Team Silver – Monday Lab, Friday Discussion
Project Report
Sam Jones, Ranjay Krishna, Daniel WyleczukStern, Mengxian Jiang, TaeSung Jung
04 May 2011
Team Silver
Project Contents
Executive Summary of the Project
Discussion of the Primary Use Context of the Device
Functional Requirements of the Prototype
Discussion of Alternative Concepts
Discussion on the Rationale for Deciding on the Final Design
Presentation of the Final Architecture
Circuit Diagrams for Each Subsystem
Table of Components and Estimate Cost
Discussion of Possible Failure Modes for the Device
Discussion of the Methods Used to Verify and Validate the Design
Conclusion
Executive Summary of Project
This project is a finger-based plethysmograph. A light is shined at receiver and, when a
finger is placed between the two, a buzzer and light respond on every pulse. In addition, the signal
can be saved on a computer and put through a simulated digital circuit to give a pseudo-real time
pulse count.
At a high level, the project involves the use of photo diodes and photo detectors, passive
and active filters, anolog circuit design, anolog to digital conversion and digital system design. As a
person inserted their finger into the clip, it would detect the subjects heartbeat and output a signal
into the amplifier. The amplifier would then amplify the signal and send it to the filter to remove all
the low and high frequency disturbances and noise. The filter would then send the clean signal
across to the analof to digital convertor to create a digital signature corresponding to the heartbeat.
This digital signal was then responsible for running the buzzer, LED and the digital readout.
Design Summary: the design of the project focused on breaking the project into individual
components and assigning each of those components to a team member. The project was broken
into five parts with “owners” as follows:
Digital component - Mengxiang: The digital readout uses four different counters to keep
track of the heart rate. The 3.75 second counter keeps the most immediate value while the 60
second counter keeps the most accurate value, and the 15 and 30 second counters act as
intermediates that trade off accuracy to immediacy. The circuit selects the best value to display
based on different factors such as how much the heart rate has changed and how long the heart rate
has been monitored.
Clip (light, phototransistor, and associated components) - Sam: The clip uses an IR LED and a
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phototransistor in order detect a person’s heartbeat. The components are placed at opposite sides
of a protoboard and face each other. When a person’s finger is placed in between the two
components a voltage change occurs from the phototransistor. This is passed into the next part of
the circuit.
Amplifier - Ranjay: This then goes to an amplifier which turns the voltage, which as
previously of the order of 1mV with the finger inside into to a usable voltage of around 3 V. This
amplification was completed in two stages, using two inverting op-amps with a gain of -33 V/V each,
ensuring that the output would be inverted twice to ensure a non-inverting sun-system.
Filter - Ranjay: The output from the amplifier goes into a Sallen-key filter. This filter is
designed to remove most of the noise from the circuit and allow a clearer picture of the signal. The
filter consisted of two Sallen Key active filter designs in series cascaded together.
A/D Converter - Daniel: The A/D converter is created using a simple comparator. The input
voltage goes into V+ and the threshold voltage goes into V-. The threshold voltage is created with a
voltage divider and a potentiometer. The potentiometer allows us to vary the threshold voltage
from person to person.
Buzzer and LED - Tae Sung: A relaxation oscillator was used to power the buzzer and LED.
The output of the A/D convertor is used to power the op amp. If the convertor gives high output,
the oscillator output is passed to the buzzer and LED which both turn on synchronously.
Construction: Each part was constructed by their respective owners initially. Due to complications
that arose, certain parts were finished early and people would rotate among different parts as
described below.
Clip: The clip involved a fair amount of work initially because it involved circuitry that was
unfamiliar to most team members. On 1 April, the clip design was finalized. It output a lot of noise,
but discern-able differences in the signal based on the pulse were observed. The signal was
observed using a simple LabVIEW VI.
Filter: The filter was a Sallen-key filter. Construction was impeded by decisions regarding
cut-off frequencies as well as the steepness of the drop-off. Ideally, a very steep drop-off is ideal
because the noise is a much higher frequency than the pulse signal. Thus, the Sallen-key filter
seemed the most appropriate. We cascaded two Sallen key filters in series, one low pass at 5Hz and
the other a high pass at 0.5Hz. The design of the low pass filter kept altering as we were trying to
eliminate the more amount of noise possible without affecting the heartbeat. We finally decided on
5Hz as the best frequency.
Amplifier: Construction of the amplifier was done using a inverting op amp amplifier. The
only trouble with construction was a result of a broken op amp. After overcoming that, the two
inverting op-amp amplifiers were cascaded in series with a gain of -33 V/V individually. The total gain
of the sub-system was expected to be around 1089 V/V.
A/D Converter: Construction was simple. Once the design was decided upon, there was vary
little difficulties in construction. Some research was required in how to properly use a
potentiometer, but once that was full researched, construction required around 15 minutes.
