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Final Project: Apnea Detector using Capacitive Humidity Sensor Ariston Gonzalez, 2006-02657 Alvin Merck de Leon, 2006-17739 Joy Santos, 2006-18825 Department of Electrical and Electronics Engineering University of the Philippines Diliman Abstract – This project is an apnea detector that is interfaced using a capacitive humidity sensor. When a person stops breathing for approximately 10 seconds, it will be indicated in the hyperterminal as an occurrence of apnea. The varying capacitance of the sensor is mapped into the 500mV to 2V range. The voltage range is interfaced in the microcontroller using it ’s analog-todigital converter. Apnea is detected when a 10% drop from the average breathing relative humidity of the subject.
comprehensive recording of the biophysiological changes that occur during sleep. An apnea monitor can and will aid in this test to identify the sleep disorder. Also by making it able to identify the sleep disorder, prevention is possible thanks to the apnea monitor. Although apnea monitors are designed using sensors simultaneously simultaneously monitoring a subject; in this project, only the relative humidity of the breath is checked. By this statement, the breathing pattern is analyzed by measuring the relative humidity of the breath because the human breath is 95% humid.
I. INTRODUCTION
A
PNEA is a term for suspension of external breathing. During this event, the movement of the muscles of respiration come to a halt and the volume of the lungs initially remains unchanged. Flow of gas between the lungs and the environment depend on the openness of the airways. Apnea can be voluntarily achieved by a person holding his/her breath, druginduced, mechanically induced by strangulation or choking. Prolonged apnea leads to severe lack of oxygen in the blood circulation. Permanent brain damage can occur after just as little as three minutes and even death for longer time than that. The volume of each breath is tightly regulated to maintain constant values of CO 2 tension and pH of the blood. In apnea, CO2 is not removed through the lungs and is collected in the blood. The rise in CO2 tension and drop in pH result in stimulation of the respiratory center in the brain which eventually cannot be overcome voluntarily. Sleep apnea is a sleep disorder wherein breathing is paused during sleep. Each episode, called an apnea, lasts long enough so that one or more breaths are missed. This is the more general knowledge which people associate apnea to. Such episodes occur repeatedly throughout sleep. It is standardized to be called an apneic event when either a 10 second interval between breaths occur or blood oxygen destaturation of 3-4% or higher. Sleep apnea is diagnosed with an overnight sleep test called a polysomnogram or a “sleep study”. Polysomnogram is a
II. MOTIVATION
Apnea is a period of time during which breathing stops or is markedly reduced. It usually occurs during sleep, and when it occurs, sleep is usually disrupted and the person may go into a shallow level of sleep or even wake up completely. Some of the complications of having sleep apnea are post-sleep fatigue, stress, or even general weakness of the muscles. An estimate of the severity of apnea is calculated by dividing the number of apneas by the number of hours of sleep, giving an apnea index (AI). The bigger the apnea index, the more severe the apnea is. The apnea index can then be closely observed by using an apnea monitor. An apnea monitor is a piece of equipment that records breathing patterns of a person. The main purpose of this project is to monitor and record the breathing pattern of a patient and eventually give signals while apnea is occurring. Obstructive sleep apnea (OSA) is the most common category of sleep-disordered breathing. The muscle tone of the body ordinarily relaxes during sleep, and at the level of the throat the human airway is composed of collapsible walls of soft tissue which can obstruct breathing during sleep. Mild occasional sleep apnea, such as many people experience during an upper respiratory infection, may not be important, but chronic severe obstructive sleep apnea requires treatment to prevent low blood oxygen hypoxemia, sleep deprivation, and other complications. The most serious complication
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is a severe form of congestive heart failure called cor pulmonale. Individuals with low muscle tone and soft tissue around the airway (e.g., because of obesity) and structural features that give rise to a narrowed airway are at high risk for obstructive sleep apnea. The elderly are more likely to have OSA than young people. Men are more typical sleep apnea sufferers than women and children, although it is not uncommon in the latter two. There has been, in recent years, an increasing awareness of the incidence as what is now known as sudden infant death syndrome, or crib death. In some instances, this tragedy has been attributed to apnea, or a cessation of breathing, of the infant that usually lasts for more than 15 seconds. Usually it happens to premature infants and is also called Apnea of prematurity. This project may then be used in Medical Instrumentation for both children and adults in prevention of worse occurrences.
