Read temperature, humidity and pressure from a BME280 over I2C with an ESP32

The BME280 is a small sensor that measures three things: temperature, relative humidity and air pressure. It talks to a microcontroller over I2C, which needs only two data wires. It makes an excellent first sensor, because in one go you learn a bus protocol, a library, and a real-world problem: heat from your own board.

The examples use an ESP32 DevKit (ESP32-WROOM-32) with Arduino IDE 2 and a common small BME280 breakout board, following on from the previous article on GPIO.

How I2C works

I2C uses two lines: SDA carries data and SCL carries the clock. The ESP32 is in charge. Every device hangs in parallel on the same pair of wires, and each has its own number, its address. The ESP32 calls an address and that device answers.

The lines need pull-up resistors to the supply, because devices can only pull a line low. Most ready-made modules already have pull-ups on board, so you normally add nothing.

Wiring

BME280 module pin	ESP32
VIN (or VCC)	3V3
GND	GND
SCL	GPIO22
SDA	GPIO21

GPIO21 and GPIO22 are the default I2C pins for the original ESP32 in the Arduino core. On an ESP32-S3 or C3 the defaults differ, so check your board’s pinout.

About power: some modules have a voltage regulator and accept 5 V; others do not, and 5 V can kill the sensor. The BME280 chip itself is rated up to 3.6 V. 3V3 is safe for both kinds, so when in doubt, use 3V3.

Step one: find the address with an I2C scanner

Before writing the real code, check that the wiring works and which address the sensor uses. This sketch calls every address and prints the ones that answer.

#include <Wire.h>

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);  // SDA, SCL
  for (uint8_t addr = 1; addr < 127; addr++) {
    Wire.beginTransmission(addr);
    if (Wire.endTransmission() == 0) {
      Serial.printf("found device at 0x%02X\n", addr);
    }
  }
  Serial.println("scan done");
}

void loop() {}

Open the Serial Monitor at 115200 baud. You should see 0x76 or 0x77, depending on whether the chip’s SDO pin is tied to ground (0x76) or to the supply (0x77). Many common modules use 0x76. If nothing shows up at all, the problem is the wiring, not the code.

Read values with the Adafruit library

In Arduino IDE, open the Library Manager, search for Adafruit BME280 Library and install it, accepting the libraries it depends on (Adafruit Unified Sensor and Adafruit BusIO).

#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_BME280.h>

Adafruit_BME280 bme;

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);           // SDA, SCL
  if (!bme.begin(0x76)) {       // try 0x77 if this fails
    Serial.println("BME280 not found: check wiring and address");
    while (true) delay(1000);
  }
}

void loop() {
  float t = bme.readTemperature();          // °C
  float h = bme.readHumidity();             // %RH
  float p = bme.readPressure() / 100.0F;    // Pa -> hPa
  Serial.printf("T=%.2f C  H=%.1f %%  P=%.1f hPa\n", t, h, p);
  delay(2000);
}

readPressure() returns pascals; divide by 100 for hPa, the unit weather forecasts use. Standard sea-level pressure is 1013.25 hPa; real readings vary with your altitude and the weather.

The scanner finds it but the library does not

This one is very common. The number one cause: the chip on the module is a BMP280, not a BME280. They share addresses and pinouts, some shops mix them up, but the BMP280 has no humidity sensor.

The library reads the chip ID before starting: 0x60 for a BME280, 0x58 for a BMP280. Anything other than 0x60 and begin() returns false. You can tell them apart by eye too: the BME280 is a square package of about 2.5 × 2.5 mm, the BMP280 a rectangle of about 2.0 × 2.5 mm. If you have a BMP280, use the Adafruit BMP280 library instead; you get temperature and pressure only.

Why the temperature reads too high

The BME280 is accurate enough for general use, but it measures the temperature where it is. Mounted right next to an ESP32, which warms up while running, especially with Wi-Fi on, it reads above the room. And because relative humidity depends on temperature, the humidity reading comes out low as well.

A temperature sensor measures the heat around it, including the heat of the board it sits on.

What works:

• Put the sensor on wires, away from the main board.
• In a case, give the sensor its own vented compartment, separate from the ESP32.
• Read less often, or put the ESP32 into deep sleep between readings, so the board stays cooler.

Common mistakes

Symptom	Usual cause	Fix
Scanner finds nothing	SDA and SCL swapped, or GND missing	Check each wire
Finds 0x77, code uses 0x76	Address mismatch	Change to bme.begin(0x77)
Scanner finds it, begin() fails	It is a BMP280	Use the BMP280 library
Reads work, then fail at random	Wires too long or loose	Shorter wires, firm connections
Temperature several degrees above the room	Heat from the board	Move the sensor away

Next, we design a 3D-printed case for this board, with a vented pocket that keeps the sensor away from the ESP32’s heat.