Lesson 1: Button & OLED Countdown

/*
  DIY KIT - ESP32 Fall-Detection Learning Prototype
  Lesson 1 of 5: Button & OLED Countdown

  WIRING (unplug the USB cable before you wire)
    OLED  VCC -> 3V3    GND -> GND    SDA -> GPIO21    SCL -> GPIO22
    Push button: one leg -> GPIO26, the leg diagonally opposite -> GND
    (The ESP32's built-in pull-up keeps GPIO26 HIGH until you press.)

  WHAT SHOULD HAPPEN
    The OLED says "press to start". Press the button: a 20-second countdown
    runs in big numbers. Press again to cancel. If it reaches 0 the screen
    says EXPIRED - in Lesson 5 this is where the alert starts.
    Serial Monitor at 115200 baud shows the same messages.
*/

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

const int SDA_PIN = 21;
const int SCL_PIN = 22;
const int BUTTON_PIN = 26;
const int LED_PIN = 2;               // the small LED on many ESP32 boards; yours may not have one
const int COUNTDOWN_S = 20;          // seconds - try 5 while testing
const unsigned long DEBOUNCE_MS = 35;

Adafruit_SSD1306 display(128, 64, &Wire, -1);
bool displayOk = false;

bool counting = false;
unsigned long countdownStart = 0;
int lastShown = -1;
bool rawButton = HIGH, stableButton = HIGH;
unsigned long rawSince = 0;

// True once each time the button is pressed (not while it is held).
// A contact "bounces" for a few ms, so we wait DEBOUNCE_MS before believing it.
bool pressedNow() {
  bool reading = digitalRead(BUTTON_PIN);
  if (reading != rawButton) {
    rawButton = reading;
    rawSince = millis();
  }
  if (millis() - rawSince > DEBOUNCE_MS && reading != stableButton) {
    stableButton = reading;
    return stableButton == LOW;      // pull-up: LOW means pressed
  }
  return false;
}

// Most of these displays answer at I2C address 0x3C, some at 0x3D. Try both.
bool startDisplay() {
  if (display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) { Serial.println("OLED found at 0x3C"); return true; }
  if (display.begin(SSD1306_SWITCHCAPVCC, 0x3D)) { Serial.println("OLED found at 0x3D"); return true; }
  Serial.println("OLED not found - unplug USB and check VCC, GND, SDA (GPIO21), SCL (GPIO22)");
  return false;
}

// Two lines of small text and, if seconds >= 0, a big number.
void show(const char *line1, const char *line2, int seconds) {
  if (!displayOk) return;
  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.println("COUNTDOWN LAB");
  display.println(line1);
  if (seconds >= 0) {
    display.setTextSize(3);
    display.setCursor(0, 22);
    display.println(seconds);
  }
  display.setTextSize(1);
  display.setCursor(0, 56);
  display.println(line2);
  display.display();                 // nothing appears until display() is called
}

void setup() {
  Serial.begin(115200);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  pinMode(LED_PIN, OUTPUT);
  Wire.begin(SDA_PIN, SCL_PIN);
  displayOk = startDisplay();
  show("ready", "press to start", -1);
  Serial.println("Lesson 1: press the button to start the countdown, press again to cancel");
}

void loop() {
  if (pressedNow()) {
    if (counting) {
      counting = false;
      Serial.println("CANCELLED");
      show("cancelled", "press to start", -1);
    } else {
      counting = true;
      countdownStart = millis();
      lastShown = -1;
      Serial.println("COUNTDOWN started");
    }
  }

  if (counting) {
    unsigned long used = millis() - countdownStart;
    int remaining = COUNTDOWN_S - (int)(used / 1000);
    unsigned long quarterSeconds = millis() / 250;
    digitalWrite(LED_PIN, quarterSeconds % 2 == 0 ? HIGH : LOW);   // blink while counting
    if (remaining <= 0) {
      counting = false;
      digitalWrite(LED_PIN, LOW);
      Serial.println("EXPIRED");
      show("EXPIRED", "press to start again", 0);
    } else if (remaining != lastShown) {
      lastShown = remaining;
      Serial.print("seconds left: ");
      Serial.println(remaining);
      show("counting down", "press to cancel", remaining);
    }
  } else {
    digitalWrite(LED_PIN, LOW);
  }
  delay(5);
}