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What does the heart rate signify? It’s a window into your muscles and lungs; it reveals how hard they are working. Your heart pounds to pump oxygen-rich blood to your muscles and to carry cell waste products away from your muscles. The more you demand of your muscles, the harder your heart has to work to perform these tasks. That means your heart must beat faster to deliver more blood. The heart rate gives a good indication during exercise routines of how effective that routine is improving your health.
Once only used by elite athletes, heart rate monitors are now becoming an essential tool for everyone from the casual athlete to the personal trainer. Heart monitors provide an easy and scientific measure of the effort you are putting into your workouts. A heart rate monitor is simply a device that takes a sample of heartbeats and computes the beats per minute so that the information can easily be used to track heart condition. Current technolo gy consists of optical and electrical monitors. The electrical method provides a bulky strap around one’s chest. The optical method does not require the strap and can be used more conveniently than the electrical method.
There are many constraints in producing a heart monitor. First, the technology used to measure the pulse has to be determined. A cost efficient way of measuring the pulse is the combination of a LED and photo-sensor. With the LED technology, ambient light causes excess noise. Therefore, a filter would be needed to attenuate the noise in order that the pulse signal can be extracted. The device must have a display or some way to observe the heart rate. The device should be accurate and easy to use to be attractive to the general pub lic. To make an impact on the market, the design must be small, lightweight, durable and affordable. With these constraints noted, one can propose a design to produce a heart monitor.
Using the design constraints described above, we can now state how we will approach our design. The basis of our design is to construct an efficient and affordable heart monitor. A LED and photo-sensor will be used to measure the pulse by measuring the change in blood flow through one of the index fingers. A noise filter will be designed to filter out any unwanted noise and interference from ambient light. A micro-controller will be programmed to count the pulse rate and control a LED display to show the pulse rate. The device will operate in the power range of 3 volts to facilitate battery operation.
With this low power consumption, it will last for a reasonably long time under normal use. The final product will be packaged in a small, lightweight, and durable package that will be approximately 2.5” x 1.7” x 1”. The production cost will not exceed $90.00.
Heart monitors exist today, so our design is not the first heart monitor to be built.
However, our device will apply to all ages of people who want to monitor their heart rate for any reason. It will provide fast and accurate readings. Because of its portable size, our design can be used at home or the office or any desired location. Under the time constraints, we will only be able to produce a prototype meeting the general constrains above. Many features could be added such as wireless monitoring, ECG technology, and alarm features.
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Table of Contents
Abstract ____________________________________________________________ 4
1. Introduction_______________________________________________________ 4
2. Problem __________________________________________________________ 4
3. Objectives_________________________________________________________ 5
3.1 Pulse Detection __________________________________________________ 6
3.2 Signal Extraction_________________________________________________ 6
3.3 Pulse Digitization ________________________________________________ 7
3.4 Display ________________________________________________________ 7
3.5 Accuracy _______________________________________________________ 7
4. Approach _________________________________________________________ 8
4.1 Optical transmitter and receiving circuit_______________________________ 9
4.2 Filtering_______________________________________________________ 11
4.3 Digitization of the pulse rate signal _________________________________ 12
4.4 Micro-controller ________________________________________________ 14
4.5 Complete Design________________________________________________ 16
4.6 Cost Analysis __________________________________________________ 17
5. Test Specifications _________________________________________________ 18
6. Test Certification _________________________________________________ 19
7. Summary ________________________________________________________ 35
8. Ackno wledgements ________________________________________________ 35
9. Individual Contributions ___________________________________________ 36
10. References ______________________________________________________ 37
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Heart related disease is rising among today’s population. The need for an accurate yet affordable heart monitor is essential to ensure one’s health quality. However, in today’s market, most heart monitors are expensive and are not user friendly. Our goal is to design and build an affordable heart monitor that is user friendly. In order to make our goal possible, we will utilize optical technology to construct an accurate and inexpensive means for one to monitor his heart rate and stay in touch with his physical condition.
The goal of our project is to design a low powered heart monitor that will provide an accurate reading of ones heart rate. The monitor will be easy to use, portable, and affordable. It will measure the heart rate from an index finger using an LED and a photosensor to detect changes in blood flow in an index finger. The heart rate will be displayed on a LED display for easy monitoring. The significance of the heart monitor is that it provides an inexpensive and accurate means of measuring one’s heart rate at his/her convenience.
