Pulse Sensor

Individual Progress Report

TA: Kevin Chen

ECE 445

March 31, 2015

Name: Ying Wang

NETID: ywang360

I. Overview

1. Objective

This project intends to realize a device that can read the human pulse rate from a fingertip. The

pulse rate in the unit of beats per minute (BPM) and a calculated Target Heart Rate will be

displayed by a 16 by 2 LCD display. According to the National Institute of Health, the average

resting heart rate for children 10 years and older, and adults (including seniors) is 60 - 100 beats

per minute. Well-trained athletes is 40 - 60 beats per minute. A real-time plot of pulse signal versus

time will be displayed on a laptop screen.

2. Modules and Module Input/Output Requirements

There are four modules in this design: Power Module, Sensor Module, Microcontroller, and

Display Module. Figure I-1 shows a block diagram of their relations.

Figure I-1 Modules and module relation

2-1. Power Module

The required power supply for this device is DC 5V. In order to obtain a constant and stable

5V output, a regulator circuit is designed. The regulator circuit will take an input voltage of 9V

battery and output a constant 5V voltage. For details and simulation results please see the module

design part below.

2-2. Sensor Module

The sensor is in direct contact with a human finger (input). It first contains a sensor that can

sense the blood circulation and convert it to electrical signals. This will be followed by a circuit

for amplification and noise reduction. The output of the sensor in this design will be a real-time

analog voltage signal that swings within a reasonable range between 0 V and Vcc = 5 V. The Sensor

Module will be placed on a PCB board for the final device.

2-3. Microcontroller

The Microcontroller (will be Arduino Uno in this design) takes the output of the Sensor Module.

The output of Sensor Module will first subject to A/D conversion. Through Microcontroller

programming, the digital signal is processed, the pulse rate (BPM) should be calculated then sent

to the output ports of the Microcontroller and a calculated result of Target Heart Rate in beats per

minute will also be sent to the output port of the microcontroller. The output signal needs to be

compatible with the input requirement of a 16 by 2 LCD display unit. The Microcontroller also

needs to handle the communication with a PC via a serial port in order to send the real-time pulse

signal to PC for display. A free software called Processing1 can be used to handle the display for

data coming from a serial port.

2.4 Display Module

Display Module takes the output of Microcontroller. It will contains two components. The first

is an on-board display unit which shows the Pulse Rate and Target Heart Rate. The second is a

window opened on a PC screen that can do real-time display of pulse signal transported from the

serial port. This is also to confirm the device has received reasonable signal.

II. Module Design

1. Power Module

The regulator circuit will take an input voltage of 9V battery and output a constant 5V voltage.

Schematics: using a zener diode D1, resistor R1 and transistor Q1 to form a voltage regulator, R2

is the load resistor.

1 https://processing.org/reference/libraries/serial/serialEvent_.html

D1

Zener Voltage = 5.6

Qbreakn

Q1

R1

100

5.606V

9.000V 4.909V

0

V19Vdc

V2

FREQ = 100VAMPL = 0.2VOFF = 0

AC =

0V

0V

R2

10

V2

V1

V3

VV

Figure II-0 voltage regulator circuit (5V)

Simulation Result: Q1 is using 2N3055 mode, other parameters are shown in the figure above.

The vibration of the input voltage is modeled by a sine signal.

The green line is the total input voltage and the red is the output voltage of the regulator, which

is fixed at ~ 4.9 V.

Tolerance Analysis:

We can calculate the output voltage to be:

V3 = ((V1-0.6)/R1 + 5/ZZT)*(beta + 1)/(1/R2 + (beta+1)/R1 + (beta+1)/ZZT), where ZZT = 11

ohm from the Zener model and beta = 130.

The load resistance is estimated to be 10 ohm, leading to a current of 0.5 A. Assuming V3 = 5 ±

0.4 V, this lead to R1 should be larger than ~80 ohm.

The minimum current passing R1 should be 0.5 mA/(beta+1) + Is = 0.0048 A. This lead to the

maximum R1 allowed to be about 700 ohm.

2. Sensor Module

The sensor I will use to convert blood pulse into electrical signal is a pair of infrared (IR) emitter

and detector. The emitter and detector will be put side by side facing upwards. When a fingertip is

place on top of both, IR light will be reflected by the finger and the blood circulation inside the

finger can cause a periodic change of IR intensity reflected, therefore, a period current response in

the detector. The response is rather weak and could be noisy. Shown below by Figure II-1 is the

original design from makezine.com2. This design has some problems and cannot satisfy the

requirement stated above. I will do two modifications.

