🌡️ 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

        <h3>I²C Wiring</h3>
        Standard I²C connections:
        <ul>
          <li><strong>VCC</strong> → 3.3V (not 5V!)</li>
          <li><strong>GND</strong> → GND</li>
          <li><strong>SDA</strong> → GPIO21 (or configured pin)</li>
          <li><strong>SCL</strong> → GPIO22 (or configured pin)</li>
          <li><strong>Pull-ups</strong>: 10kΩ resistors on SDA and SCL to 3.3V</li>
        </ul>

        <h3>Address Scanning</h3>
        First step: verify sensor is connected:
        <ol>
          <li>Scan I²C bus for all addresses (0x08–0x77)</li>
          <li>Look for ACK responses</li>
          <li>Verify expected address matches datasheet</li>
        </ol>

        <h3>Common Issues</h3>
        <ul>
          <li><strong>No device found</strong>: Check wiring, pull-ups, power</li>
          <li><strong>Wrong address</strong>: Some sensors have address-select pins</li>
          <li><strong>NACK errors</strong>: Sensor not ready, check timing</li>
        </ul>

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