Minimising CO2 Sensor Power Consumption
20Pages

APPLICATION NOTE AN004: Minimising CO2 Sensor Power Consumption ABSTRACT All GSS sensors use a technique called non-dispersive infra-red (NDIR) sensing where light is injected into the optical measurement chamber, which contains the gas of interest. The light that passes through the optical cavity is detected by the photo diode. The signal from the photo-diode is digitised by the microcontroller and compared with a reference level stored in memory. The microcontroller can then calculate the level of CO2 in the optical measurement chamber. The power consumed by the CO2 sensor depends on several interrelated factors. The type and efficiency of the light source is the largest contributor to the power consumed by the sensor. In comparison to many other sensors that use incandescent light sources, GSS sensors use a highly efficient Light Emitting Diode (LED) to illuminate the CO2 gas. LEDs can (but not always) be much more efficient in converting electrical power into light than conventional light sources. The length of time the light source is active is also a major contributor to how much power is consumed by the sensor. The light source in a CO2 sensor can be run continuously or pulsed to minimise overall power consumption. Today’s semiconductor devices consume much less power than their forebears due to lower supply voltages, effective power management and shrinking feature sizes. Nevertheless, improvements to signal processing strategies for filtering, data handling and communications can improve power savings. This application note describes the fundamentals that underpin the low power design of GSS sensors and how to calculate the power consumption of each device. Finally, strategies for reducing power consumption whilst optimising overall performance are presented, including the new CozIR®-Blink sensor. Gas Sensing Solutions Ltd. Page | 1 For regular updates, sign up at https://gassensing.co.uk Revision 1.1, 29 March 2021 Copyright © 2021 Gas Sensing Solutions Ltd

Gas Sensing Solutions Ltd. Page | 2 For regular updates, sign up at https://gassensing.co.uk Revision 1.1, 29 March 2021 Copyright © 2021 Gas Sensing Solutions Ltd

APPLICATION NOTE AN004: Minimising CO2 Sensor Power Consumption METHOD OF OPERATION Gas Sensing Solutions’ CO2 sensors uses a solid state, ultra-low power LED light source. This type of light source has several advantages, particularly for low power applications. A CO2 sensor module typically consists of four major functions, a mid-IR LED light source, a photo-diode detector, an optical light path measurement chamber and a microcontroller. Figure 1: Block diagram of typical GSS solid-state LED NDIR sensor Mid-IR 4.25µm light is strongly absorbed by CO2 gas. Light from the LED is injected...

APPLICATION NOTE AN004: Minimising CO2 Sensor Power Consumption LED EFFICIENCY ADVANTAGES When the primary goal is to minimise power consumption of a sensor, choosing one with an LED light source is the optimum place to start. LEDs are semiconductor devices that emit light via the recombination of electrons and holes within the active region of the device (a sequence of specially designed layers manufactured using a process called molecular beam epitaxy). There are several metrics to describe the efficiency of converting electrons into photons, some of which are beyond the scope of this...

APPLICATION NOTE AN004: Minimising CO2 Sensor Power Consumption CO2 MEASUREMENT – PRINCIPLE OF OPERATION GSS CO2 sensors have been optimised for use in battery power applications where the short start-up time and low power consumption offer significant advantages over standard NDIR sensing technology. Regardless of the type of light source, measurement of the CO2 level is determined using the Beer-Lambert law. Beer-Lambert's law states the loss of light intensity when it propagates in a medium is directly proportional to intensity and path length. CO2 molecules absorb infrared radiation at...

APPLICATION NOTE AN004: Minimising CO2 Sensor Power Consumption POWER CONSUMPTION OF DIFFERENT OPERATING MODES The behaviour of GSS sensors depends on the type of control and data interface (UART or l2C) and product family. Common to almost all sensors are several different operating power states. Outlined in this section is how the operating power consumption of the device can be calculated for each stage in the sensing cycle. The different sensing stages are sleep; wake-up; measurement (LED emission and detection); and data processing and transfer. Table 1: List of available modes of...

APPLICATION NOTE AN004: Minimising CO2 Sensor Power Consumption SLEEP STATE Except for the CozIR®-LP3 which has a standby function, for most GSS sensors, the lowest power state is when the sensor is in a Sleep state. The LED and sensors are not active, only the internal clock is being powered, whilst the controller awaits notification to wake-up, change sensor settings or take a reading. The measured average current consumption for a typical GSS sensor in this state is approximately 10 µA, equivalent to 33µW whilst operating in this quiescent state with a 3.3V supply. GSS sensors must be...

APPLICATION NOTEAN004: Minimising CO2 Sensor Power Consumption PARAMETER Figure 2: CozIR®-LP3 wake-up timing sequence in the case of the CozlR®-Blink, the sensor automatically starts taking measurements and goes into the Sleep state once it has calculated a single CO2 reading. The sensor must be power cycled before it can start taking measurements again. MEASUREMENT (LED EMISSION AND DETECTION) Once the sensor has been triggered to wake-up, the on-board controller initialises charging of the LED capacitor bank for pulsed emission and subsequent detection of the transmitted light by the...

APPLICATION NOTE AN004: Minimising CO2 Sensor Power Consumption SIGNAL PROCESSING AND DATA TRANSFER The power consumed at this stage is dependent on how much data is recorded and transferred. The data transfer method depends on which interface method is chosen. The UART interface allows data to be streamed continuously or polled on request. If the sensor is operating in l2C mode, the data can only be polled on request. The typical power consumption during this phase of the sensor measurement cycle will depend on how much data is transferred across the interface. Measurements are based on a...