feat: BMP180 read and sea-level pressure conversion

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/WeatherPredictor.ino

@@ -2,6 +2,7 @@
 #include <Wire.h>
 #include "config.h"
 #include "display_ui.h"
+#include "sensors.h"
 
 static void i2cScan() {
   Serial.println(F("I2C scan:"));
@@ -25,7 +26,14 @@ void setup() {
 
   displayBegin();
   displaySplash("Weather", "Predictor v0.1");
+
+  if (!sensorsBegin()) Serial.println(F("BMP180 not found!"));
 }
 
 void loop() {
+  float abs_ = readAbsPressureHpa();
+  float msl  = toSeaLevelHpa(abs_, DEFAULT_ALTITUDE_M);
+  Serial.printf("abs=%.1f hPa  msl=%.1f hPa  t=%.1f C\n",
+                abs_, msl, readTemperatureC());
+  delay(2000);
 }

WeatherPredictor/sensors.cpp

@@ -0,0 +1,22 @@
+#include <Adafruit_BMP085.h>
+#include <math.h>
+#include "sensors.h"
+
+static Adafruit_BMP085 bmp;
+
+bool sensorsBegin() {
+  return bmp.begin();          // uses Wire (already started on D6/D1), addr 0x77
+}
+
+float readAbsPressureHpa() {
+  return bmp.readPressure() / 100.0f;   // Pa -> hPa
+}
+
+float readTemperatureC() {
+  return bmp.readTemperature();
+}
+
+// Standard barometric reduction to sea level.
+float toSeaLevelHpa(float absHpa, float altitudeM) {
+  return absHpa / powf(1.0f - (altitudeM / 44330.0f), 5.255f);
+}

WeatherPredictor/sensors.h

@@ -0,0 +1,7 @@
+#pragma once
+#include <Arduino.h>
+
+bool  sensorsBegin();
+float readAbsPressureHpa();                          // absolute (station) pressure, hPa
+float readTemperatureC();                            // degrees C
+float toSeaLevelHpa(float absHpa, float altitudeM);  // mean-sea-level pressure, hPa