Making A Combo Lock

For the next PCOMP lab, I made a combination lock that would “open” (light a green LED) after the button was pressed four times. This was not that hard to set up on the bread board as much as getting the programming right. The logic and syntax was pretty new to me, but Jeff was kind enough to offer just enough advice to let me figure it out. The big revelation was using currentButtonState and previousButtonState as a way to track the state of the button, mainly if the state had changed at any moment since the loop restarted ( previousButtonState = currentButtonState; ). This gave the loop a sense of incremental time so that the number of button presses (or state changes) could be counted by incrementing the pressCount variable inside of an IF statement.

Here is the code:

// a combination lock which opens after 4 button presses
//three LEDs, one switch

int switchPatch = 2;
int yellowLED = 3;
int redLED = 4;
int greenLED = 5;
int currentButtonState;
int previousButtonState;
int pressCount = 0;

void setup() {
  pinMode(switchPatch, INPUT);
  pinMode(yellowLED, OUTPUT);
  pinMode(redLED, OUTPUT);
  pinMode(greenLED, OUTPUT);

void loop() {

  //read the button at dig pin 2
  currentButtonState = digitalRead(switchPatch);
  //if the state of the button changes
  if (currentButtonState != previousButtonState) {
    // and if the currentButtonState is not equal to the previous state
    // the button has either been pressed or released (something has changed)
    // so if the button stage has changed AND that state reads HIGH, then
    // the button has been pressed
        if (currentButtonState == HIGH) {
          // so turn on the yellowLED
          digitalWrite(yellowLED, HIGH);
          // and turn off the redLED
          digitalWrite(redLED, LOW);
          // and increase pressCount to track how many times this will happen
          } else {
            //otherwise the button has not been pressed
            // so keep the red LED on and yellow off
            digitalWrite(redLED, HIGH);
            digitalWrite(yellowLED, LOW);
    // if we press the button 4 times then...
    if (pressCount == 4) {
      //turn the green led ON
      digitalWrite(greenLED, HIGH);
      // and reset the counter
      pressCount = 0;
      } else {
        // otherwise keep the green LED off
        digitalWrite(greenLED, LOW);
    // make the currentButtonState = to the previousButtonState just before
    // the loop starts over
    previousButtonState = currentButtonState;
    // print the pressCount value in the Serial Monitor