An Arduino controlled LED that can be turned on and off and flickers like a candle.

Please download the Arduino software from their website https://www.arduino.cc/

(You have to click "Just download")

• Windows: installer or .zip
• Mac: .zip
• Linux: {apt, yum, pacman} install arduino

• Long horizontal power supply connections. Split into left an right half.
• Shorter vertical groups of five pins.

Put together the following circuit on your breadboards:

If the LED does not light you should try swapping its leads around.

Remove the connection to the Arduino 5V pin and attach it to the Arduino pin 11 instead.

Open the Arduino IDE, configure your Board and enter the following program:

 1 void setup() {
 2   pinMode(11, OUTPUT);
 3 }
 4 
 5 void loop() {
 6   digitalWrite(11, HIGH);
 7   delay(500);
 8   digitalWrite(11, LOW);
 9   delay(500);
10 }

After uploading the program, the LED should blink with a frequency of 1Hz.

The statement delay(500) in the uploaded program determines how long the microcontroller should wait between the LED state changes.

In the case above it waits 500ms.

Lets see what happens if we decrease both delays by a factor of 100.

The Human eye does no longer perceive the LED as blinking.

Instead the LED seems to be lit less brightly than before.

By varying the proportion between the LED beeing on and the LED beeing of how bright the LED seems to be lit.

The Process of varying the on/off proportion to change the perceived brightness is called pulse width modulation (PWM).

The Microcontroller used on the Arduino can also do PWM in hardware.

The corresponding function is rather missleadingly called analogWrite(pin, value).

The following program will cycle through different led brightnesses.

 1 void setup() {
 2   pinMode(11, OUTPUT);
 3 }
 4 
 5 void loop() {
 6   analogWrite(11, 0); delay(1000);
 7   analogWrite(11, 63); delay(1000);
 8   analogWrite(11, 127); delay(1000);
 9   analogWrite(11, 191); delay(1000);
10   analogWrite(11, 255); delay(1000);
11 }

• Connect one pin of a button to one Arduino GND pin.
• Connect the other pin on the same side of the button to Arduino pin 10.

The LED gets dimmer when the button is pressed! It was supposed to be the other way around!

Executing pinMode(10, INPUT_PULLUP) connects an internal resistor to pin 10 pulling the voltage up
(digitalRead(10) == 1).

When the button is pressed the button shorts pin 10 to 0V (GND)
(digitalRead(10) == 0).

To invert the return value of digitalRead use
if (!digitalRead(10)) {.

 1 boolean is_on= true;
 2 void loop() {
 3   if (!digitalRead(10)) {
 4     is_on= !is_on;
 5   }
 6   if (is_on) {
 7     analogWrite(11, 255);
 8   }
 9   else {
10     analogWrite(11, 5);
11   }
12 }

…

The LEDs toggle between on and off while the button is pressed.

We only want it to toggle once each time the button is pressed.

⇒ We need to track the last button state.

…

To simulate the random flickering of an actual candle we need a source of randomness.

The Arduino environement supplies the random(min, max) function for this case.

In the program below rand_var is randomly set to either 10, 11, 12, 13 or 14* each time loop is executed.

1 void setup() {
2 }
3 
4 void loop() {
5   int rand_var= random(10, 15);
6 }

* The lower bound is inclusive. The upper bound is exclusive.

You can now try to combine the analogWrite, analogRead, random and delay functions to create a nice looking Candle animation.

1 if (is_on) {
2   analogWrite(11, random(10, 255));
3   delay(random(20, 120));
4 }
5 else {
6   analogWrite(11, 0);
7   delay(100);
8 }

…

What happens when we click the Upload button?

Firstly the textual representation of the program is translated into a representation the microcontroller understands.

00000128 <main>:
128:  87 b3   in      r24, 0x17       ; 23
12a:  88 61   ori     r24, 0x18       ; 24
12c:  87 bb   out     0x17, r24       ; 23
12e:  30 d1   rcall   .+608           ; 0x390 <uart_init>

Then the Computer instructs the microcontroller on the Arduino to restart by setting the special RESET pin to 0V.

Upon startup a special program on the microcontroller is executed.

This program is called the bootloader. It flashes the LED on the Arduino board and waits for programming instructions.

The Computer then starts sending the compiled program to the Arduino.

Once the upload is successfull the uploaded program is started.