Buzzer+LED: Since Buzzer+LED required only a small number of components; a few different
designs were completely built at the start of the project. After other subsystems were built, the
prepared designs were tested and the best one was chosen.
Digital Circuit: Many frequently used sub-components that were not included in the
components provided by Multisim such as 12-bit adders were first created as sub circuit
components using smaller components such as 4-bit adders. Then these sub-components are tested
and then wired with the main circuit.
Testing was done on a piece by piece basis. So, whenever a part was built, it was first tested with
the function generator. Then, once the clip was running, it was tested with the clip. For instance,
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once the filter was done, it was hooked up to the function generator and a scope. Once it was
determined that the filter worked, it was put aside until the clip worked. Once the clip was
determined to work, the clip signal was put into the filter and the output of that was read. This
sequence of events continued for every component until it was determined the entire
plethysmograph worked.
Clip Testing: Testing on the clip was initially done part by part. In the beginning there was
not much knowledge regarding how each LED and phototransistor functioned, so first the best
LED/Phototransistor combination had to be found. Initial testing was difficult because there was no
set output to look for. Without a working filter and amplifier, the pulse could not be read yet. Initial
testing made sure there was some change in the clip output when a finger was inserted in between
the LED and the phototransistor. The next step was to integrate the clip with the filter/amplifier.
That brought about lots of needed changes. Some of those changes included: adding a passive high
pass, using rectifier diodes and using a voltage divider in the log amplifier. Once the clip worked with
the filter and amplifier, the mechanical portion of the clip. Once created, it was tested to ensure that
it created less noise. In conclusion, there were three major steps needed to be taken in testing the
clip: first getting a working subsystem, then testing with the filter/amplifier and finally testing the
mechanical clip.
Amplifier testing: The initial testing for the amplifier was done by first checking the resistor
values that were chosen and ensuring that those values were closest to the ideal values as possible.
Second, testing for the gain of each amplifier and making sure that the total gain is around 1089 V/V.
However, after testing, the gain was measured to be around 930V instead. Fortunately, this did not
affect the functionality of the circuit as it was sufficient to drive the filter.
Filter testing: The filter initially had a low pass cut-off of around 3Hz. This caused a large junk
of the heartberat signal to be lost due to such a small cut-off. To fix this, the cut off was changed to
10Hz, just like the lab 4. However, this cut-off was too high and let a lot of high frequency noise to
disrupt the actual signal. Finally, the cutoff was finalized at 5Hz. The filter was then combined with
the clip and the amplifier and was outputting a clean signal of around 10 mV.
A/D Converter: Testing was first done with a function generator. A sine wave from the
function generator was put in and the signal was observed with the MyDaq oscilloscope. The
threshold voltage was adjusted. Variation in the width of the digital pulse was observed confirming
a working circuit. Testing was then done with the real pulse output. A working digital pulse was
confirmed and testing was complete.
Buzzer + LED: Testing was done by using square wave provided by a function generator. Both
the tone of buzzer and brightness of LED was adjusted prior to the integration of all subsystems.
Digital Readout: Testing was done by adding 7 segment displays to each of the subcomponents to act as indicator of the signal. Different inputs of VCC and Ground are then fed into
the sub-components individually to check if the output is correct. Once each individual subcomponent worked by themselves, they were hooked up into the larger circuit and then still tested
by the same method of adding 7 segment displays. This proved quite successful, as it is easy to
locate the sources of bugs and easily debug them. There was a specifc routine to test the Digital
Readout with .lvm files once our circuit was completed. These are the steps for testing in Multisim
using .lvm files:
1. Open up a blank VI on labview
2. Add an input DAQ assistant.
3. Choose to only include 1 voltage output.
4. Choose continuous sample, 10k sample size, and 1k sample rate.
5. Right click inside the loop (the rectangle with the DAQ assistant inside) and choose output --> Write to
Measurement File
6. In the X Value Columns, choose one column only, and leave all the options unchanged.
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7. Wire up the output of the DAQ assistant to the Write to Measurement File.
8. Click run and after ten seconds or so, click stop. The lvm file will be Written
Discussion of the Primary Use Context of the Device
Use Cases
Priority
Explanation of the Primary Use Context Tables
Patient remains connected to
the device
M
Patient reconnects
himself/herself to the device
(in case of disconnect)
M
Patient informs the nurse in
case of discomfort
L
Nurse keeps his/her ears open
for aberrant beepings
M
Nurse calls the A/S center in
case of malfunctions
L
Doctor sets up the device
Doctor reads the displayed
data
Doctor diagnoses the
symptoms
Doctor informs patient/nurse
of the diagnosis
Doctor
Patient
Device
Tech
Support
H
H
We established different relations between the
aforementioned groups. Of significant importance was how the
doctor, nurse, patient, and tech support interacted with the
device. The relation between those groups is important, but not
as important as the relation between them and the device
because the latter influences the former.