The human heart rate is a very good indicator of one’s physical condition [1]. In order to monitor the heart, one must have a way to measure his heart rate. Heart rate is measured in different ways. Two of the most common techniques to measure the heart rate are optical and electrical methods. A cost effective and convenient way of measuring the heart rate is using the optical method. The optical method measures the heart rate by sensing changes in blood flow through the index finger. The electrical method requires a bulky strap to be worn around the chest to monitor the heartbeat [4]. The electrical method has an average error of 1% and average cost of $150.00. The optical method has an accuracy rating of 15% and an average cost of $90.00. Our approach is to design a portable monitor that can be used at one’s leisure. Therefore, the optical technology will be used in our design. This approach will result in an accurate and portable package by meeting specified design requirements.
Design requirements are determined by the optical and signal processing technology.
This technology serves as a convenient alternative to other pulse measuring technology.
Other technologies are often inconvenient to use, inaccurate, or expensive. A visit to the doctor’s office will result in the use of an electrocardiogram. Another technique is a stethoscope used in conjunction with a stopwatch [6]. The ECG is accurate but expensive and not as convenient as our proposed design. The stethoscope is a good indicator, but is not as accurate as most people would like. A trip to the doctor’s office is usually expensive which leaves many people wishing for a cheap and accurate alternative that they can use at their own leisure.
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The goal of our project is to design a low powered heart monitor that will provide an accurate reading of the heart rate using optical technology. The monitor will be easy to use, portable, durable, and affordable. We will incorporate the optical technology using a standard red LED and photo-sensor. The LED and photo-sensor will used to measure the heart rate within seconds from an index finger. A micro-controller will be programmed to count the pulse rate. The heart rate will be digitally displayed on a LED display controlled by the same micro-controller that counts the pulse rate. The significance of the heart monitor is that it provides an inexpensive and accurate means of measuring one’s heart rate at his or her convenience. With this in mind, one can begin to see the impact of the heart monitor.
The heart monitor will have a significant impact on everybody that is willing to use it
[10]. The low price of the heart monitor will make it accessible to every household. One will be able to monitor his heart at home safely, which will eliminate doctor visits unless an abnormality is detected. Studies show that SIDS could be primarily a cardiovascular disorder [3]. With the heart monitor, parents with newborn infants showing symptoms of
SIDS could detect any abnormality in the infant’s heart rate [7]. The heart monitor would also have an impact on athletes. Once athletes start monitoring their heart rates, they’ll probably discover the answers to most of the ir fitness questions [8]. With the aid of the heart monitor, athletes can learn to exercise at an ideal heart rate for their training and health maintenance [5]. In athletic training, athletes will be able to use their heart rates to determine how hard they were working [2]. Monitoring the heart rate would help decrease over-training that can lead to injuries and months of rehabilitation. Stress can also be monitored using the heart rate monitor. A person at work could frequently check his or her pulse rate to reduce the onset of stress. In summation, the heart monitor we will design can help monitor anyone’s health at his or her leisure.
1. Pulse Detection : Our device will use a red LED and a photo-sensor paired in a manner that will be able to measure ones heart rate through blood reflectivity changes in the index finger. Attenuation will be on average 80% of transmitted light.
Voltage created will vary between 0 and 10 mV with respect to each heart pulse.
2. Signa l Extraction : We will use a low pass filter to remove any interference caused by ambient light and level detection distortions. The filter used will have a cutoff frequency of 4 Hz, a roll off rate of 20 dB/decade, and pass-band frequencies amplified by a factor of 40dB.
3. Pulse Digitization : The conditioned signal will be analyzed by a comparator to provide a digital pulse of amplitude 1Vpp to be fed into the micro-controller digital input. This stage of the design will require that the amplified and filtered heart pulse signal have a SNR of 20 dB.
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4. Display : The heart rate will be displayed on a 3-digit LED display. A microcontroller will be programmed to operate the LED display. A 3 digit LED display will be used because it will allow the full range pulses detected by the device to be displayed.
5. Accuracy : Our design should give accurate readings with no more than +/- 10% error. The accuracy of current optical heart monitoring devices is typically 10-15%.
6. Power : The device will operate using a 3-volt lithium coin cell battery source because of the small packaging being utilized by our design. Our target battery is the commonly used CR2477, which has a current capacity of 1000mAh. The device will consume 30mA or less. The battery should last one year under normal use.
7. Durability : The device will be designed to operate in a standard temperature environment of –30 C to 80 C. The device will be shock and water-resistant.
8. Physical Packaging : The final packaged dimensions shall be no larger than a
2.5” x 1.7” x 1” (H x W x D). This small packaging will be achieved by taking advantage of the small footprint of current SMT devices.
9. Cost : Cost will be kept to a minimum to maintain a competitive edge with currently available products. A maximum estimate for production at this point is
$90.00. This cost will be achieved through the unique design approach and component selection we will employ.
