Arduino Crash Course – Lesson 5

Parts Needed

• Arduino Uno
•  Servo Motor 

If you didn’t buy the kit from Elegoo (or some similar kit made for arduino), please ensure the Servo motor can run with 5V in and does not require more current than an Arduino Uno GPIO pin can supply (40mA per pin).

PWM – Pulse Width Modulation

Pulse Width Modulation (will be referred to as PWM from here on in this Arduino Crash Course) is just a fancy word that represents turning on/off a digital signal usually at a high rate.  PWM is used in many different ways, such as controlling motors, reducing energy waste when converting power, to controlling ICs or even LEDs. 

Before we go further, we must understand some terms.  A period is the amount of time in one cycle and is measured in Hz (Hertz).  So if you have a 100KHz frequency PWM at 50% duty cycle, that means that each period is 10µs consisting of 5µs of digital on (HIGH) and 5µs digital off (LOW).

We can see other examples in the following picture (from wikipedia):

By looking at this, you may be visualizing how it can be used in a way to be more efficient in power conversions.  Hopefully you can also visualize how you can use PWM as a way to communicate.  If you have ever seen what have been called NeoPixel LED strips, the WS2812B IC connected to each LED uses a similar communication protocol to control the Red/Green/Blue of each LED.  We may get to controlling these in a future lesson in this Arduino Crash Course (or part of a more advanced post), but if you are interested the strip looks like this (I have this exact set and works great):

NOTE: You cannot power these (or most LED strips) with Arduino’s directly, but you can control them.  There are plenty of resources online if you want to try to build something with them.

PWM can also be used to simulate Analog, in essence you can make a simplified DAC (digital to analog conversion).  But being that DAC ICs are relatively cheap, and much more accurate than an Arduino can be, building a DAC using PWM from Arduino should be done strictly as a learning exercise and not part of a final design solution.

Back To Servo Motors

As stated earlier in this lesson, a servo motor has 3 connection wires: a ground (usually brown or black), power (usually red), and a control/signal (usually orange). 

Luckily for us, there is a Servo library available to allow us to focus on what we want to do as opposed to bog down on controlling the servo via PWM: https://www.arduino.cc/en/reference/servo. But as should be expected if you have been following along with this Arduino crash course, we don’t simple but we want to learn.  So let’s delve into the library code to see exactly what it is doing (https://github.com/arduino-libraries/Servo).

Let’s start with the Servo.cpp write function:

void Servo::write(int value)
{
  if(value < MIN_PULSE_WIDTH)
  {  // treat values less than 544 as angles in degrees (valid values in microseconds are handled as microseconds)
    if(value < 0) value = 0;
    if(value > 180) value = 180;
    value = map(value, 0, 180, SERVO_MIN(),  SERVO_MAX());
  }
  this->writeMicroseconds(value);
}

In this function, we see that we are resetting any value < 0 to 0 and > 180 to 180.  Then we see the map function which converts the degrees value (from 0º to 180º) to the servo min and max modulation (more details on this map function https://www.arduino.cc/reference/en/language/functions/math/map/).

If you peer into the writeMicroseconds function of the Servo class, you will notice a call of analogWrite().  analogWrite is the PWM magic (handled by Arduino library for you): https://www.arduino.cc/reference/en/language/functions/analog-io/analogwrite/.  

Please note, the Arduino Uno uses PWM on pins 3, 5, 6, 9, 10, 11 with a frequency: 490 Hz (pins 5 and 6: 980 Hz).  Also, since Arduino is 8 bit, the range you can send via PWM is 0-255 (2^8 – 1).  

Side Note: You may notice as you are perusing the documentation that the analogRead takes values between 0 and 1023.  Why? Well because the Arduino has a 10bit ADC (analog-to-digital converter).

Let’s Build

The wiring is very simple.  So why did we choose pin 9?  Well we need to use a pin that has PWM (again, reference https://www.arduino.cc/reference/en/language/functions/analog-io/analogwrite/ to know what pins can send PWM).  

Like said earlier in this lesson, and hopefully as you perused through the Servo library, you have confirmed that the Servo motor’s position is controlled by the pulse width.  The smaller the pulse width, the more counter-clockwise the servo motor turns and vice versa. 

Now that we have wired up our servo motor, try to think of code that will make the motor sweep from min to max to min and repeat.

Great! All Wired Up! Let’s Code!

Let’s go through the GPIO pins we are using on the Arduino and note where it’s connected:

• Pin 9 – Output – Control of Servo (orange wire)

For the code, we look at the reference for Servo again and declare our Servo and a variable to hold the current position: 

https://www.arduino.cc/en/reference/servo

#define SERVOPIN 9

#include <Servo.h>

Servo theMotor;
int pos = 0;

Now our setup method is rather straightforward, we want to call the attach to our motor instance to the pin the motor is attached to.

void setup() {
  theMotor.attach(SERVOPIN)
}

Finally, we want to write our loop function.  Remember, servos can only go from 0º to roughly 180º.  So to go from min to max back to min, we just want to loop through each degree (we will increment/decrement by 5º at a time).

Let’s get writing!

void loop() {
  for (pos = 0; pos <= 180; pos += 5) { 
    theMotor.write(pos);              
    delay(30);                       
  }
  for (pos = 180; pos >= 0; pos -= 5) { 
    theMotor.write(pos);              
    delay(30);                       
  }
}

What’s with the delay?  Well, even though we set are commanding the motor to move 5º at a time, it takes time for the motor to physically move from where it was to where it needs to be.  One thing to take note of is the speed of the servo is the same no matter how many degrees it needs to rotate (try it out!).

What’s Next

Great! Another lesson down in the Arduino Crash Course.  We now know how to control servo motors! How can we expand this simple wiring to be more beneficial?

• Connect 2 buttons to the arduino: a forward and reverse button, if the forward button is pushed, move the servo xº clockwise (increase the position) and if the reverse button is pushed, move the servo xº counter-clockwise.
• Use the serial communication to print out the position (in degrees) that represents the state of the servo
• If you are brave, try controlling the Servo using just the analogWrite function (i.e. don’t use the Servo library).

I hope you enjoyed this lesson and the excitement is continuously growing as you work through this Arduino Crash Course.  See you next time!  Again, if you are enjoying this Arduino Crash Course, please share with others and join us on Facebook.