BMP280 Altimeter

// BMP280 Altimeter - barometric pressure turns into altitude. The BMP280
// measures air pressure, and every ~8 meters you climb the pressure drops
// by about 1 hPa. The sketch converts pressure to altitude, remembers a
// baseline, and shows how far above or below it you are.
// Click the BMP280 on the bench and drag its P slider to 'fly' up and down.
// Press the button to zero the altimeter at the current altitude.

#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>
#include <Adafruit_BMP280.h>

#define ZERO_PIN 14   // push button: set baseline to current altitude
#define CLIMB_LED 4   // lights while climbing

Adafruit_BMP280 bmp;
Adafruit_SSD1306 display(128, 64, &Wire, -1);

float baseline = 0;
float lastRel = 0;

void setup() {
  Serial.begin(115200);
  pinMode(ZERO_PIN, INPUT_PULLUP);
  pinMode(CLIMB_LED, OUTPUT);
  Wire.begin();
  display.begin(SSD1306_SWITCHCAPVCC, 0x3C);
  bmp.begin(0x76);
  delay(100);
  baseline = bmp.readAltitude(1013.25);
  Serial.println("BMP280 Altimeter ready - button zeroes the altitude");
}

void loop() {
  float p = bmp.readPressure() / 100.0;        // Pa -> hPa
  float alt = bmp.readAltitude(1013.25);       // meters above sea level
  float rel = alt - baseline;                  // meters above the baseline

  if (digitalRead(ZERO_PIN) == LOW) {
    baseline = alt;
    rel = 0;
    lastRel = 0;
    Serial.println("Altimeter zeroed at current altitude");
    delay(300);   // simple debounce
  }

  float vspeed = (rel - lastRel) * 2.0;        // m/s (loop runs every 500 ms)
  lastRel = rel;

  const char* trend = "LEVEL";
  if (vspeed > 0.5) {
    trend = "CLIMBING";
  } else if (vspeed < -0.5) {
    trend = "DESCENDING";
  }

  if (vspeed > 0.5) {
    digitalWrite(CLIMB_LED, HIGH);
  } else {
    digitalWrite(CLIMB_LED, LOW);
  }

  float feet = rel * 3.281;

  display.clearDisplay();
  display.setTextColor(SSD1306_WHITE);
  display.setTextSize(1);
  display.setCursor(0, 0);
  display.print("BMP280 ALTIMETER");
  display.setTextSize(3);
  display.setCursor(0, 12);
  display.print(rel, 1);
  display.setTextSize(1);
  display.print(" m");
  display.setCursor(0, 40);
  display.print(feet, 0);
  display.print(" ft   ");
  display.print(trend);
  display.setCursor(0, 52);
  display.print("P ");
  display.print(p, 1);
  display.print(" hPa  Btn=zero");
  display.display();

  Serial.print("P=");
  Serial.print(p, 1);
  Serial.print(" hPa  alt=");
  Serial.print(rel, 1);
  Serial.print(" m  ");
  Serial.println(trend);
  delay(500);
}