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Sensor Net

Sensors

Sensor Net uses two external sensor breakout boards: the TMP102 for temperature and the BMP280 for atmospheric pressure. Both communicate with the microcontroller over the I2C bus. This page describes each sensor and how the firmware reads data from it.

TMP102 Temperature Sensor

Section titled “TMP102 Temperature Sensor”

The TMP102 is a 12-bit digital temperature sensor manufactured by Texas Instruments. It is commonly available as a small breakout board from SparkFun and other vendors.

TMP102

Key Specifications

Section titled “Key Specifications”
PropertyValue
ManufacturerTexas Instruments
InterfaceI2C
Default I2C address0x48 (with ADD0 pin connected to GND)
Resolution0.0625 degrees C (12-bit)
Accuracy+/- 0.5 degrees C (typical)
Range-40 degrees C to +125 degrees C
Supply voltage1.4V to 3.6V

How the Firmware Reads It

Section titled “How the Firmware Reads It”

The firmware’s TMP102Sensor driver reads the temperature register directly over I2C:

  1. Write 0x00 to the sensor to select the temperature register.
  2. Read 2 bytes back (MSB first, then LSB).
  3. Combine the upper 12 bits of the 16-bit word to get the raw value.
  4. Multiply the raw value by 0.0625 to get the temperature in degrees Celsius.

Example calculation:

MSB = 0x17, LSB = 0x80
raw = (0x17 << 4) | (0x80 >> 4) = 0x178 = 376
temperature = 376 x 0.0625 = 23.5 degrees C

Alternate I2C Address

Section titled “Alternate I2C Address”

If you connect the ADD0 pin to VCC instead of GND, the I2C address changes to 0x49. This is useful if you want to attach two TMP102 sensors to the same bus. Update the TMP102_ADDRESS constant in config.h to match.

BMP280 Pressure Sensor

Section titled “BMP280 Pressure Sensor”

The BMP280 is a digital barometric pressure sensor manufactured by Bosch. It is widely available as a breakout board from Adafruit, SparkFun, and many generic vendors.

BMP280

Key Specifications

Section titled “Key Specifications”
PropertyValue
ManufacturerBosch Sensortec
InterfaceI2C (also supports SPI, but Sensor Net uses I2C)
Default I2C address0x76 (with SDO pin connected to GND)
Pressure range300 to 1100 hPa
Pressure resolution0.16 Pa
Accuracy+/- 1 hPa (typical)
Supply voltage1.71V to 3.6V

How the Firmware Reads It

Section titled “How the Firmware Reads It”

The firmware’s BMP280Sensor driver uses the Adafruit BMP280 library, which handles all the internal compensation calculations that the BMP280 requires. The library’s readPressure() method returns pressure in Pascals, so the driver divides by 100 to convert to hectopascals (hPa), which is the standard meteorological unit.

1 hPa = 100 Pa
Standard atmosphere = 1013.25 hPa

Alternate I2C Address

Section titled “Alternate I2C Address”

If you connect the SDO pin to VCC instead of GND, the I2C address changes to 0x77. Update the BMP280_ADDRESS constant in config.h to match.

Sensor Driver Interface

Section titled “Sensor Driver Interface”

Both sensor drivers implement a common abstract interface defined in the firmware. This means the main firmware code does not need to know which specific sensor is attached — it simply calls sensor.read() and gets a value back.

class Sensor {
public:
    virtual bool          begin() = 0;  // Initialize hardware
    virtual SensorReading read()  = 0;  // Take one reading
    virtual const char   *name()  = 0;  // e.g. "TMP102"
    virtual const char   *unit()  = 0;  // e.g. "C"
};

The NODE_TYPE setting in config.h determines which concrete sensor class is compiled into the firmware for a given build. See the Configuration page for details.

Next Step

Section titled “Next Step”

Now that you understand the sensors, proceed to Assembly for step-by-step wiring instructions.

© 2026 Andrew Lemons