Arduino’s Blink

The very first project everybody makes in any Arduino starter kit is the blinking LED project, “Blink.” In this post, we will have a look at the theory behind this simple circuit and program.

The Circuit

This circuit is very simple and contains only two components. An LED’s anode is connected to a current limiting resistor, which is connected to the Arduino’s digital output pin 13. The LED’s cathode is connected to the Arduino’s ground as in the following circuit:

When the Arduino’s digital output pin is set to HIGH or 5 volts, current flows through the resistor and LED making the LED emit light. When the Arduino’s digital output pin is set to LOW or 0 volt, current stops flowing and the LED does not emit light.

Each component will be described later in the post. The value of the resistor may vary from one kit to another. I will explain how to compute its value later in the post.

Breadboarding

The following picture depicts how to connect the different parts using a solderless breadboard, jumper wires, an LED and a 330Ω resistor.

The Program

We start with the simple blinker program found in most if not all Arduino kits. If it is not included in your kit, you can always download a copy of Blink on the Arduino site. The following is an excerpt from the actual listing, some comments have been removed for clarity.

/* Blink: turns an LED on for one second, then off for one second,
   repeatedly.
   This example code is in the public domain.
   http://www.arduino.cc/en/Tutorial/Blink  */
 
 // the setup function runs once when you press reset
 void setup() {
   // initialize digital pin LED_BUILTIN as an output.
   pinMode(LED_BUILTIN, OUTPUT);
 }
 
 // the loop function runs over and over again forever
 void loop() {
   // turn the LED on by making the voltage HIGH, wait a second
   digitalWrite(LED_BUILTIN, HIGH);
   delay(1000);
   // turn the LED off by making the voltage LOW, wait a second
   digitalWrite(LED_BUILTIN, LOW);
   delay(1000);
 }

The language used within the Arduino IDE (Integrated Development Environment) is actual C++. The major difference is that the Arduino system does not call a main( ) function like it does under normal Windows or Linux console applications. instead, the system expect the programmer to define two ‘C’ functions:

void setup();

and

void loop();

The setup( ) function (or method) is called once the first time the program is downloaded, once after the Arduino board reset button has been depressed, or once the Arduino board has been powered up. The loop( ) function is called repeatedly and indefinitely after setup( ) has been called once.

The Arduino system also comes with a set of predefined function libraries. In our first example, two functions are used to access the digital output pin to make the LED blink. The first function’s signature (this is how we call a function’s description) is

void pinMode ( int pin, int mode );

Where pin is an integer specifying the digital pin to be setup and mode is an integer specifying the mode this pin will be used in. In the program above, the pin number used is LED_BUILTIN which corresponds to the pin number connected to the built-in Arduino board LED, pin 13 on the Arduino UNO. The mode can have one of three values: INPUT, OUTPUT, or INPUT_PULLUP, all constant values that can be used as the digital I/O pin mode. In the program, we use OUTPUT, allowing us to use the pin as a digital output. The pinMode( ) function is used as part of the setup( ) function body to prepare the LED_BUILTIN digital I/O pin to be outputted to.

The second function signature is

void digitalWrite ( int pin, int value );

Where pin is an integer specifying the digital pin we are outputting to, while value is an integer whose value can be 0 or 1, LOW or HIGH. digitalWrite( ) can only be used if the pin mode was previously set to OUTPUT.

The last function signature used within our simple piece of code is

void delay( int value )

where value is an integer specifying the amount of time in milliseconds (thousandths of a second) the program is to wait and do nothing. In the program above it waits twice for a full second each time, leaving the LED on for a full second then turning it off for a full second.

Components

Two electronic components are used within this circuit, a resistor and an LED. We will describe each in turn in the following paragraphs

Resistors

Resistors are electronic devices that restrict current flow according to Ohm’s law. Ohm’s law states that the current through a conductor between two points is directly proportional to the voltage across the two points. In mathematical terms, Ohm’s law can be restated

I = V / R

Where I is the current in amperes (A), V is the voltage across the conductor in volts (V) and R is the resistance of the conductor in ohms (Ω). Hence, a 330Ω resistor with 5V across its leads will let a current flow of 5V / 330Ω, 0.01515A, or 15.15mA.

Resistors restrict current by dissipating energy in the form of heat. This is an important factor as resistors have to be selected not only in terms of their resistance, but also for their capacity to dissipate heat, or power rating. The energy dissipated per second by a resistor, the electrical power, is expressed in watts (W). In mathematical terms, Power can be computed as

P = VI

Where P is the power in watts (W), V is the voltage across the resistor in volts (V) and I is the current, in amperes (A) flowing through the resistor. in the previous example, the power dissipated by the 330Ω resistor is 5V • 0.01515A, 0.07575W, or 75.75mW. The resistors supplied with most kits have a power rating of 1/4W (250mW). A 1/4W power rating is sufficient for our current example and most Arduino circuits you will encounter.

LED

LEDs are semiconductor devices that emit light when current flows through them. as its name suggests, an LED only allows current to flow in one direction, from anode to cathode, in the direction of the arrow of the diode symbol. Unlike resistors, an LEDs voltage across it leads is not linearly proportional to the current through the device and we have to look at its specification sheet to determine how the LED will perform within a circuit.

Here is a sample datasheet for a standard 20mA LED:

As can be seen from the data sheet, the forward voltage across the LED, that is the voltage across the LED when 20mA of current is flowing from the anode to the cathode, is between 1.8 and 2.2 volts. Also, it is suggested to limit the current between 16 to 18 mA in normal operation.

Computing the Resistor Value

Apart from Ohm’s law and the power rating formula, we need a bit more information to find a resistor value that will limit the current flow within the LED to a value not exceeding 18mA. Kirchkoff’s Voltage Law states that the sum of voltages around a closed circuit loop is 0V. Current goes in the counter-clockwise direction in this circuit, from the positive end of the power source to the negative end and the voltage drop around the power source is negative since voltage does not drop but increases around it. Redrawing our LED circuit when the digital output pin is HIGH and replacing the pin by a 5V source we get:

The sum of all voltages around the circuit is:

-5V + 1.8V + V_R1 = 0V

Solving for V_R1, we get

V_R1 = 5V – 1.8V = 3.2V

Thus, we know that we want 18mA to flow through both LED and resistor and that the voltage across the resistor is 3.2V. Going back to Ohm’s law we know that

R1 = V_R1 / I = 3.2V / 0.018A = 177Ω

This is the smallest resistor value to obtain the maximum optimal amount of current through the circuit. Resistors don’t come in all possible values. The most common values are the following values multiplied by powers of 10: 10, 15, 22, 33, 47, 68. The value just higher than 177Ω is 220Ω. Using this new value, we get

I = V_R1 / R1 = 3.2V / 220Ω = 14.5mA

A value close to the optimal value suggested by the LED manufacturer. The value shown in the first circuit, 330Ω is the value used in the SparkFun kit I used for this experiment. This value limits the current to a value of 9.7mA which does allow the LED to emit light, but at a dimmer intensity.