Our device will use a red LED and a photo-sensor paired in a manner that will be able to measure ones heart rate through blood reflectivity changes in the index finger.
The power transmitted by the LED will be matched to the photo sensor in such a way that the resistance will vary within the range capable of the photo sensor after attenuations through the index finger. Since attenuations will vary somewhat depending on the person using the device, our specifications will assume the attenuation will be on average 80% of the light transmitted. A resistance network will be used with the sensor to transform the changes in resistance to changes in voltage. The voltage created will vary between 0 and 10 mV with respect to each heart pulse.
We will use a low pass filter to remove any interference caused by ambient light and level detection distortions. The filter used will have a cutoff frequency of 4 Hz to allow a maximum heart rate of 200 BPM (Beats/Minute) to be measured by the device with minimal loss in accuracy. Since the filter roll off is not required to be
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HEART MONITOR Page 7 of 38 high performance, the filter will use a roll off rate of 20 dB/decade. This roll off will provide an attenuation of the greatest source of noise, 60Hz, by 23.5dB. The pulse will have a –14 dB SNR before it will pass through the filter. The pass-band frequencies will be amplified by a factor of 40 dB with the aid of a small signal amplifier. DC blocking will be used to prevent immeasurable pulses caused by a high dc offset from ambient light.
The conditioned signal will be analyzed by comparator to provide a digital pulse of amplitude 1Vpp to be fed into the micro-controller digital input. The comparator will detect the peak of each pulse and create a corresponding digital pulse. This stage of the design will require that the amplified and filtered heart pulse signal have a SNR of
20 dB to obtain a clean digital pulse. The time between each successive rising digital pulse edge will be interpreted by the micro-controller as the period between each heart pulse.
The heart rate will be displayed on a 3-digit LED display. A micro-controller will be programmed to operate the LED display. Since improper finger placement could cause problems with the user, a flashing segment on the LED display will be used to help the user identify when a heart rate is being measured. A 3 digit LED display will be used because it will allow the full range pulses detected by the device to be displayed.
Our design should give accurate readings with no more than +/- 10% error. This accuracy is not quite as tight as the tolerance of ECG monitors (+/- 1%) but a significant cost reduction will be achieved by taking our approach. The accuracy of current optical heart monitoring devices is typically 10-15%. The accuracy of our device is controlled primarily by the amount of time used to average the pulse rate from the user. The more time used means more samples and thus greater accuracy.
Our device will use a method that will have a dynamic time for pulse gathering. The pulse will be averaged after a set number of pulses are obtained, five in our case, by the micro-controller. This will allow a better distribution of accuracy over the range of pulses that can be measured by the device.
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The device will operate using a 3-volt lithium coin cell battery source because of the small packaging being utilized by our design. The use of SMT devices will help in the reduction of power needed and thus power source size. A standard watch battery form will be used to aid in compact packaging. Our target battery is the commonly used CR2477. This choice of battery will provide us with a nominal current capacity of 1000 mAh. Given that our device will consume no more tha n 30 mA, the duration of the battery should be 33 hours. The battery should last one year under normal use; however, this will vary from between users. Typical usage of 5 minutes per day will allow the device to operate for 1 year.
The device will be designed to operate in a standard temperature environment of –30
C to 80 C. The device packaging will be designed to hold up under normal usage.
The packaging will be shock and water-resistant.
Our design is a small and lightweight device that will be the size of a standard stopwatch. It will be approximately the size of one’s palm for easy usage. The final packaged dimensions shall be no larger than a 2.5” x 1.7” x 1” (H x W x D). This small packaging will be achieved by taking advantage of the small footprint of current SMT devices.
Cost will be kept to a minimum to maintain a competitive edge with currently available products. A maximum estimate for components at this point is $30.00.
This cost will be achieved through the unique design approach and component selection we will employ. Our design will have a market price of $90.00 dollars.
To successfully simulate this project, design will be done in stages. The first stage will be design a circuit that will receive the pulse rate. Next, the signal will be extracted using a filter. Next, the signal must be digitized so that it can be counted.
Once these three stages are designed, they will be tied together to show a working design through simulation. If this is done successfully, we should have proof that our design will work. Next, a micro-controller will be used to count the pulse rate and
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HEART MONITOR Page 9 of 38 display it on a three digit LED display. Finally, each stage will be put together to form a working project.
