🛠 Interroger un capteur I2C (BME280) sur le bus du 40 broches

Objectives

Enable the I2C bus on the Model B+ (I2C0 is available by default on the 40-pin connector), scan the bus with i2cdetect, then read the identification number of a BME280 sensor in Python — proving that the hardware/software chain works before using the measurements.

Hardware

• Raspberry Pi Model B+ running Raspberry Pi OS legacy 32-bit
• 1 BME280 (or BMP280) module with a 4-pin connector (VCC, GND, SCK/SCL, SDI/SDA)
• 4 Dupont wires; power the module with 3.3 V (pin 1), never with 5 V if the module is not 5 V tolerant

Complete code

# Activer I2C : raspi-config > Interfaces de connexion > I2C, ou :
echo "dtparam=i2c_arm=on" | sudo tee -a /boot/config.txt
sudo reboot
sudo apt install -y i2c-tools python3-smbus
i2cdetect -y 1        # le BME280 doit répondre à 0x76 (ou 0x77)

#!/usr/bin/env python3
"""Lecture de l'ID du BME280 sur i2c-1 — Model B+."""
import smbus

ADDR = 0x76                      # 0x77 si l'adresse SDO est montée
bus = smbus.SMBus(1)
chip_id = bus.read_byte_data(ADDR, 0xD0)
print(f"BME280 chip id: 0x{chip_id:02X}")   # attendu 0x60 (BMP280 : 0x58)

Expected verification

i2cdetect -y 1 displays a grid where the cell 76 is marked. The Python script prints “BME280 chip id: 0x60”. With a BMP280, 0x58 confirms a neighboring sensor; 0x55 or 0xFF indicates a power supply issue.

Common errors

• I2C not enabled or reboot forgotten: “Could not open bus: No such file or directory”.
• SDA/SCL swapped: i2cdetect shows nothing; on the B+ pins 3/5 are fixed to I2C0.
• Non-5V-tolerant module powered with 5 V while using 3.3 V logic: readings of 0xFF, or even a dead module.
• Address 0x77 wired but script using 0x76: adjust ADDR according to the i2cdetect scan.
• No onboard ADC: a raw analog sensor cannot be connected directly; it requires an I2C converter (PCF8591, ADS1115).