DESIGN HEART RATE MEASURING DEVICE FROM
FINGERTIP USING A LOW COST MICRO-CONTROLLER
Dharmendra Pancholi*, ,Mr.Pramod Kumar Jain**, Mr. D.S.Ajnar***
*M.Tech.Student, Dept of Electronic & Instrumentation , S.G.S.I.T.S, Indore, India, Email: engg.d.pancholi@gmail.com
**Assoc Prof, Dept of Electron ic & Instrumentation, S.G.S.I.T.S, Indore, India .Email:eng.pkjain@gmail.com
***.Assoc Prof, Dept of Electronic & Instrumentation, S.G.S.I.T.S, Indore, India.
ABSTRACT:- In this paper, we presented the design
of a new integrated device for measuring heart rate
from fingertip using a low cost microcontroller to
improve estimating the heart rate. Our proposed
Heart Rate Measuring (HRM) device is economical
and user friendly and uses optical technology to
detect the flow of blood through index finger. . Heart
rate of the subject is measured from the finger using
optical sensors and the rate is then averaged and
displayed on a text based LCD.
Keywords- Heart pulse rate, finger pulse rate,
microcontroller.
I. INTRODUCTION
Heart rate measurement is one of the very
important parameters of the human cardiovascular
system. The heart rate of a healthy adult [1] at rest
is around 72 beats per minute (bpm). Athletes
normally have lower heart rates than less active
people. Babies have a much higher heart rate at
around 120 bpm, while older children have heart
rates at around 90 bpm. The heart rate rises
gradually during exercises [2] and returns slowly to
the rest value after exercise. The rate when the
pulse returns to normal is an indication of the
fitness of the person. Lower than normal heart rates
are usually an indication of a condition known as
bradycardia, while higher than normal heart rates
are known as tachycardia.
Heart rate is simply and traditionally measured by
placing the thumb over the subject’s arterial
pulsation, and feeling, timing and counting the
pulses usually in a 30 second period. Heart rate
(bpm) of the subject is then found by multiplying
the obtained number by 2. This method although
simple, is not accurate and can give errors when the
rate is high. More sophisticated methods to
measure the heart rate utilize electronic techniques.
Electro-cardiogram (ECG) is [3,4] one of
frequently used and accurate methods for
measuring the heart rate. ECG is an expensive
device and its use for the measurement of the heart
rate only is not economical. Low-cost devices in
the form of wrist watches [5,6] are also available
for the instantaneous measurement of the heart rate.
Such devices can give accurate measurements but
their cost is usually in excess of several hundred
dollars, making them uneconomical. Most hospitals
and clinics in the UK use integrated devices
designed to measure the heart rate, blood pressure,
and temperature of the subject. Although such
devices are useful, their cost is usually high and
beyond the reach of individuals.
This paper describes the design of a very low -cost
device which measures the heart rate of the subject
by clipping sensors on one of the fingers and then
displaying the result on a text based LCD. The
device has the advantage that it is microcontroller
based and thus can be programmed to display
various quantities, such as the average, maximum
and minimum rates over a period of time and so on.
Another advantage of such a design is that it can be
expanded and can easily be connected to a
recording device or a PC to collect and analyse the
data for over a period of time. The building cost of
the proposed device is around $20. One similar
basic device from Cosy Communications [7] with
no extension capabilities costs around $100.
II. THE MEASUREMENT DEVICE
Figure 1 shows the block diagram of the proposed
device. Basically, the device consists of an infrared
transmitter LED and an infrared sensor photo transistor. The transmitter-sensor pair is clipped on
one of the fingers of the subject (see Figure 2). The
LED emits infrared light to the finger of the
subject. The photo-transistor detects this light beam
and measures the change of blood volume through
the finger artery. This signal, which is in the form
of pulses is then amplified and filtered suitably and
is fed to a low-cost microcontroller for analysis and
display. The microcontroller counts the number of
pulses over a fixed time interval and thus obtains
the heart rate of the subject. Several such readings
are obtained over a known period of time and the
results are averaged to give a more accurate reading
of the heart rate. The calculated heart rate is
displayed on an LCD in beats-per-minute in the
following format:
Rate = nnn bpm
Where nnn is an integer between 1 and 999.
Figure 1. Block diagram of the measuring device
The cut-off frequency of the filter was chosen as
2Hz. Figure 4 shows the frequency and phase
responses of the amplifier together with the filter.