In order for our design to work, the pulse rate must be measured through changes of blood flow through an index finger. Each pulse of blood from the heart will increase the density of blood in the finger pulsatile tissue and cause a decrease in light power received by the photo-sensor. The Figure 4.1-1 shows a diagram of tissues that will contribute to the received signal. The photo-sensor will not pick up a purely AC signal since there are some DC components received from other non-pulsatile tissues and ambient light levels. The varying light levels received will be converted into a varying resistance in the photo-sensor. The varying resistance will be converted into a varying voltage by using a resistance network and power source. In order to do this, a red led will be used in combination with a photo-sensor to detect and transmit the pulse rate. Since the tissue in the human body acts a filter for red light (Figure 4.1-2), a red LED was chosen to allow the maximum amount of light energy to pass through the index finger. The circuit shown in figure 4.1-3 was designed to do this. The red
LED is basically a stand-alone system.
Figure 4.1-1 Pulsatile index finger tissues
Figure 4.1-2 Attenuation Levels of Light through blood fluids
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The circuit in Figure 4.1-3 shows our pulse-rate to voltage converter and our source of constant red light that was designed and constructed to gather real- world data for our analysis and design of our pulse detection system. The red LED is simply forward biased through a resistor to create a current flow. The value of R2 was chosen based on the current needed by our 3000 mCandela LED to produce a maximum amount of light output but that was within the current limits of the device.
The value calculated was then approximated to a resistance value that is commonly available. The photo-resistor is placed in series with a resistor to reduce the current drawn by the detection system and to prevent shorting the battery when no light is detected by the photoresistor.
Figure 4.1-3 Pulse receiving circuit
The diagram in Figure 4.1-4 shows the configuration of the LED and photo-sensor in relation to the index finger. They are placed in such a way that the light has to pass through finger tissue before it enters the photo-sensor.
Figure 4.1-4 Finger positioning
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Once the pulse rate has been detected, it will need to be extracted. A second order butterworth filter will be used to do this. Using P-SPICE and MATLAB, we will simulate the filter to make sure it will operate under our specified design requirements.
Due to interference caused by ambient light and level detection distortions, the desired signal can be extracted from the noisy signal using a second order low pass
Butterworth filter. This filter is chosen because it is characterized by a magnitude response that is maximally flat in the passband and monotonic overall. Butterworth filters sacrifice roll-off steepness for monotonicity in the passband and stopband.
Since the heart signal we are measuring is low in amplitude, we do not want any attenuation in the passband and therefore the second order Butterworth filter will meet our design objectives.
The frequency response of the filter is obtained by using the following equation:
__________
H(jw) = K / v1 + (w/w c
)
2n
The pole- zero plot for the filter is obtained and shown below;
Figure 4.2-1 Pole-Zero plot of 2 nd
Order Low Pass Filter
The gain of the filter used is 1, and R1 = mR, R2 = R, C1 = C, C2 = nC. Since it is a lowpass Butterworth filter, m = 0.229 and n = 3.3. By setting C2 = 0.068 µ F, the
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HEART MONITOR Page 12 of 38 value of the resistance R1 and R2 and capacitance C1 are obtained using the following equation:
___
Figure 4.2-1 2 nd
order Butterworth Filter
In order to use a micro-controller to count the pulse rate, the signal must be digitized.
A phase-shift comparator will be used to find rising edges of the filtered signal received by the photo-sensor. A one-bit Analog to Digital converter should be adequate to digitize the pulse signal. This will allow one pin of the micro-controller to be used as an input. The time between rising pulse edges will be determined by the microcontroller so that the frequency of the heart rate can be measured. We will design a comparator that has a performance that will produce a reliable output for a relatively low input SNR. The designed circuit is shown in Figure 4.3-1.
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Figure 4.3-1 Comparator
The output from this comparator should be a digital signal that the micro-controller can monitor easily. The circuit operates on the principle that the original signal is subtracted from a phase-shifted version of the original. The phase shifting is obtained by passing the original signal through a first order passive low pass filter. This subtraction is positive for a rising slope and negative for a decreasing slope. Since the op-amp used for the comparison is configured with no feedback the output will change to its maximum positive value for rising edges and change to its maximum negative value for falling edges.
The input to the comparator design sho wn in Figure 4.3-1 is required to be on the order of 20 dB. The required SNR level will be obtained from the previous low pass filtering stage. An input signal with an SNR any lower than the 20 dB limit will cause the comparator to falsely trigger on the noise present in the signal. Thus, the low pass filtering of the raw signal is crucial to the proper operation of the comparator circuitry.
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A micro-controller is an economical means of counting the pulse rate and controlling a LED display. The circuit diagram in figure 4.4-1 shows the combined microcontroller and display interconnection. Since board space is at a premium in our design, we directed our design toward reducing the number of large chips used.
