Non-contact breath sensor based on a Doppler detector

Respiratory activity is an important parameter for observation of human body activity. A normal adult has a respiratory rate of 12–15 breaths/min in normal conditions. Breath sensor based on non-contact Doppler detector allows for the measurement and detection and the absence of the breathing action, which may cause death in a few minutes. The monitoring and detection of some respiratory abnormality presents interest in certain situations (i.e. patients from intensive therapy, of newborn children and many others). The paper covers a new technical solution of the low-cost breath sensor, which consists of the microwave generator together with a resonant patch antenna. There is only one antenna for transmitting and receiving. The technical solution of the oscillator is based on a single FET transistor. The microwave oscillator may be tuned by change of the antenna dimensions. The solution presented here is designed for 2.4GHz (ISM band). The respiratory activity is mainly detected due to the motion of the body. The wave reflected by the moving body surface is mixed with the oscillating frequency by FET transistor junction. Filtering the low frequency signals gives a component that represents the Doppler frequency due to body surface motion. Next, it is processed with high a resolution analogue-digital converter and digitally filtered, and time-frequency analyzed as well. The processing enables the detection of the respiration rate with accuracy 1 beat/min or 0.016Hz. Signal processing in digital domain includes removal of DC offset and out-of-band noise. Experimental results confirm the possibility of using a microwave Doppler detector to measure and analyze respiratory activity signals. This method permits to measure the breathing rate period.