The output time response of the amplifier and filter
circuit is shown in Figure 5 which consists of
pulses. An LED, connected to the output of the
operational amplifiers flashes as the pulses are
received and amplified by the circuit.
The output of the amplifier and filter circuit was
fed to one of the digital inputs of a PIC16F84 type
microcontroller [8]. In order to reduce the cost of
the circuit the microcontroller is operated from a
4MHz resonator. The microcontroller output ports
drive the LCD as shown in Figure 3. The circuit
operates when a push-button switch connected to
RB1 port of the microcontroller is pressed.
III.
Figure 2. Infrared transmitter and received sensor
pair
The circuit diagram of the measurement device
is shown in Figure 3. The circuit basically consists
of 2 operational amplifiers, a low-pass filter, a
microcontroller, and an LCD. The first amplifier is
set for a gain of just over 100, while the gain of the
second amplifier is around 560. During the
laboratory trials it was found necessary to use a low
pass filter in the circuit to filter out any unwanted
high frequency noise from nearby equipment.
Figure.3.circuit diagram of device
METHODOLOGY
A. Optical Transmitter and Receiver Circuit
HRM measures the pulse rate through changes of
blood flow through an index finger. Each pulse of
blood from the heart increases the density of blood
in the finger pulsatile tissue and causes a decrease
in light power received by the photo-sensor. The
photo-sensor does not pick up a purely AC signal
as there are some DC components received from
other non-pulsatile tissues and ambient light levels.
The varying light levels received are converted into
a varying resistance in the photo-sensor. The
varying resistance is converted into a varying
voltage by using a resistance network and power
source. In order to do this, two red LEDs are 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, two red
LEDs were chosen to allow the maximum amount
of light energy to pass through the index finger.
The circuit shown in Fig. 4 is designed to achieve
this.
The circuit in Fig. 4 shows our pulse -rate to
voltage converter and our source of constant red
light that is designed and constructed to gather realworld data. The red LED is forward biased through
a resistor to create a current flow. The value of R2
is chosen in a way so that it produces a maximum
amount of light output. The calculated value is
approximated to a resistance value that is
commonly available. The photo-resistor is placed
in series with a resistor to r educe the current drawn
by the detection system and to prevent shorting the
power supply when no light is detected by the
photo resistor.
frequencies are boxed in by movement artifacts at
the low end (generated by the peg moving an d
distorting the underlying tissues; light pegs are
better) and at the top end by mains-hum
interference. The circuit runs from a single 5 Volt
battery and the output zero is offset by about 1 Volt
by referring everything to an internal common line
at a voltage set by a pair of forward-biased silicon
diodes. This is convenient for interfaces with a 0-5
Volt input. The potentiometer allows the overall
gain to be adjusted so as to prevent clipping on
large signals. Components are not critical but the
two 2.2 µF capacitors must be able to stand some
reverse bi as so they should be non-polarized or
tantalum. The circuit can easily be made up on a
small piece of strip board.
C. Microcontroller
A microcontroller is an economical means of
counting the pulse rate and controlling a LED
display. The method used below allows the
displays to be driven without the use of a display
driver. The displays are set and refreshed by
multiplexing the segment lines to the same I/O pins
on the microcontroller.
Figure 4. Pulse receiving circuit used in HRM device
B. Amplification of Pulse Rate Signal
To let the microcontroller counting the pulse
rate, the signal must be amplified. An amplifier is
used to fin d rising edges of the filtered signal
received by the photo sensor. This allows one pin
of the microcontroller to be used as a n input. The
time between rising pulse edges is determined by
the microcontroller so that the frequency of the
heart rate can be measured. The designed circuit is
shown in Fi g. 5.
Programming the microcontroller involves
developing a calculation algorithm to count the
pulse rate. The calculation algorithm for counting
the pulse r ate will be easy to develop using
Firmware. The microcontroller will continuously
be checking if a signal is fed into it. Once a signal
is detected, the algorithm will begin according to
the flow chart in Fig. 6.