The method used below allows the displays to be driven without the use of a display driver. The displays will be set and refreshed by multiplexing the segment lines to the same I/O pins on the microcontroller.
Figure 4.4-1 Micro -controller
Programming the micro-controller will involve developing a calculation algorithm to count the pulse rate. The calculation algorithm for counting the pulse rate will be easy to develop using Firmware.
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The micro-controller will continuously be checking if a signal is fed into it. Once a signal is detected the algorithm will begin as follow:
1) Set the timer to zero.
2) Increment the timer for every peak of the digitized signal.
3) Repeat step two until timer equals 5, and the time taken to reach the 5 counts (Na) is stored.
4) Calculate the beats per minute using equation: BPM = 12Na.
5) Send the heart beat rate to the LED display.
6) If the reset button is pushed the routine will reset.
Figure 4.4-2 Algorithm for detecting pulse rate
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Once each stage of the design has been simulated to prove that each stage will work, the design will be tied together to form a complete working design. The signal will be transmitted and received using the circuit in Figure 4.1-3. Next, using the lowpass filter in Figure 4.2-1, the signal will extracted so that it can be digitized using the comparator circuit in Figure 4.3-1. Once the signal is digitized, it will be counted and displayed using the micro-controller in Figure 4.4-1. In Figure 4.5-1 below, this process is illustrated.
82
Figure 4.5-1 Flowchart of design
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At this point, we have documented the necessary components along with component prices to construct the design that we have simulated. Our design constraint for component cost is $30.00. The chart below shows that this requirement has been met at this point.
Red LED (RS-2760307)
$1.99
Photo-Sensor (RS-2761657)
Micro-controller (PIC16F84)
741 Op-Amp (LM324D)
3 volt battery (614-CR2477N)
Capacitors and resistors
LED display (153-1005-ND)
PCB board
Plastic package (SCRA-ND)
Battery Clip (RS-2710777)
Transistor (2N2222)
Table 4.6-1 Cost Analysis for components
$0.45
$7.33
$0.56
$2.90
$3.85
$5.70
$1.99
$3.06
$0.79
$0.99
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In order to obtain specifications for our design, several preliminary tests will be conducted. These tests will consist of both analog and digital applications. Hardware and software will be used to perform these tests. We will construct prototypes of each stage of our design to ensure that they interface properly.
In table 1 below, a description of the tests used to verify our requirements is illustrated. By performing these tests, we will produce the needed data to ensure that our design will interface and work properly.
Requirement
Optical
Transmit and
Pulse Detection
Receive Test
•
Signal Extraction
•
Circuit
Simulation
•
Noise Filtering Firmware Test Physical
Packaging
•
•
Accuracy
•
Display •
Duration •
Power •
Durability
•
Packaging
•
Cost
• •
Table 1. A general description of the tests to be used in the design of a heart rate monitor.
The transmit and receive testing will be done using the LED and photo-sensor. A simple circuit using these components will be constructed and tested to obtain necessary data. From this data, we will be able to examine pulse detection and experiment with signal extraction.
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This test will help analyze several requirements. Using P-spice, we will construct schematics in different stages of our design. Designing a filter to attenuate the noise will simulate signal extraction. Power and duration requirements will be determined by simulation. Once we have a complete schematic of our design, the accuracy requirement can be tested. From this simulation, specifications calculated will determine which parts to buy that will address the cost requirement.
This test will begin by capturing raw data from the LED and photo-sensor. An oscilloscope will be used to detect noise from ambient light and other sources. This test will determine the amount of noise that will be attenuated to extract the pulse signal. Matlab will be used for some signal analysis.
The prototype heart monitor was tested so that it meets all the test specifications set in the beginning of the project. This is done by comparing the hardware results with the simulated results to verify its functionality. The tests are conducted in several parts to ensure that each individual component of the design is functioning.
Before the hardware was build, every single part of the design was first simulated.
The simulations are very important in making sure that the design will function. In this design, the simulations were divided into several parts: optical receive and transmit circuit, filter circuit, comparator circuit and finally, the entire integrated circuit.
A simple circuit containing a red LED matched to the photo sensor is connected to an oscilloscope to measure the signal received by the photo sensor.
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Figure 6.1.1-1 Pulse Receiving Circuit
The pulse receiving circuit shown above is used to obtain real data using the O-
Scopes in the Communications Laboratory. The scope plot in Figure 6.1.1-2 is obtained from the pulse receiving circuit under low ambient light noise condition.
This is achieved in an environme nt where there is no ambient light source in the vicinity. This plot contains the heart beat signal, which is around 2Hz and some
120Hz noise.
