The Scentinel utilises the BME688 environmental sensor for gas measurements, and is supported by the LIS2DH
accelerometer for additional context. The power systems use the INA228 for power measurements, while the battery
utilises the MAX17260 fuel gauge to monitor the battery.
Gas and environment
I²C / SPI
The BME688 is metal-oxide gas sensor with temperature, relative-humidity and
barometric-pressure sensing. Its gas readings change in response to different mixtures of volatile organic
compounds,
while the temperature, humidity and pressure sensors provide additional context.
These combined readings can be fingerprinted and used to train a machine learning model for pattern
recognition.
Scentinel drives the gas heater through ten programmed temperatures and records one resistance value at each
step. Different heater temperatures alter the sensor’s sensitivity, turning a single reading into a repeatable
ten-dimensional fingerprint:
The LIS2DH is an ultra-low-power three-axis accelerometer. Selectable ±2 g, ±4 g, ±8 g and ±16 g ranges allow
it to detect anything from gentle handling to stronger shocks, while embedded motion, orientation and
free-fall functions can generate interrupts without continuous processor polling.
Implementation
The LIS2DH detects motion events and physical disturbances, providing more context to gas
readings and helping to interpret unusual sensor behavior or readings.
The INA228 is a 20-bit bidirectional current, voltage and power monitor. It measures the voltage drop across
an external 50mΩ shunt together with bus voltage to calculate current and power.
A key feature of the INA228 is its hardware power accumulator which allows it to measure total power and
current
usage over time without requiring polling.
Implementation
The use of two INA228 ICs in Scentinel allow it to measure the solar generated and the
system load separately. It allows instantaneous and power flow measurements, as well as interval energy
totals. Bidirectional sensing preserves the direction of current, which is important when distinguishing
battery charging from discharging.
The MAX17260 is a single cell battery monitor. By using Analog Devices’ ModelGauge m5 algorithm, it can
accurately estimate the state of charge by combining coulomb counting with a voltage-based model and
temperature compensation. This is a key part of being able to
accurately determine the battery's state of charge because LiFePO4 batteries have a very flat
discharge curve, making voltage alone a poor indicator of remaining capacity.
Implementation
The gauge supplies battery voltage, current, temperature, state of charge and estimated time to empty
for the LiFePO4 18650 battery across a 10mΩ shunt resistor.
