Design of a Non-Dispersive Infra-Red (NDIR) based CO2 sensor to detect the human respiratory CO2

Abstract

Respiratory Carbon dioxide (CO2) contains substantial amount of information that can be used to diagnose and treat pulmonary diseases. Many devices have been developed for this purpose, such as capnography, vital monitor, peak flow meter, spirometer etc. There are many CO2 sensor are available in the market but among them NDIR based sensors are considered to be most inexpensive with its accuracy in terms of sensitivity and fast response time. There are commonly two types of technology available for detection; mainstream and sidestream. Mainstream technology is preferable than sidestream because sidestream is not applicable in intubated patients and at the same time it tends to give delay in detection due to longer transmission tube. Most of the NDIR CO2 sensor are being used for the environmental CO2 detection and there are very few mainstream NDIR based CO2 sensor are available in the market. These sensor have a vast number of advantages with some disadvantages as well; such as high response time, thermal noise, temperature increase and others. This project proposed the specification of the electrical circuit of the NDIR CO2 sensor combined with a gas chamber to detect human respiratory CO2. To determine the specification of the CO2 sensor circuit, the components value has been calculated and then the circuit design has been carried out by using Multisim Software. The overall CO2 sensor circuit has six circuit blocks named oscillator, driver circuit, preamplifier, voltage regulator, rectifier, LPF and each of the blocks were built and simulated in the Multisim software. After the simulation the circuit has been built on breadboard to test the output. An IR source from International Light Technologies (ILT) 4115-2A and pyroelectric photodetector L2100X2020 from laser component were used for this project as NDIR components. After the successful simulation from breadboard a gas acquisition cell has been designed to acquire the human CO2 gas. The design has been done by using Solid Works software and printed from a 3D printing machine. The material used for this chamber was ABS. After placing all the calculated components with the source and detector the output has been observed on the digital oscilloscope as a capnograph wave form showing the voltage range. These waveforms are being used in a capnometer determining respiratory diseases. The circuit shows a response time of 6 second with less noise and the waveform showed clear view of detected CO2 without any temperature increase.