M
Nurse
directs
Doctor
Nurse
H
In terms of deciding the primary uses of the device, we
identified the various types of people who would interact with
the device once constructed. We determined that patients,
doctors, nurses, and technical experts on the device (e.g.
employees of the same company who produced the device who
are responsible for troubleshooting and fixing the device). We
decided that the relations between the doctor and the device
were very important. It is the doctor who decides to purchase
the device and it is the doctor who will use the device. Also of
importance is the relation between the patient and the device.
The patient has to feel comfortable when wearing the device.
If they aren’t, they will complain to the doctor who will not
purchase further devices.
seeks direction
receives medical
attention
attracts
requests services
alerts and provides
info
gives technical advice
Patient
diagnose/treats
Device
purchases
monitors
prepares
is monitored
by
Tech
Support
request
help
monitor
fixes
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Functional Requirements of Prototype
Requirement
type
Clip
Filter
Amplifier
Main function
Logarithmically
read heartbeat
of every
person
Remove
noise
caused by
ambient
light
Amplify the
signal by a set
amount
Output signal
around 1 mV
Remove
noise
caused by
patient
movement
Have cutoff
frequencies
of 0.5 Hz
and 5 Hz
Need an
input
voltage of
about 3V
and the
output was
in mV
Secondary
function
Tertiary
function
Input/Output
constraints
A/D Converter
Light + Buzzer
Turn on an LED
and buzzer at
the peak of the
user's pulse
Do not distort
the waveform
Convert the
analog signal
to a 1 bit
digital signal
Have the
conversion
based on a set
voltage
threshold that
is easily
changed by the
user
Digital Readout
Display an
accurate
measurement
of the beats per
minute as
determined by
the input data
Insure neither
the buzzer of
the LED is
annoying to
the user
Read .lvm data
saved by the
user
Input needed
to be in mV
and output
needed to be
around 5V
Minimize jitter
Input needed
to be in the 1V
- 8V range and
the output
should be high
(9V) or low (9V)
Input needed
to be around
5-10V and
have a square
waveform
Input needed to
be in an .lvm
data file with
appropriate
formatting
Explanation:
Starting the project, the team needed to decided not only what the subsystems were, but
what the requirements of each subsystem were. Some decisions were self-explanatory: the
amplifier should amplify. But others, such as should the filter also amplify, required more thought.
Thus, we established requirements for each substyem.
Before constructing any device, it was necessary that each person was aware of how the
other subsystems worked. Because everything would eventually be linked together, it was
necessary that every team member let the others be aware of the required inputs of their device.
That way, they would be able to tailor their device’s output to the necessary input of the next
subsystem. Thus, each owner could then generate specific technical requirements for their
subsystem.
For the clip, it was necessary that it perform the logarithmic scaling because it had the best
access to the pulse’s original signal. In addition, it was necessary that the output of the clip be in the
mV range. Much lower and any amplification could fail to distinguish the pulse waveform from any
background noise.
It was decided that, for the filter, it would need to remove both unwanted high and low
frequencies caused by movement, ambient light, and any other sources. The cutoff frequencies
were determined after testing various other frequencies. It was decided that there would be no gain
on the filter in order to simplify the circuit and prevent unforeseeable interference. Thus, the filter
would take in an input in the range of around 3 V and output in the mV range.
The amplifier had simplifier requirements. It needed to not distort the signal (invert it, etc)
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and it should take in a signal in the mV range and output a signal in the V range.
The A/D converter needed to take in the amplified and filter signal and convert it to a digital
signal. The conversion should be consistent and not jitter back in forth. There needs to be a one to
one correspondence between the user’s heartbeat and the digital pulse. In order for the A/D
converter to work cleanly, it needed a certain voltage range to operate. It also needed to output a
voltage range varied enough so that the digital pulse was clearly distinguishable.
The light and buzzer needed to turn on at the peak of the digital voltage so the user had an
immediate auditory and visual indication of their pulse rate. It also needed to not annoy the user.
Various buzzer frequencies caused the user to become annoyed, so testing was required to fix this.
The digital readout could not operate in real time under the current constraints. Thus, it
needed to be able to read in an .lvm file saved by the user. It needed to display a reasonable
calculation of the BPM.