III. OBJECTIVES
Figure 3. Basic Integrator Op-amp configuration Solving for the KCL equation at the inverting terminal of the op-amp we will have:
The output of the basic op-amp integrator circuit has a voltage output that drifts incrementally. To eliminate this voltage drift at the output, resistor R 2 is placed in parallel with the feedback capacitor C1 (Figure 4).
To be able to observe breathing patterns. (Inhale/Exhale detection) To be able to detect apnea occurrence To be able to make data logging/record breathing patterns IV. METHODOLOGY
The design’s block diagram is shown in Figure 1.
Signal Conditioning
Sensing
uController Interface
PC interface
Figure 4. Integrator Circuit with Drift regulator
Figure 1. Apnea Detector Block Diagram The sensing and the signal conditioning blocks comprise the Instrumentation stage of the project. This stage will be the bulk of the hardware of the project and is divided into three main stages (Figure 2).
Integrator Circuits
Instrumentation Amplifier
Peak Detector
Figure 2. Hardware Block Diagram The basic configuration of an op-amp integrator circuit is shown in Figure 3.
Any change that the capacitive sensor experienced has little effect on the variation of the amplitude of the integrated output. Therefore a need to amplify this change without amplifying the whole output is introduced in the circuit. A difference amplifier was the first choice for this task, but the many advantages that an instrumentation amplifier offers made us decide to use the latter. The inputs that will be fed to the instrumentation amplifier will come from two integrating op-amp circuits. The first integrator circuit bears the capacitive sensor as the feedback while the other integrator circuit has a fixed valued capacitor. The second integrator will serve as the reference integrator circuit which will be subtracted from the first integrator. The expected output of the instrumentation amplifier will then contain, at the very least, the small change or variation that the sensor will introduce if any change in humidity is sensed. The instrumentation amplifier should
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also have a gain control in order to change the magnitude of the small change that is captured.
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The operation of the peak detector is based on the idea of holding off the charge that a capacitor holds. The diode will conduct positive half-cycles, charging the capacitor to the waveform’s peak. When the input waveform falls below the DC peak stored on the capacitor, the diode is reversed biased, blocking current flow from capacitor back to the source. Thus, the capacitor retains the peak value even as the waveform drops to zero. V. HARDWARE AND SCHEMATIC
Figure 5. Instrumentation Amplifier Circuit
Where, R8=R7, R10=R9, R5=R6, Rpot = VR1. VR1 is a 10kΩ potentiometer place in between the difference op amps to serve as the gain of the overall instrumentation amplifier. (Figure 5)
Figure 7. Schematic Full
Figure 8. Actual Circuit VI. RESULTS AND ANALYSIS
Figure 6. Modified Peak Detector Circuit Overall, the effect of any variation in humidity has been translated into a change in the amplitude of a voltage output. The instrumentation amplifier stage’s output is still an imperfect sawtooth waveform with varying amplitude depending on the relative humidity sensed by HS101. To isolate the variation we are interested in further, a peak detector is introduced as a mediator between the instrumentation part of the project and the uController.
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From the relative humidity curve of rough rice (also tested in the project),
VII. CONCLUSION
The project was successful in detecting apnea and monitoring breathing patterns using only a humidity sensor. Apnea may not be detected successfully through this project but can be prevented at a greater probability. Hardware construction was not very tedious once the required stages were planned accordingly. This project also aided in proving the instrumentation amplifier from the lecture class because of the accurate output of the circuit. Software programming helped in giving a more reliable and real time measurement. The actual sensor also assisted the project because it easily fitted the breathing mask generally available in the public (does not need a specific breathing mask).
VIII. RECOMMENDATIONS
The project could have been more marketable if it was contained in a mobile package wherein it only requires a single voltage supply. To do this, the operation amplifiers that were used could be replaced with single supplied operational amplifiers like the LM358. An alarm system can be interfaced in the project when a warning needs to be noticed using a simple buzzer. A more in depth data recording could also improve this project. IX. REFERENCES
Dry Rice Wet Salt Human Breath
1.1V 1.4V 1.5V-1.6V
From the table, the characteristic equation is
25%RH 75%RH 95%RH
[1] [2] [3]
C-5Rice Drying Principles.pdf. HS1101 Capacitive Humidity Sensor Datasheet LM741 Datasheet
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