Figure II-1 Original sensor module design

2-1. Load Resistors of Emitter and Detector

The first modification is rather trivial. I need to adjust the value of load resistors (shown in

Figure II-2) for the emitter and detector to ensure they work fine and more importantly to make

sure a reasonable DC component of the detector output (not too close to ground and not too close

to Vcc). The recommended forward voltage of the emitter from the datasheet is 1.2 V with a forward

current of 20 mA. This leads to a load resistance

R1 = (5 V – 1.2 V) / 20 mA = 190 Ω

This happens to be not very different from 220 Ω in the original design so I will just keep R1 =

220 Ω.

2 Makezine.com/projects/ir-pulse-sensor

Figure II-2 Sensor composed by an IR emitter and detector.

In order to get a reasonable R2, I use a voltmeter (from the toolbox) to measure Vo in Figure

II-2. It was found that when I use R2 = 8.2 kΩ, Vo has a reasonable range between ~2 V to ~3 V

when a finger is placed on the sensor.

2-2. Re-design the circuit after detector

The circuit in Figure II-1 will not work properly mainly because of two reasons. 1. The first

OPAMP, in my opinion, is for noise reduction. But a quick analysis of its transfer function reveals

that it is band reject filter that reject the frequency from ~1.6 Hz to ~194 Hz. This doesn’t make

sense to me because there are still useful signal above 1.6 Hz(for example, pulse above 96). 2. The

second OPAMP is to amplify the signal however, it can be calculated that the gain at f = 1 Hz is

about 100, which is possibly too large that could result in cut-off (clipping) of the real signal. This

is indeed observed as will be shown in the Test and Verification Section.

Before design my own circuit, I first look at the real-time output Vo of the detector (with a

fingertip on the sensor), which will serve as the input of the following circuit. Figure II-3 shows

the Vo versus time recorded by Arduino Uno and displayed on my laptop by Processing. Although

the signal shown here is pretty clean, sometimes noisy signal was seen. Details of how the test was

done will be provided in the Test and Verification Section. Basically, by using Arduino and

Processing, the laptop can be used as an oscilloscope. As we can see the output voltage has a DC

level of ~ 2 V a peak-to-peak voltage is ~ 0.13 V. This tells me that I need to design the amplifier

gain to be ~10 to 20. A gain of 100 is definitely too high!

Figure II-3 Pulse signal measured from the output Vo of the detector with a finger on. No load was

added to Vo.

Directly after the IR detector output, I plan to first add a low-pass filter to filter out the high-

frequency noise. I choose to use the Sallen-Key structure3.

Figure II-4 Butterworth low-pass filter using Sallen-Key architecture

Figure II-4 shows a low-pass filter using Sallen-Key architecture. The output resistance of the

IR detector is just R2 = 8.2 kΩ. It is decided for now that any signal with f > ~20 Hz should be

suppressed. Guided by the design rule in the footnote document, we assume R4 = R, R2+R3 =

mR, C1 = C, and C2 = nC. The cut-off frequency fc = 1/(2πRC(mn)0.5) and for Butterworth filter

Q = (mn)0.5/(m+1) = 0.707. So in order to meet fc = 20 Hz, the following design can be

calculated:

n m C1 C2 R3 R4

3.3 0.229 0.2 uF 0.66 uF 11.4 kΩ 50 kΩ

3 http://www.ti.com/lit/an/sloa049b/sloa049b.pdf

U1

OPAMP

+

-

OUT

C2

C1

R3 R4R2

8.2k

V10.13Vac

2Vdc

output to amplif ier

Open Circtui output of IR detector

Load of IR

detector

The next stage will be an amplifier that has a gain of ~ 10 at f = 1 Hz. In addition, the DC

component should not be amplified, only the AC component needs to be. So the amplifier needs

to have a high-pass feature which is reject the very low frequency signal. It is decided here the

low-frequency roll-off frequency to be 0.1 Hz. The circuit can be realized as in Figure II-5.

Figure II-5 amplifier circuit with a high-pass design to only amplify AC component

The transfer function of Figure II-5 is

𝐻 = 𝑅7 + 𝑅6

𝑅7

𝑠 𝑅5 𝐶3

1 + 𝑠 𝑅5 𝐶3

To realize a gain of 10, R6 can be 9 kΩ and R7 can be 1 kΩ. The low-frequency roll-off fc =

1/(2πR5C3) = 0.1 Hz. So it can be designed that R5 = 160 kΩ and C3 = 100 uF.

