Sensing technologies for monitoring the physical world are indispensable in various technical fields. Understand four common environmental sensors and their working principles.
Every time a thermostat adjusts building temperature, a ventilator responds to a patient's breathing, or a mobile phone triggers an air quality alert, it is sensors that make it possible. These small devices perform seemingly simple tasks: converting physical quantities such as heat, pressure, gas concentration, humidity, or particulate mass into electrical signals that circuits can operate with.
The physics behind this conversion varies depending on the sensor type, and understanding it helps engineers select appropriate devices, explain their limitations, and avoid common specification errors. This article covers five sensor categories that appear in automotive, medical, industrial, and environmental applications: temperature, pressure, carbon dioxide, humidity, and particulate matter in the air.
Temperature sensors convert thermal energy into electrical signals. The two most common passive types are NTC thermistors (whose resistance decreases with increasing temperature) and PTC thermistors (whose resistance increases with increasing temperature), both widely used in embedded systems for precise, low-cost measurements. For applications where contact is impossible or unnecessary—such as measuring rotating shafts, molten materials, or without contacting patient skin—non-contact infrared sensors detect emitted thermal radiation according to the Stefan-Boltzmann law. A third variant is the current-limiting thermistor, which functions more as a protective device than a measurement instrument: when placed in series with a power supply, its high cold-state resistance limits the surge current that could otherwise stress capacitors and rectifiers during switch-on.
Micro-Electro-Mechanical Systems (MEMS) pressure sensors use semiconductor manufacturing processes to etch a tiny silicon diaphragm onto a chip. When pressure causes the diaphragm to bend, the piezoresistors embedded in the silicon change their resistance proportionally—a phenomenon known as the piezoresistive effect. Wheatstone bridges convert these resistance changes into millivolt-level differential voltages. MEMS sensors can be made extremely small, with low power consumption and high precision, making them the dominant technology for medical blood pressure monitors, automotive manifold absolute pressure (MAP) sensors, weather instruments, and industrial process control, with measurement ranges from fractions of an inch of water column to thousands of pounds per square inch (PSI).
Non-dispersive infrared (NDIR) is the standard method for measuring CO₂ concentration. A broadband infrared light source illuminates through a sample chamber; CO₂ molecules strongly absorb light at 4.26 µm, while the reference channel remains constant at a non-absorbing wavelength. The ratio of the two channels yields concentrations in parts per million (ppm). NDIR sensors offer selectivity, long-term stability, and precision down to single-digit ppm—characteristics that make them the standard for indoor air quality monitoring, greenhouse climate control, safety alerts in fermentation or storage facilities, and automotive HVAC systems in vehicles. Drift over time is a known challenge; self-calibration designs automatically compensate by periodically referencing known clean outdoor air.
Relative humidity sensors typically employ a capacitive polymer film whose dielectric constant changes as it absorbs or releases water vapor—a predictable, reversible physical change that can be read via capacitance variation. The integrated circuit combines this capacitive element with an on-chip temperature sensor (used to calculate relative humidity based on absolute humidity content) and signal conditioning, outputting calibrated digital readings via I²C or SPI interfaces, or analog voltages.
For environments with condensation, chemical exposure, or wide temperature fluctuations, probe-type designs encapsulate the sensing element in a protective housing while still allowing for vapor exchange. Humidity measurement is crucial for HVAC efficiency, pharmaceutical storage compliance, food production, and meteorological instruments.