Discussion of Alternative Concepts
Clip: One decision for the clip was to use a red LED or an IR LED. We chose an IR LED because
it did not emit visible light to annoy the user, but the red LED worked as well. In addition, the initial
design revolved around either using a logarithmic amplifier or a Common Emitter amplifier. We
chose a logarithmic amplifier for our final design. However, we explored and tested a simple
Common Emitter amplifier. Instead of a diode connected to the collector, a resistor was connected
there. This produced some different output voltages and in some case were better than the outputs
of the log amplifier depending on the resistor, but in the end the log amplifier excelled at minimizing
output change from user to user.
Filter: there were a number of alternative designs for the filter. The first design considered
was to use a standard bandpass which is a low pass filter in series with a high pass filter. The second
design was a Sallen-key filter which combines the effects of a low and high pass filter into one
circuit. The third design was to use a passive RC bandpass filter. This filter does not involve an op
amp but instead just resistors and capacitors. The fourth design considered was to use an amp band
pass filter without any gain.
Amplifier: there were five different designs for the amplifier. The first design was to build a
non-inverting operational amplifier with gain. This would use resistor ratios to determine an
amplification ratio. The second design was to cascade two inverting amplifiers. The inversions
would cancel out and the amplifiation would stack such that Atotal = A1 * A2. The third design was
to use a MOSFET. The fourth design was to use a JFET. The final design was to use build an inverting
amplifier.
A/D Converter: There were three possible implementations considered: a Schmitt Trigger, a
comparator, and a transistor. The Schmitt Trigger involved setting a low and high threshold voltage
and anything higher would result in a high output, anything lower than the low would result in a low
output. Anything in between would hold the previous output. A comparator is a simple op amp
function. The input voltage goes into V+ and the threshold voltage goes into V-. If V+ is more than
V-, a high is output. If it’s lower, a low is output. V- is controlled by a voltage divider on Vcc. The
voltage divider includes a potentiometer which allows V- to be varied quickly and easily. A transistor
is simply using a transistor. The input goes into the middle pin of the transistor. If the voltage is
high, the transistor allows a high signal (Vcc) to be passed through. If the voltage is low, no voltage
is passed.
Buzzer and LED: the primary decision involved here was what oscillator to use. The first
design considered was a non-inverting Schmitt Trigger and integrator. This has a standard noninverting amplifier hooked into an integrator. The output of the integrator goes into the input of the
first op-amp. This gives a triangle wave. Another oscillator design considered was a Wein bridge
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oscillator. This is again similar to a non-inverting amplifier with some changes. A capacitor in series
with a resistor connects V- and the output. Another capacitor in parallel with another resistor
connects V- to GND. This gives a sine wave that slowly grows until it hits a max.
Other than the oscillator, a transistor could be used both to power the op amp and
effectively shut off any current leakage during each heart beat. Adding a transistor would have been
useful if our A/D converter were not good enough to provide clean signals to drive the Buzzer+LED
subsystem.
Digital Circuit: There are many ways of creating a digital readout for the heart rate using
digital logic. The simplest implementation of the heart rate is probably one in which a slow clock
with period of 1 minute acts as a reset signal for a counter while the heart rate is used as the actual
input clock for the counter, therefore the counter will always have the correct count. However, the
problem with this simplest design is that it will take 1 full minute before a reading of the heart rate is
displayed. Therefore, an alternative is to use a faster slow clock such as one with 30 second period
and therefore multiply the count by 2 to get the heart rate. However the problem with this approach
is that once we get down to 3.75 seconds, the heart rate will be in multiples of 16 which is a fairly
huge error margin. One solution to get rid of this error margin is to actually implement multiple
counters in which the counter have different periods and therefore gradually reduce the error of the
count as time goes on, which was our approach.
We used 4 counters (3.75, 15, 30, 60) which would switch over to the long time periods as
time went on. However, this approach seems quite wasteful since once the 60 seconds has passed,
the other counters contribute nothing to the display. Since a heart rate can change rapidly, 60
seconds is still a long time to wait for an updated display. Therefore, we also implemented a
selection system in which if the 60 second value is off from the 3.75 second one by a certain
threshold (we set it to 32), it would change to the 30 second value, and if the 30 second is also off,
then 15, and then finally the 3.75 if the 15 is also off.
An alternative design to this multiple clock that also solves the problem of reducing error
margin as well as fast update time is to use the heart rate as the reset for the counter and to use a
very fast clock as the counter clock input. This approach does not directly count the heart rate but
rather the heart beat period, which is then fed into a look-up table which translates the period into a
heart rate. This is extremely fast as it requires only to heart beats for a display which should
normally take only a few seconds. The error is also low if the very fast clock is extremely fast.
However, there is also a trade off in terms of the very fast clock speed and the size of the look-up
table since a faster clock would require a bigger table. One drawback of the look-up table design is
that there is no memory of what the heart rate was in the past. Therefore a potential problem is that
the heart rate display could give a reading such as 67, 73, 69, 75, 71, 65 where the value keeps
jumping from each heart rate. This may be undesirable if one merely wishes to check whether the
heart rate is just within a certain range. Like how an air conditioning unit does not merely turn on or
off when a certain temperature is reached but rather when the temperature has went past it, this
may be the case for the heart rate monitor as well. In this case, our design of multiple clocks would
work better than the look-up table.