The total Sensor Module design should connect Figure II-3, II-4 and II-5 together.

2-3. Re-design the circuit after filter

After further testing of the pulse sensor circuit, I find that the current gain 10 is too big. So I

modified the gain to 4 and redesigned the amplifier part of the circuit as following:

C3

R5 R6

R7

U2

OPAMP

+

-

OUT go to Arduino A0

output of f ilter

C3

R51

R6

R71

U2

OPAMP

+

-

OUT go to Arduino A0

output of f ilter

5V

R52 R53

R72

C3 = 100 uF, R51=160 kΩ, R52 = R53 = 9 kΩ, R6 = 9 kΩ, R71=1.8 kΩ, R72=1 kΩ, now the

gain is (R6+R71+R72)/(R71+R72) = 11.8/2.8 = 4.2

The testing result could be seen in the following testing part.

3. Microcontroller Module

I am going to use Arduino Uno in this design. The Microcontroller takes the output of the Sensor

Module, which is periodic analog voltage signal.

3-1. Arduino Uno Inputs and Outputs

The inputs and outputs of an Arduino Uno is shown below as Figure II-6.

Figure II-6 Input and Output ports of Arduino Uno

Arduino Uno has 6 analog inputs A0 to A5 (inputs between 0 V - 5 V), the analog signal is

sampled and digitalized and then output to the serial port on the top right of Figure II-6. The

digitalization has 1024 levels thus one sample is a number between 0 and 1023. Each output data

consists of 5 characters in total: a 4-digit decimal number, represented by four characters, and a

new line character. Each character uses 10 bits, 8 of which are for the actual character and the

other 2 are for other use. The bit rate of the serial port is 9600 Hz by default. The sampling rate

therefore can be calculated by 9600/5/10 = 192 samples/sec.

As have been mentioned before the serial port can be read by a PC using Processing. Processing

can also draw a dynamically updated graph each time a sample arrives.

About the output for the 7-segment displays, three displays will be used because a pulse rate

could not exceed 999 BPM. I am planning to use the 12 of the 14 digital I/O ports, denoted here

as O0, O1, … , O11 to output the binary coded decimal (BCD) signal. O0, O1, O2 and O3 will be

used for the lowest decimal digit. O4, O5, O6, O7 for the second decimal digital. O8, O9, O10,

and O11 for the third.

The inputs and output connections have also been shown in Figure II-6.

3-2. Obtaining Pulse Rate by Programming

Here is the basic algorithm I am going to use to get the pulse rate (BPM). Imaging a pulse

signal that looks like what is show in Figure II-3,

First, the digital signal might be subject to an averaging to make it very smooth

Second, I am planning to record all the local maxima including their time stamps T[i] and

values V[i] with certain time interval, say, 2 seconds. See Figure II-7 below for the flow chart.

Figure II-7 Flow chart for recording all local maxima with a time interval of 2 seconds.

Next, we can scan V[i] and discard those maxima that are apparently smaller than others. Only

keep the V[i] that are the largest and within certain variation range. Then the pulse rate can be

calculated by the two arrays T[i], i = 1 to m:

BPM = 60 ∑ (𝑇[𝑖] − 𝑇[𝑖 − 1])/(𝑚 − 1)𝑚𝑖=1

Finally, the BPM number, which should be a 3-digit integer here, needs to have each of the three

digits into a BCD number and then output to O0 to O11, which can be read by Display Module.

This is rather simple and can be illustrated by Figure II-8 below. After this the program should go

to calculate the averaged pulse rate of the next 2 seconds.

Figure II-8 convert BPM to output that can be read by Display module

Target Heart Rate Range Calculation Formula: (220-age)*50%-------(220-age)*80%. This

formula is concluded from the data information on American Heart Association website :

http://www.heart.org/HEARTORG/GettingHealthy/PhysicalActivity/FitnessBasics/Target-Heart-

Rates_UCM_434341_Article.jsp

4. Display Module

For the display module I will use a 16 by 2 LCD display (LCM 1602 16-by-2) for displaying

the pulse rate and Target Heart Rate.

III. Test and Verification

1. Test/verification plan

Requirement Verification Points

Power Supply

Output a stable 5V(+/-0.4V)

voltage to the rest of circuit

and within +/-0.2A of needed

current

Put a voltmeter across the

output of regulator circuit, the

voltage should be within

5V+-0.4V. The components

of the circuit should all work

properly without any short or

overload.

