Commonly used humidity and CO2 sensors operate at high temperatures or require infrared transceivers. These sensors—which include optical sensors, piezoelectric devices, carbon nanotubes, metal oxide devices and polymer composites—have complicated circuits which makes them expensive. Additionally, they are mostly powered by batteries, which are inconvenient and expensive to replace.
To address these issues, researchers from the Graduate Institute of Electronics Engineering at National Taiwan University(NTU GIEE) have developed a low-cost and low-energy humidity and CO2 sensing platform that can be printed by inexpensive ink-jet printers at room temperature. This new innovation in sensor technology, based on organic sensing film, brings a number of key advantages compared to traditional gas sensors.
The researchers’ sensors are able to operate at room temperature without infrared transceivers and heating dissipation,enabling very low power consumption (as low as 10 μW for humidity sensors and 5 μW for CO2 sensors). “To our knowledge,this is the first multiple environmental parameters(Temperature/CO2/Humidity) sensing platform that demonstrates a true self-powering functionality for long-term operations,” the team stated in its article, published in the journal Sensors.
Since polymer composites have high sensitivity, low production costs, low power consumption and reasonable response time, the team adopted polymer composites for its humidity and CO2 measurement sensors. A p-type silicon-on-insulator (SOI) wafer was used as the device substrate. The substrate was cleaned and dried using nitrogen gas, and then heated to remove humidity from the surface.
The sensors contain a sensing film in-betweentwo parallel electrodes (Cr/Au) .The humidity sensor was based on the organic materials poly-3,4-ethylenedioxythiophene and poly-styrene -sulfonate (PEDOT:PSS) blended with aluminum–zinc oxide (AZO). The CO2 sensor used a sensing film made from emeraldine base-polyaniline EB-PANI and PEDOT: PSS with a 1/1 ratio.The sensing range of CO2 sensor is from 500 ppm to 10,000 ppm.
Since the sensors operate with very low power, there searchers designed a Complementary Metal-Oxide-Semiconductor system-on-chip (SoC) to amplify and read out the signals obtained by the sensors. The SoC contains an analog-front-end interface circuit, an analog-to-digital convertor, a digital controller anda power management unit (PMU).
The digital controller is employed to power the sensing circuit with a small duty-cycle to reduce power consumption to 3.2 μW. The PMU converts and adjusts the power from a dye sensitized solar cell module into required supply voltages for SoC circuits. The SoC system can be powered by a rechargeable Li-ion battery charged by a photovoltaic transducer through an on-chip integrated charger circuit. Measurement information can be displayed on electronic-paper or on a web browser and stored on a cloud server by using a wireless module.
The sensors developed by the NTU GIEE team is perfectly suited to applications in which the concentration of humidity and CO2 is crucial, such as food safety, retail, agriculture, pharmaceuticals and health, warehouse and storage. They could also be used as part of monitoring systems widely used in many Internet-of-Things applications, such as monitoring air for quality, soil and data centers.
You can learn more about the technology by reading the journal article titled “A Self-Sustained Wireless Multi-Sensor Platform Integrated with Printable Organic Sensors for Indoor Environmental Monitoring.”