Discussion on the Rationale for Deciding on the Final Design
Clip: The clip consisted of a photo-diode and a photo-detector followed by signal diodes. The
signal diodes minimized change in output from user to user in the form of a logarithmic amplifier.
The final design of the clip also included a passive RC filter to remove the DC offset that we were
receiving. The mechanical portion needed to limit the amount of noise we received from users
making small movements. This was achieved by using a spring loaded clip. Also padding was added
on the clip to ensure comfort for the user.
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Filter: The Sallen-key filter was chosen because it allowed us to build both a high pass and a
low pass separately, allowing us to test the two filters individually. Even though the Sallen-key
design is much more complicated to implement, its steeper roll-off guaranteed a better filter,
especially since the frequencies are so close together. Also, we decided against having the filters
amplify because it created harder test cases for the filter and also cause the voltage to hit the rails if
the voltage was amplified too much.
Amplifier: The amplifier receives its input from the clip and is around 1-5 micro-Volts. So, an
amplification of around 900-1000 is required to output a voltage in the milli-volt range. After
considering all the alternatives in the Pugh matrix, two inverting amplifiers in series displayed the
most stable and predictable behavior.
A/D Converter: A comparator was chosen for a number of reasons. Primarily, it allowed us
to easily adjust the threshold voltage. A transistor did not give us this functionality while the Schmitt
trigger was much harder to adjust. The comparator also used fewer parts than the Schmitt trigger
and it was also easier to construct. The transistor was easier to use than both of those, but it had
other flaws. In terms of accuracy, the comparator and Schmitt trigger gave equally good results
while the transistor gave poor results. We valued functionality and the number of parts, with fewer
being better, the most. Therefore, the comparator was chosen.
Buzzer + LED: For the buzzer, three potential designs - Schmitt trigger and integrator,
relaxation oscillator, wien bridge oscillator - were compared using Pugh matrix. These three were
the candidates for Buzzer subsystem because they are the most widely used designs that do not
contain an inductor. Using a design containing an inductor was not desirable because it would create
electromagnetic fields that increase the overall noise of the circuit. After going through Pugh matrix
comparison, relaxation oscillator was chosen, primarily because it is the design that contains the
least amount of components.
A transistor in common collector mode was initially chosen to provide current to LED. However,
using a transistor would significantly deform the shape of the digital pulse. So a simple series
resistance was connected with LED, with power from the digital signal being used to provide current
to it.
Digital Circuit: For the digital readout, four potential designs were compared using a Pugh
matrix. The designs were a 3.75 second period counter, a 15 second period counter, a 3.75 and 15
counter, and a 3.75, 15, 30, and 60 counter. A fifth candidate, the heart rate as reset with extremely
fast clock, also known as the look-up table design, was not considered due to the fact that it initially
looked like it required a microcontroller which broke the project constraint. After going through
Pugh matrix comparison, the four counter design was chosen due to the fact that it allowed the
smallest error margin in the heart rate display (within 1 heart beat) as well as providing the fastest
output (every 3.75 seconds). It is true that the fifth candidate, the look-up table could potentially be
faster since it requires only the time for two heart beats, it is limited by the fact that it uses only two
samples to extrapolate the heart beat. This means that the look-up table display will very likely
change on every heart beat rather than displaying a constant output for a long period of time even
though the heart rate has not changed considerably. Like how an air conditioning unit does not
merely turn on or off when the set temperature is reached but rather when the temperature has
went past it by a certain amount, our readout design reacts to the heart rate in a similar fashion
while the look-up table display will needlessly switch between relatively close values.
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Test Thread Specification
Output event ("buzz and beep")
The system shall buzz a buzzer and turn
on an LED.
LED and Buzzer Output System
Analog Circuit
Software Requirements Specification
Information event ("store on computer")
The system shall take the digital signal and store it as
data for the computer
Digital Input System
Digital Circuit
Outputting
data
Awaiting
input
System
State
Test Thread Specification
Output event ("digital display")
The system shall display a real-time digital Outputting
pulse count
Data
Digital Output System
.5 Seconds
Timing Target
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Presentation of the Final Architecture
-Operational Description Template
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Information event ("voltage input")
The system shall detect that a finger was placed into the clip and read
the changes in voltage.
Clip System
Hardware Requirements
Specification
Data event ("filter")
The system shall filter the signal.
