and ones) and transmits them sequentially over a communication channel.

LED (light-emitting diode) is a diode that produces light when an electric current passes through it. Being a diode, it only allows current to flow in one direction.

RGB is an abbreviation for the primary colors: red, green, and blue. Combining these colors virtually any color can be produced.

RGB LED is a device where both a red, green and blue LED has been placed in one package.

 

Modules

1. Buying Parts
2. Preparing the Arduino
3. Setting up Python on your PC
4. Learning Python
5. Arduino for Data Collection
6. Ultrasonic Range Sensing
7. Pendulum Experiment

 

Updated 2021-05-04  CEW