🌡️ Sensor Selection + I²C Wiring

Why it matters

The capstone uses an I²C environmental sensor (temperature, humidity, pressure). Proper wiring and address scanning are critical.

The idea

Sensor Options

Common I²C environmental sensors:

• SHT31: Temperature + humidity, 0x44 address
• BME280: Temperature + humidity + pressure, 0x76 or 0x77
• SHT40: Temperature + humidity, improved accuracy

I²C Wiring

Standard I²C connections:

• VCC → 3.3V (not 5V!)
• GND → GND
• SDA → GPIO21 (or configured pin)
• SCL → GPIO22 (or configured pin)
• Pull-ups: 10kΩ resistors on SDA and SCL to 3.3V

Address Scanning

First step: verify sensor is connected:

1. Scan I²C bus for all addresses (0x08–0x77)
2. Look for ACK responses
3. Verify expected address matches datasheet

Common Issues

• No device found: Check wiring, pull-ups, power
• Wrong address: Some sensors have address-select pins
• NACK errors: Sensor not ready, check timing

Demo

Sensor wiring is physical, not visual. Review this before connecting your sensor.

Key takeaways

• Use 3.3V sensors (not 5V) for direct ESP32 connection
• I²C requires pull-up resistors (10kΩ) on SDA and SCL
• Always scan I²C bus to verify sensor address
• Check datasheet for address-select pins if address doesn’t match

Going deeper

SHT31 has excellent accuracy (±2% RH, ±0.3°C) and low power consumption (~2.7mA active, <1µA sleep). BME280 adds pressure sensing but uses more power. For battery-powered devices, SHT31 is often the better choice. Always check sensor power consumption in datasheet.

Math details

I²C pull-up calculation:
  V_CC = 3.3V
  I_max = 3mA (I²C spec)
  R_min = V_CC / I_max = 3.3V / 0.003A = 1100Ω

  Use 10kΩ (standard value, provides ~330µA current)

Capacitance limit:
  C_bus_max = 400pF (I²C spec)
  With 10kΩ pull-ups: works up to ~1m cable length

Sensor power:
  SHT31: 2.7mA active, <1µA sleep
  BME280: 3.6µA sleep, 338µA active

Implementation

LLM Prompt: I²C Address Scanner

Write Rust code for ESP32 I²C address scanner using esp-hal.
Scan addresses 0x08–0x77, attempt read/write, report which addresses
respond with ACK. Include timeout handling and error reporting.
Useful for debugging I²C connections.

Lab Exercise

1. Connect SHT31: VCC→3.3V, GND→GND, SDA→GPIO21, SCL→GPIO22
2. Add 10kΩ pull-ups on SDA and SCL to 3.3V
3. Run I²C address scanner — verify sensor at 0x44
4. Read temperature register — verify reasonable values
5. Read humidity register — verify 0–100% range