Filter System
Hardware Requirements
Specification
Data event ("amplify")
The system shall amplify the signal
Amplification System
Hardware Requirements Specification
Data event ("convert")
The system shall convert the analog signal to
a digital signal
A/D Converter System
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Human Interface Specification
Saves data on computer using
available software
Information event ("finger placed")
Patient places finger into the clip
Operator (Patient)
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Reasons and Rationale Behind the Final Architecture
Operational Description Template: there was a clear order of steps in the workings of the
plethysmograph, so our operational description template (ODT) codifies that. Initially, the main, and
most important, user action is the placing of the finger into the clip, i.e. the space between the
system’s IR LED and phototransistor. Then, the next system is the filtering system. This takes the
output from the clip and removes the high frequency noise. It then outputs the filtered signal to the
amplifier. The amplifier amplifies the signal to a usable level because, previously, the signal was in
the millivolt range and a signal in the volt range is preferred. The amplified signal then goes two
places as demonstrated in the ODT. It goes to the buzzer and LED analog display, which gives a realtime output of whenever the pulse peaks. The amplified signal can also be saved as data on a
computer, which is moved to the next system, the digital system. The digital system runs the data as
if it were real time and gives a pseudo real time pulse count.
The even types were decided based on whether data was moved from one discreet system
to another (say, finger to hardware or hardware to output). So the filtering of the signal was an
information event whereas the buzzer emitting sound was an output event. Also, the systems were
classified into user interface, hardware, software, and test in order to best illuminate how the entire
plethysmograph can be broken up into four main parts.
Functional Flow Diagram: this diagram presents an easy to understand picture of the steps
of the system. These steps can be read in the description for the ODT, so they are omitted for
brevity. The choice of dotted or solid lines demonstrates where real time, user driven effects on the
system can cause changes or delays. For example, the user can choose to not save the digital data
and completely omit the digital display step.
State Change Diagram: There were five different details that were chosen to describe each
state: the presence of a finger, if analog processing was occurring, if digital processing was occurring,
if there was analog output, and if there was digital output. These characteristics were chosen
because they easily allow one to see breaks in the four different system classifications (user
interaction, hardware, software, and output). The states also represent where user action can
impact which state happens next (the user choice to examine digital data or not).
Circuit Diagrams for Each Subsystem
Clip:
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The functional behavior of the clip is that it outputs a voltage signal with a hidden AC pulse and that
it will be comfortable for the user. The user interacts with the clip by placing his or her finger in
between both pads on the spring clip. By placing his or her finger there, it is in between the IR LED
and Phototransistor. The light travelling through the finger creates a base current in the
phototransistor which then creates a respective collector current. When the heart beats, blood flows
through the finger in a way that allows less light to pass through the finger, which then creates a
smaller base current. Because the collector current will then be smaller, the output voltage will be
higher (see circuit diagram). There are two diodes placed from Vcc to the collector terminal to create
a logarithmic amplifier so the output voltage does not vary too much from user to user. The diode is
controlled by the Shockley Equation:
and the inverse relation is:
. It can be seen that the voltage will not vary that much because of the
log relation. At the collector of the phototransistor a passive high pass with a very low cut-off is
placed to get rid of any DC bias. The output is then fed to the filter/amplifier. This meets the
functional behavior by producing a consistent hidden AC pulse through a log amplifier and passive
high pass. Also it is comfortable for the user through the padded spring mechanical design.
Filter:
The filter had two Sallen Key designs cascaded in series. The first op-amp served as the low pass
filter with a cut-off frequency of 5 Hz. The second op-amp was a high pass with cut-off at 0.5 Hz. The
low pass removed all the excess high frequency noise from the clip and kept the heartbeat signal
intact which the high pass took care of all the low frequency disturbances occurring because of
finger movements. The circuit was also very compact as it used only 0.1 micro-farad capacitors and
didnt require larger capacitors because of the use of high resistances. The filter did not amplify the
signal because after testing it was found to be much more reliable with any gain associated with it.
Amplifier:
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The amplifier had two levels of amplification, both inverting with a gain of -33 V/V. Overall, the
amplifier subsystem created a non - inverting gain of 1089 V/V. It used four resistors: two of 1K-Ohm
and two of 33 K-Ohms. The input expected at the amplifier was approximately 1mV and was
expected to output 1-2 V signal after amplification.
A/D Converter:
The functional behaviour is that Vout is positive at the peak of the heartbeat and zero or negative
otherwise. Voltage in is the heartbeat signal coming from the amplifier. Because this signal varies
from around zero volts to seven volts, we chose to vary V- with a voltage divider on Vcc. An equation
for Vout is given below. Rp = the resistance of the potentiometer. Vout=Vcc if V+>VV- = (Vcc*Rp)/(10k + Rp) If Rp is at its max (10k), V- = 5V. If Rp is at its minimum, V- = 0V. We can
therefore vary V- between zero and five volts easily. This allows us a decent range of values, which
are between the ranges of Vin. This gives us complete control on the threshold voltage. Therefore,
we can insure that Vout only goes high if and only if the heartbeat peaked.