15

Pulse Sensor

The pulse sensor should

output an accurate periodic

analog signal to

microcontroller. The periodic

The output of the pulse sensor

circuit could be verified by

using a free software called

“Processing” It could sketch

the output of the pulse sensor

40

wave should correspond to

the pulse of the testing people

within 5 beats per minute.

circuit synchronously to a PC

screen. Then we can count

the pulse per minute based on

the waveform and compare it

with the result of iPhone App

“Heart Rate”

Microcontroller

The Microcontroller takes the

output of the Sensor Module

and calculate the beats per

minute and output to the

display module. The beats per

minute should be accurate

within 5 beats/min

The calculated beats per

minute (binary value) can be

displayed by processing then

we can compare the output of

the microcontroller with the

waveform generated by pulse

module.

20

Display Module

Takes the output of the

microcontroller and convert it

to seven segment display

To be compared with the

binary value number

displayed by “processing”

25

2. Schedule

March 1st week Test the Sensor Module output, modify circuit design if necessary.

Should be a periodic signal that swings within a reasonable range.

This can be verified by using Arduino and Processing to do real-

time monitoring.

March 2nd week Simulate Power supply module, Microcontroller programming and

test the LED display. Both can be verified by the real-time signal

recorded by Arduino and Processing. March 3rd week

March 4th week Finalize circuit design and start to design PCB board

April 1st week Prepare for mock Demo, finalize PCB board

April 2nd week Test PCB board

April 3rd week Demonstration

April 4th week Write final report

3. Initial Test Results

3-1. Test setup using Arduino and Processing.

Figure III-1 Circuit test setup, using Arduino and Processing as an Oscilloscope.

Figure III-1 shows the test setup used for the Sensor Module. How to connect Arduino has been

explained previously. The code for creating such dynamic figure using Processing can be found

on the Arduino website4. Basically, whenever the serial port receives a number, the code will

update the figure with a new date point.

The voltage output of the IR detector has been shown by Figure II-3. Here I show in Figure III-

2 the results of Sensor Module output using the original and problematic design.

Figure III-2 Output of the original design

4 http://arduino.cc/en/tutorial/Graph

As can be seen here, the signal is very noisy. In addition, when compare Figure III-2 to Figure

II-3, we can see the output of the original design is significantly distorted, which I believe is due

to that the gain is too high. The new design has been described in the text above.

4. Further Testing Results

After modifying the pulse sensor circuit and change the gain to 5, I did the testing and the

result is as following:

The above figure is clean but some peaks got clipped off because the gain is still a little bit too high.

So I modified the circuit again so the gain is reduced to 4. I did the testing again as following:

The above figure is showing the pulse rate with a light press of finger on the IR emitter and detector.

The following figure is showing the pulse rate with a harder press of finger on the IR emitter and

detector. As could be seen from the two figures, the IR emitter and detector are sensitive to the

pressure of the finger, the harder press of the finger gives a better and cleaner waveform.

IV. Cost Analysis

Labor

Name Hourly Rate Total hours invested Total = Hourly Rate x

2.5 x Total Hours

Invested

Ying Wang $27.50 600 $41250

Parts List

Part name Numbers Total cost ($)

IR emitter 1 2

IR detector 1 2

Arduino Uno 1 30.00

OPAMP LM324 1 2

LCM 1602 16-by-2 1 Free

5.6 V Zener diode 1 0.10

2N3005 NPN transistor 1 0.84

9V battery 1 1.37

9V battery clip 1 0.22

25 pin solder tail strip socket 2 10.64

Resistors, Capacitors and

LEDs

Free

Total 49.17

Total cost: $41298

V. Safety

The safety of this pulse sensor is one of the most important aspect when designing this device.

This pulse sensor will be powered by 9V battery so it is safe for people to use. The IR emitter

and detector may contain Arsenide which could be hazardous if broken. They are safe under

normal conditions. Children should not use this device without an adult’s guidance. It is also

important to note that there are many factors that could affect people’s pulse, like emotions,

temperature, body position, body size and medication use. This device is not a substitute for

professional diagnostics. If your pulse is very high or if you have frequent episodes of

unexplained fast heart rates, especially if they cause you to feel weak or dizzy or faint, tell your

doctor, who can decide if it’s an emergency.

VI. Ethics

This pulse sensor is designed to improve the quality of people’s life. The pulse rate is an

important indicator of people’s health condition. So measuring the pulse rate accurately and

anytime they want at home or outside satisfies people’s needs and benefit them in many ways.

VII Work to be done

So far I have performed the pulse sensor module testing, power module simulation and prepared

the PCB board. Further work need to be done on power module testing, Arduino programming

and display in the next few weeks.