Figure 6.1.1-2 Real Pulse with low noise
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Matlab was then used to simulate the obtained pulse rate with low noise data and figure 6.1.1-3 shows the plot obtained from the simulation.
v t
H
Figure 6.1.1-3 Matlab simulated plot for real pulse rate with low noise
In Matlab, the FFT of the low noise signal was obtained to measure its SNR. From the plot above, the SNR is found to be 9.54 dB.
The second scope plot shown in Figure 6.1.1-4 on the other hand is obtained under noisy ambient light condition. This plot contains the heart beat signal and a large sum of 120Hz noise.
Figure 6.1.1-4 Real pulse rate with heavy 120 Hz noise
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Again, Matlab was used to simulate the obtained pulse rate with heavy 120 Hz noise data, shown in figure 6.1.1-4. v t
Hz
Figure 6.1.1-5 Matlab simulated plot for real pulse rate with heavy 120 Hz noise
From the plot above, the SNR is calculated to be –18dB. The plot shows the signal is mainly influenced by 120Hz noise and its harmonics at 240Hz.
A second-order butterworth low-pass filter was used to filter the noise from the signal. Figure 6.1.2-1 shows the Butterworth 2 nd
order low-pass filter circuitry. The filter will have a cutoff frequency of 4Hz to allow a maximum heart rate of 200 beats per minute. The filter also has a roll off rate of 20dB/decade. Referring to the output of the filter simulation at figure 6.1.2-2, this roll off will provide an attenuation of the greatest source of noise at 120Hz by 60dB
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Figure 6.1.2-1 Butterworth 2 nd
Order Low-Pass Filter
Figure 6.1.2-2 Output from the filter simulations
The filter is also being simulated using Matlab to verify its functionality. The raw heart pulse signal obtained from section 6.1 is used in the simulation of the
Butterworth 2 nd
order low-pass filter. Figure 6.1.2-3 shows the Matlab simulated plot for the filtered signal with low noise.
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Hz
Figure 6.1.2-3 Matlab simulated plot for filtered signal with low noise.
From the plot above, the SNR of the filtered signal with low noise is calculated to
34dB.Before the filtering process this signal had a SNR of 9.54dB.
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Figure 6.1.2-4 below shows the Matlab simulated plot for the filtered signal with heavy noise.
Figure 6.1.2-4 Matlab simulated plot for filtered signal with heavy noise.
From the plot above, the SNR of the filtered signal with low noise is calculated to
23.5dB.Before the filtering process this signal had a SNR of –18dB. This plot also proves that the Butterworth 2 nd
Order low-pass filter has successfully removed the
120Hz noise.
The design of the comparator shown in Figure 6.1.3-1 is used to convert the continuous signal to a discrete signal of 1Vpp. The comparator requires the incoming signal to have a SNR of at least 20dB so that it can successfully digitize the signal.
An input signal with an SNR of any lower than 20dB will cause the comparator to falsely trigger on the noise present in the signal. The plot in Figure 6.1.3-2 shows the
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Figure 6.1.3-1 Comparator circuit
Figure 6.1.3-2 Comparator Simulated Output
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The comparator is also being simulated using Matlab to verify its functionality. The raw filtered heart pulse signal obtained from section 6.1.2 is used in the simulation of the comparator. Figure 6.1.3-3 shows the matlab simulated comparator output for the filtered signal with low noise. Figure 6.1.3-4 shows the matlab simulated comparator output for the filtered signal with heavy noise.
Figure 6.1.3-3 Matlab simulated comparator output for signal with low noise
Figure 6.1.3-4 Matlab simulated comparator output for signal with heavy noise.
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This stage of simulation contains the pulse receiving circuit, the filter and comparator.
Using P-SPICE, the pulse rate is simulated using a 60 Hz and 120 Hz noise under ideal conditions. This simulation shows that under ideal conditions, our design is successful. The figure below shows three plots that show the process of detecting, filtering, and digitizing the pulse-rate signal.
Figure 6.1
-
P-SPICE simulations using theoretical noise
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The first phase of the prototype, the optical receiver and transmitter, is constructed and tested. The output of the receiver is connected to an O-scope to obtained the heart beat signal. Figure 6.2.1-1 shows the heart beat signal obtained from the prototype.
As expected from our simulation, the obtained signal contains 120Hz noise. This signal has a SNR of –9dB.