Buzzer/LED:
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This subsystem is turned on at the positive peak of the signal that is fed from the A/D converter
subsystem. When turned on, the circuit provides enough power for the LED to shine brightly, and
provide a voltage signal at an appropriate oscillating frequency for the buzzer to beep at an
appropriate tone.
The subsystem was able to deliver enough current to the LED without use of any transistors.
The oscillating frequency was given by the formula
The frequency was originally adjusted at 2kHz but lowered later on after testing multiple
times with our own ears. The final values of resistors and capacitors were chosen to make the
frequency to be around 370 Hz, which was the tone found to be most soothing to our ears.
Digital Circuit:
This subsystem consists of four differently clocked counters, one at a 3.75 second period, one
at 15, one at 30, and one at 60. The counts of each of the not 60 counters are already the heart rate in
beats per minute since multiplication is done ahead of time. This is done through the use of
multiplying the input heart rate frequency. The simplified circuit diagram explains how the frequency
multiplication works.
Basically the heart rate acts as a reset for a counter with a very fast clock that counts to what
you wish to multiply the heart rate by. Once that the counter reaches that amount, a mux switches the
output from the fast clock to the ground thus multiplies the heart rate frequency. The heart rates of
each counter are fed into logic that selects the best out of them. The selection works as follows: first it
checks whether 15, 30, or 60 seconds has passed yet, and if so chooses the longest timed counter
based on time passed. However, if the 60 second counter differs from the 3.75 by a certain amount
(we set ours to 32), use the 30 second, and if the 30 also differ, use the 15, and if the 15 also differ,
use the 3.75 value for display. One critical component required for this selection logic is the use of an
absolute value subtractor. The simplified circuit diagram for the subtractor is shown below:
Rather than subtracting two values and trying to take the absolute value which requires that
the comparator be able to tell negative values, the circuit first compares the two values, take the
negative of the smaller one and put them into the adder, therefore guaranteeing a positive difference.
This difference is then compared with a threshold (which we set to 32 for our project) for the selection
logic to determine which heart rate to output.
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Table of Components and Estimate Cost
**Most price estimations are based on purchasing quantities between 100 and 1000**
Table of Components and Estimate Cost (Analog Circuit)
Component
P/N (If applicable)
Resistor
.68uF Capacitor
100nF Capacitor
Op Amp IC
100k-ohm Variable Resistor
IR LED
Piezo Buzzer
Clothes Pin
Padding
Copper Wire
Phototransistor
Rectifier Diodes
Red LED
LF353
276-143
PDF:142KB
PNZ150-ND
1N4007
160-162-ND
Quantity
21
3
5
4
1
1
1
1
1
1
1
2
1
Price Per Component Total Price
$0.20
$4.20
$0.65
$1.95
$0.35
$1.75
$0.60
$2.40
$0.75
$0.75
$0.50
$0.50
$0.50
$0.50
$0.04
$0.04
$0.10
$0.10
$0.20
$0.20
$0.75
$0.75
$0.06
$0.12
$0.30
$0.30
Total:
$13.56
Table of Components and Estimate Cost (Digital Circuit)
Component
Counter
Flip Flop
NAND gate
7 segment disp.
Clock
P/N (if applicable)
74LS163
74AHCT1G79GW
74HC1G00GW
HDSP-C3E1
CD4046BPWR
Quantity
Price Per Component
Total Price
15
$0.75
$11.25
50
$0.05
$2.65
300
$0.05
$13.50
3
$0.38
$1.14
1
$0.16
$0.16
Total:
$28.70
Discussion of Possible Failure Modes
Failure
Identification
Mode
of Item or
No.
Function
Failure Mode
F.1
Pulse Reader Failure to detect light
F.2
Failure to emit light
Signal
Processing
F.3
System
Failure to filter signal
F.4
F.5
F.6
Failure to amplify signal
Analog
Output
System
Failure to buzz
Failure to display output LED
Failure Effects (a. local; b.
system; c mission)
b. pulse not read c. failed mission
b. pulse not read c. failed mission
b. signal not usable c. failed
mission
b. signal not usable c. failed
mission
b. no user communication c.
failed mission
b. no user communication c.
failed mission
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F.7
F.8
F.9
Power
System
Digital
Output
System
Failure to provide adequate power
or provides too much power
b. system cannot turn on c. failed
mission
Failure to convert from analog to
digital signal
Failure to accurately display pulse
count
b. signal not usable by digital
system c. failed mission
b. no accurate information
conveyed to user c. failed mission
Discussion of the Methods Used to Verify and Validate the Design
After constructing each of our subsystems, we used the criteria described below to verify that the
designs were good enough to be used for the final product and no further changes were necessary.
