This week’s EYE on NPI (video) is a breath of fresh air! How do we know it’s fresh? Because we have a brand new Sensirion SGP40 air quality sensor hooked up and ready to be our digital nose. We loooove Sensirion sensors, and we stock a whole bunch in the Adafruit shop. They are renowned for their excellent prices and quality on temperature, humidity and gas sensors, so when we saw this pop up on Digi-Key, we quickly picked up a few pieces to check out.
The SGP40 is Sensirion’s digital volatile organic compounds (VOC) sensor designed for easy integration into air treatment devices and air quality monitors. Based on Sensirion’s CMOSens® technology, the SGP40 offers a complete sensor system on a single chip and features a digital I²C interface, a temperature-controlled micro-hotplate, and a humidity-compensated indoor air quality signal. In combination with Sensirion’s powerful VOC algorithm, the sensor signal can be directly used to evaluate indoor air quality for applications such as triggering the gradual fan control of an air treatment device.
Both the SGP40 chip and VOC algorithm feature unrivaled robustness in the final application over their lifetimes. Pushing the multi-pixel and multi-hotplate approach to the next level of individual control of each sensing element, the SGP40 enables a drastic reduction in power consumption, making it suitable for battery-driven applications as well. Sensirion’s state-of-the-art production process guarantees high reproducibility and reliability.
Gas sensors are really interesting, they are all essentially resistive sensors, where the exposed silicon has a metal oxide layer (MOX) that is susceptible to ambient gasses. There’s a heater element that you need to turn on, then wait till the silicon chunk can react with the air. The resistance changes with the amount of organic compounds in the air and you can measure the resistive differences.
Now for many gas sensors, you basically get 4 pins: two for the heater, and two for the resistor. You have to power the heater, then use a resistor divider or op-amp to read the resistance and back calculate. There’s no built-in calibration available. What we like about the SGPx0 series is that you don’t need the power control or analog input.
This isn’t Sensirion’s first gas sensor, we’ve already stocked the SGP30 for a while, and there are a few things we really liked about that sensor that are the same on the SGP40. First, is a dead-simple I2C interface, you send it some commands, it responds with the normalized resistance data reading. No complex code required, it’s trivial to port to any microcontroller.
With a humidity sensor on the same microcontroller, you can feed the ambient RH to the SGP40 and it will improve the resistance calculation.
See Sensirion’s video below.