Figure 6.2
-
Signal
A second-order butterworth low-pass filter is used to filter the noise from the heart beat signal. The low pass filter has a cutoff frequency of 4Hz and a roll-off rate of
20dB/decade. Figure 6.2.2-1 shows the real output obtained after the noisy heart signal is passed through the filter. Comparing figure 6.2.1-1 and figure 6.2.2-1, it is clear that the high frequency noise of 120Hz from ambient lights has been filtered out as expected. The filtered signal is required to have a SNR of 20dB or greater, to ensure that the comparator is able to correctly convert the continuous signal to a digitize form without producing false trigger due to noise. The filtered signal below has a SNR of approximately 24dB, and this will allow the comparator to properly digitize the heartbeat. This test shows that the filter is able to remove high frequency noise from the heartbeat signal.
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Figure 6.2
-
Filter Output
A phase-shift comparator is used to detect rising edges of the filtered signal. The comparator will convert the continuous signal from the low pass filter to a digital signal of 1Vpp and then feed this digitized signal into the micro-controller. The comparator will be able to produce a digital output if the input signal has a SNR of order 20dB or higher. Figure 6.2.3-1 is the real output of the comparator, where the continuous heart signal was successfully converted to a digitized form.
From figure 6.2.3-1, three spikes occurred due to the comparator falsely triggering the signal. This happened because the photon sensor of the prototype does not have a stable position on the breadboard. However, at the final packaging, this problem would not arise because it will be fitted properly on the PC board.
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Figure 6.2.3-1 Comparator Output
The micro-controller is programmed to count the number of peaks of the input signal in 10 seconds, and the result is further multiplied by 6 to obtain the total number of peaks per minute. In the testing of the micro-controller, the LED display is connected to the micro controller and a known frequency pulse signal is fed into it. The correct number of peaks per minute value is showed on the LED display. When the microcontroller is integrated into the entire design circuitry, it is able to count the number heartbeats per minute and drive the LED display to display the counted value.
Therefore, the interface between the entire circuit and the micro-controller is functional. The accuracy of the heart monitor is also tested. The only way to perform this test is to obtain a more consistent and accurate heart pulse from an electrocardiogram (EKG) and compared the results with the one obtained using the prototype heart monitor. Table 6.2.4-1. shows the results of the comparison.
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Electrocardiogram
(beats per minute)
76
Heart Monitor
(beats per minute)
78
Percentage Error (%)
2.56
78
76
82
83
85
78
72
84
84
90
0.00
5.56
2.38
1.19
5.56
77
79
89
84
84
96
8.33
5.95
7.29
Table 6.2.4-1 Accuracy Comparison with an Electrocardiogram
The comparison shows that the accuracy of the prototype heart monitor has a mean of
4.31 and standard deviation of 2.87. However, this accuracy may defer depending on the circumference size of the finger of the user. Table 6.2.4-2 shows the percentage average error of the prototype for different users with different fingers’ circumference size.
Table 6.2.4-2 Accuracy Comparison with different finger sizes
The initial proposed power consumption was to use a CR 2477, 3-volt lithium coin cell battery source. Initial estimate of power consumption of the design was no more than 30mA. However during the process of implementing the design, the circuitry is consuming more power than expected. The reason being so, is because an amplifying stage was added in the design to amplify the incoming signal from the photo sensor.
In driving the LED display, the micro-controller is also consuming more power than was expected. The design is now consuming approximately 36mA. To solve this problem, an additional CR 2477, 3-volt lithium cell battery will be used to operate the device.
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This part of the test is not performed on the final package of the heart monitor because the final product is not functioning.
In order to fulfill the size constrained as proposed in the project, a printed circuit board (PCB) is used for the heart monitor. Surface mount technology (SMT) components are used for the design due to the small size of the board. A very difficult task was faced by the team members, in the process of soldering the SMT components onto the PCB. This is because every SMT components used are extremely small and thus, soldering the components requires tedious job and patience.
Figure 6.2.7-1 shows the PCB used for the final package of the design, while figure
6.2.7-2 shows the size comparison of one of the SMT resistors used in the final package with a through-hole resistor.
Figure 6.2.7-1 Printed Circuit Board
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Figure 6.2.7-2 Size Comparison of SMT and Through-hole resistor
However, the components are successfully mounted onto the PCB and the PCB is placed into an enclosed rectangular box with the dimension of 2.5” X 1.7” X 1” (H X
W X D). Figure 6.2.7-3 below shows the layouts of the design in its final package.
Figure 6.2.7-3 Final Package Layouts
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The final cost of the prototype including all the components used is shown in Table
6.2.8-1. However, if the heart monitor is produced in bulk, the bulk cost per unit will not exceed USD 24.86.
