Exercise 4 - Push Button Controlled LED

In this exercise we will learn how to use the STEMTera™ Breadboard to read the input from a push button and translate the input into a toggling state that can be used to control an LED.

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1. Connect the anode (longer) pin of the RED LED to 38G and cathode (shorter) pin to 39G.
2. Connect the push button switch to 32F, 34F, 32E and 34E.
3. Connect the 220Ω resistor to PIN2 and 38J.
4. Connect a jumper wire to GND and 39I.
5. Connect a jumper wire to GND and 32J.
6. Connect a jumper wire to PIN6 and 34J.

Write and Upload the Push Button Controlled LED Sketch

In order to execute the sketch for this exercise, launch Arduino IDE then

Click File -> New.

A new sketch editing window will appear. Copy the whole block of code below and paste it into the new sketch window, overwriting the empty setup() and loop() that was pre-created. Ensure that the board selected is Arduino Uno and the right COM port is chosen according to “Board Selection” chapter.

boolean LEDState=false;
int LED = 2;
int BUTTON = 6;

void setup() {
    pinMode(LED, OUTPUT);
    // set BUTTON pin as INPUT with an Internal PULLUP
    pinMode(BUTTON, INPUT_PULLUP);
}

void loop() {
    // wait for BUTTON Pin LOW
    if (digitalRead(BUTTON)==LOW){
        delay(50); // wait for 50 ms
        // if BUTTON Pin still low, then it is a solid press
        if (digitalRead(BUTTON)==LOW) {
            LEDState = !LEDState;   // toggle the LED state
            digitalWrite(LED, LEDState);    
        }
        // NOTE: this is a bad way to debounce a button
        // if the BUTTON is stuck at LOW the sketch will 
        // forever stays in the while
        while(digitalRead(BUTTON)==LOW) {
            // wait until BUTTON is read HIGH
        };  
    }
}

Click Upload, and the Arduino IDE will prompt you to Save sketch folder as. Enter Excercise_4 into the filename field and click save. Once saved, the Arduino IDE will start to compile the sketch. After the compilation is completed, the Arduino IDE will start to upload the compiled sketch into the STEMTera™ Breadboard. During this stage both yellow LEDs on the STEMTera™ Breadboard will start blinking indicating the Arduino IDE is uploading the sketch to the STEMTera™ Breadboard.

Understanding the Push Button Controlled Sketch

In order for the sketch to keep track of the state of the LED, we need to first declare a boolean variable with a default as false.

boolean LEDState=false;

Each time the LED changes its state, we will store the state in this variable. We then declare two more variables, one for the LED pin and the other one for BUTTON pin, which are 2 and 6 respectively.

int LED = 2; int BUTTON = 6;

The pin where the LED is connected to will be set as OUTPUT using pinMode(LED, OUTPUT) function and the pin where the push button is connected to will be set as INPUT_PULLUP using pinMode(BUTTON, INPUT_PULLUP) function. The constant

INPUT_PULLUP

means, other than making the pin as INPUT, it also enable the internal pull-up resistor of that pin, so that the pin will be connected to the 5V via the internal pull-up resistor and always in a HIGH state. When the push button that is connected to the BUTTON pin is pressed, it connects the BUTTON pin to ground (GND) thus making the BUTTON pin reads LOW.

When in the loop(), the code

if (digitalRead(BUTTON)==LOW)

is waiting for the BUTTON pin to go LOW, once it is low, it wait for 50ms and read the BUTTON pin again for low. This act as a simple debounce to eliminate any noise when the push button first make a contact. If the BUTTON pin is still low after 50ms, then the contact is solid and therefore the state of the LED is being toggled (false to true, or true to false) using this code.

LEDState = !LEDState;

After toggling the LEDState, we can now write the state to the LED pin. In order to prevent the toggling repeatedly fired while the push button is being depressed, we add another line of code to wait for the BUTTON pin to go HIGH when the push button is released.

while(digitalRead(BUTTON)==LOW)

This code is a lazy hack and a bad coding practice because if the push button is shorted, the sketch will forever stays in the while loop and will never execute any other code.

CHALLENGE! Write a sketch to perform the same task with button debounce, LED toggling and will never locks up even when the push button is shorted.