Clip
Criteria: The clip had to hold the finger firmly but without discomfort, so that it could accurately
detect heartbeat of the user. Also, the clip had to be designed in a way to efficiently shut out
ambient light so that the photodiode did not produce any significant electric signal in the absence of
finger.
Test/Result: The clip was tested with each member’s index finger, and for every finger, sufficiently
fulfilled all criteria.
Filter/Amplifier
Criteria: Filter/Amplifer subsystem had to filter out all noise in the electric signal it received from the
clip and amplify it large enough to be converted to digital signal. Also, the electric signal at the end
of filter/amplifier subsystem had to have a wave form that corresponds to the actual heartbeat
without any undesirable peaks or troughs.
Test/Result: The wave form produced by filter/amplifier as shown below sufficiently fulfilled all
criteria.
A/D Converter
Criteria: A/D converter had to convert the analog signal to digital signal. It had to detect heartbeats
of the analog signal without over counting or failing to count one, and convert them into a noise-less
square wave of consistent and appropriate amplitudes.
Test/Result: The wave form produced by A/D converter as shown below sufficiently fulfilled all
criteria.
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Buzzer/LED
Criteria: Buzzer/LED subsystem had to provide sound and light synchronous to the heartbeat. The
loudness and tone of the sound and brightness of the light had to be easily detectable and yet cause
no discomfort to the user.
Test/Result: The sound and light produced by the subsystem was approved by all members and TAs,
and thus sufficiently fulfilled all criteria.
Digital Readout
Criteria: The digital readout had to quickly and accurately calculate the heartbeat rate in units of
beats per minute. It also had to have the ability to ignore sudden drop or increase in the heart rate
that are caused by malfunctioning of other subsystems.
Test/Result: Several LVM files containing different data was used to test the simulation of the digital
readout. The result sufficiently fulfilled all criteria.
Conclusion
The completed plethysmograph functioned well without any noticeable problems. It was
able to consistently detect heartbeats, make appropriate displays, and accurately calculate the
beating rate. The plethysmograph could tolerate some movements of the finger, so that the user
had a certain degree of freedom to move while his finger was put inside the clip. Such achievement
can be accredited to not only the clip structure, but also the noise-reducing quality of filter, amplifier
and A/D converter subsystems. The buzzer and LED functioned synchronously and consistently, at
amplitudes big enough to distinguish yet not too big to create any significant discomfort. The digital
readout could quickly compute the heartbeat rate in beats per minute, only after 3.75 seconds after
the user’s finger is inserted inside the clip. It could also detect any abrupt changes in the heartbeat
rate and ignore them for the calculation of beats per minute.
The analog wave form of the heartbeat was shown above in the validation section. The wave
form accurately represents the heartbeat, with large peaks representing the ventricular systole and
small peaks representing the atrial systole phase. This is the real time wave form of the electric
signal after it passed through a series of filter and amplifier, and before it passes through another
amplifier and A/D converter.
The digital wave form of the heart beat is shown in the above section as well. This is the real
time wave form of the electrical signal after it passed the A/D converter. The digital signal ranges
from about -1 to 10. Because the off state of the digital signal is below zero, buzzer/LED subsystem
could be built without incorporating a switch that blocks out all electric signals in the off state of the
digital signal.
The plethysmograph was tested for fingers of different skin color and thickness. For some
fingers, the threshold values of the A/D converter had to be adjusted. This was done by changing the
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value of resistance of the potentiometer. For most fingers, however, the plethysmograph worked
well without the need of changing threshold levels.
There are several ways to improve our plethysmograph in the future. First is to make the
plethysmograph portable. This would require reducing the overall size of the circuit, by
manufacturing the circuit on the printed circuit board and minimizing the use of large capacitors.
The plethysmograph would also have to be able to operate on batteries. This is a difficult challenge
because of power consumption problems.
Second is to make the plethysmograph more stable. Although the current plethysmograph
can tolerate small movements of the finger, the tolerance level needs to be improved a lot if the
device is to be used for a wider range of applications, such as reading the heartbeat when the user is
exercising.
Third is to find a better tool to replace digital readout. The current digital readout has two
limitations; it cannot compute heartbeat in real time, and it is extremely slow. Finding a faster
software that can directly transfer data with the analog part of the circuit, or building a
microcontroller on PCB would solve these problems.
Finally, more functions could be added to the plethysmograph. These include volume
control, brightness control, the ability to alert the user in case of aberrant heartbeat, etc. Improving
our plethysmograph in such ways would bring our device one step closer to becoming competent in
the real world market.
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