Part Price (USD)
Red LED (RS-2760307)
Photo-Sensor (RS-2761657)
Micro-Controller (PIC 16F84)
741 Op-amp (LM 324D)
3 Volt Battery (614-CR2477N)
Capacitors and resistors
LED Display (153-1005-ND)
PCB Board
1.99
0.45
6.58
0.50
3.76
3.34
5.58
9.99
Plastic Package (SCRA-ND)
Battery Clip (RS-2710777)
2.99
1.56
Transistor (2N2222)
Total
Table 6.2.8-1 Final Prototype Cost
0.30
37.04
Through thorough simulation, we have shown that the pulse rate can be detected from changes of blood flow through an index finger. Simulation also shows that the pulse rate can be filtered and digitized so that it can be counted to calculate an accurate pulse rate. The results obtained from the prototype matche s the simulation results obtained in Senior Design I. During the course of implementing the prototype, only one design requirement was changed. The power requirement for the design was changed from using one 3- volt lithium cell battery to two 3-volt lithium cell battery.
However, the overall design was a success. The prototype is able to detect, filter, digitize, and display the heartbeat of a user.
We wish to acknowledge Dr. Lori Bruce for her technical support regarding this project. We also want to thank Dr. Joe Picone for his guidance through the design process.
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Todd Peacock
•
Filter and comparator design
•
Micro-controller frequency detection
•
Optical signal generation and detection
•
Matlab and Pspice simulation
•
Packaging
•
Documentation
Craig Williamson
•
Filter and comparator design
•
Optical signal generation and detection
•
Matlab and Pspice simulation
•
Packaging
•
Documentation
Chong Meng Teh
•
Updating and maintaining the webpage
•
Optical signal generation and detection
•
Matlab and Pspice Simulation
•
PCB layout design
•
Documentation
Voon Siong Sui
•
Filter design simulation
•
Micro-controller LED display programming
•
Matlab and Pspice simulation
•
PCB layout design
•
Documentation
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[1] Z. Kmietowicz, “Better coronary care reduces deaths from heart disease,” British
Medical Journal, p600, Tavistock Square, London March 2000.
[2] J. Weller, “You’ve got the beat,” Women’s Sport and Fitness, p68, USA,
September 1997.
[3] R. Bandler, “Cardiac problem at heart of SIDS?”, BBI Newsletter,
American Health Consultant, Inc., USA, April 1998.
[4] J. Hastings, “You Call This Exercise?”, Health, Vol. 14, p106, USA, June 2000.
[5] M. Bane, “Watch the Monitor,” Men’s Health, Vol. 11, p144, USA, November
1996.
[6] P. Kandela, “The stethoscope,” The Lancet Interactive, UK, p23, September
1998.
[7] J. Carroll, “SIDS: counseling parents to reduce the risk,” American Family
Physician, Leawood, KS, April 1998.
[8] J. Csatari, “The beat goes down,” Men’s Health, Vol. 10, p23, USA, September
1995.
[9] T. Fritz, “The Heart of bodybuilding,” Muscle & Fitness, USA, p77, October
1999.
[10] C. Smith, “How to become your own personal trainer with the use of a heart rate monitor,” Fire Engineering, Vol. 149, p118, USA, June 1996.
[11] J. Hilburn and D. Johnson, Manual of Active Filter Design, McGraw-Hill, Inc.,
New York, 1973.
[12] A. Roll, “PIC-based Controllers”, Electronics Australia, Vol. 62, p70, September
2000.
[13] S. Orfanidis, Introduction to Signal Processing, 1 st
edition, Prentice Hall, Inc.,
New Jersey, USA, 1995.
[14] R. Goody, Microsim Pspice for Windows: A Circuit Simulation Primer, 2 nd edition, vol. 1, Prentice Hall, Upper Saddle River, New Jersey, USA, 1998.
[15] T. Sifferlen and V. Vartanian, Digital Electronics with Engineering Applications,
Prentice-Hall, Inc., New Jersey, 1970.
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[16] M. Fogiel, Modern Microelectronics, Research & Education Association, New
York, 1972.
[17] J. Millman, Digital and Analog Circuits and Systems, McGraw-Hill, Inc., New
York, 1979.
[18] D. Comer, Modern Electronic Circuit Design, Addison-Wesley Publishing
Company, Inc., 1976.
[19] D. Defatta and J. Lucas and W. Hodgkiss, Digital Signal Processing: A System
Design Approach, John Wiley & Sons, Inc., New York, 1988.
[20] PIC Micro Microcontroller Families, http://www.microchip.com/10/lit/pline/picmicro/families/16f8x/index.htm,
Microchip Corporation, Santa Clara, California, USA.
[21] D. Hanselman and B. Littlefield, Mastering MATLAB 5, Prentice Hall, Upper
Saddle River, New Jersey, USA, 1998.
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