Figure.6.algorithm for detecting pulse rate
Once each stage of the design has been simulated
to prove their unit efficiency, they are integrated
and in Fig. 7, the working principle is illustrated
Figure 5. Amplifier Circuit used in HRM device
The amplifier uses an LM358 dual op amp to
provide two identical broadly-tuned band pass
stages with gains of 100. Again, the type of op amp
is not particularly critical, as long as it will work at
5V and drive the output rail to rail. The signal
Figure 7. Working principle of HRM device
IV. EXPERIMENTAL
RESULTS
The first phase of the device, the optical
receiver and transmitter, is constructed and tested.
The output of the receiver is connected to an O scope to obtain the heartbeat signal. Fig. 8 shows
the heartbeat signal obtained by the device for a
person using two different channels.
Figure 8. Extracted heartbeat signal
A band pass filter is used to filter the noise from
the heartbeat signal. Fig. 9 shows the output
obtained after removing the noise in the heartbeat
signal.
been filtered out as expected. The filtered signal is
required to have a SNR of 20 dB or greater, to ensure
that the amplifier is able to correctly convert the
continuous signal to a higher amplitude signal form
without producing false trigger due to noise. The
filtered signal has a SNR of approximately 24 dB, and
this allows the amplifier to properly amplify the
heartbeat. This test shows that the filter is able to
remove high frequency noise from the heartbeat
signal.
The microcontroller is programmed to count the
number of peaks of the input signal in 5 or 10
seconds, and the result is further multiplied
correspondingly by 12 or 6 to obtain the total
number of peaks per minute. The LCD is connected
to the microcontroller and a known frequency pulse
signal is fed into it. The correct number of peaks
per minute value is showed on the LCD. When the
microcontroller is integrated into the entire design
circuitry, it is able to count the number heartbeats
per minute and drive the LCD to display the
counted value.
The performance of HRM device is tested with
the output of Electrocardiogram (ECG) for 10
patients. The error rate is calculated using (1)100 |
Here,
|
Actual heart rate
Measured heart rate and
Error rate
The comparison shows that the HRM device
has accuracy with a mean of 4.31 and standard
deviation of 2.87.
Figure 9. Filtered output signal
IV. CONCLUSION
The design of a low- cost microcontroller based
device for measuring the heart pulse rate has been
described. The device has the advantage that it can
be used by non-professional people at home to
measure the heart rate easily and safely.
The device can be improved in certain areas as
listed below:
•
•
A graphical LCD can be used to display a
graph of the change of heart rate over time
Sound can be added to the device so that
a sound is output each time a pulse is
received.
•
The maximum and minimum heart rates
over a period of time can be displayed.
•
Serial output can be attached to the device
so that the heart rates can be sent to a PC
Figure 10. Amplified output signal
Comparing Fig. 9 and Fig. 10, we see that the high
frequency noise of 120 Hz from ambient lights has
(1)
for further online or offline analysis.
•
Warning or abnormalities (such as very
high or very low heart rates) can be
displayed on the LCD or indicated by an
LED or a buzzer.
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He has received the B.E. degree in
Electronics and Communication Engineering from
S.G.S.I.T.S. affiliated to D.A.V.V.University (Formerly
known as University Of Indore), Indore, India in 1993
and M .E. Degree in Digital Techniques &
Instrumentation Engineering from Rajiv Gandhi
Technical University Bhopal, India in 2000. He has been
in teaching and Research Profession since 1995. He is
now working as Associate Professor in Department of
Electronics & Instrumentation Engineering, S.G.S.I.T.S.,
Indore, India. His interest of research is in designing of
analog filter and Current conveyer.
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Available
at:
http://www.picotech.com/experiments/calculating_
heart_rate/index.html [December 27, 2009]
ABOUT THE AUTHORS
He has received the B.E. degree in Electronics and
Instrumentation Engineering from S.G.S.I.T.S. Indore
affiliated to R.G.P.V Bhopal, India in 2011. He is
currently pursuing M.Tech degree in Microelectronics
and VLSI Design from S.G.S.I.T.S. Indore.
He has received the B.E. degree in Electronics and
Communication Engineering from D.A.V.V. University
(Formerly known as University Of Indore), Indore, India
in 1987 and M .E. Degree in Digital Techniques &
Instrumentation Engineering from D.A.V.V. University,
Indore, India in 1993. He has been in teaching and
Research Profession since 1988. He is now working as
Associate Professor in Department of Electronics &
Instrumentation Engineering, S.G. S.I.T.S., Indore. He
has also worked as a computer Engineer. His interest of
research is in Analog and digital system design.