1. PRESENTED BY
JOHN GARANG DE MABIOR
BHARATH RAJ
RANJITH KUMAR
MOHAMAD MUSTAQ NAWAS
HEART RATE MONITOR
USING 8051
MICROCONTROLLER
2. Heart rate monitor using 8051
This article is about a simple heart rate monitor using
8051 microcontroller. Like the previous 8051
projects, AT89S51 is the microcontroller used here.
The device senses the heart rate from the finger tip
using IR reflection method and displays it on a three
digit seven segment display in beats per minute. The
circuit has an accuracy of 4 beats per minute and it is
very easy to use. In medical terms, the technique
used here for sensing heart rate is called
photoplethysmography.
3. Photoplethysmography is the process of optically estimating the volumetric
measurement of an organ. Pulse oximetry, cardiovascular monitoring,
respiration detection, heart rate monitoring etc are few common
applications of photoplethysmography. Let us have a look at the
application of photoplethysmography in heart rate monitoring from the
figer tip. When the heart expands (diastole) the volume of blood inside
the finger tip increases and when the heart contrcats (systole) the volume
of blood inside the finger tip decreases. The resultant pulsing of blood
volume inside the finger tip is directly proportional to the heart rate and if
you could some how count the number of pulses in one minute, that’s the
heart rate in beats per minute (bpm). For this an IR transmitter/receiver
pair placed in close contact with the finger tip. When the heart beats, the
volume of blood cells under the sensor increases and this reflects more IR
waves to sensor and when there is no beat the intensity of the reflected
beam decreases. The pulsating reflection is converted to a suitable current
or voltage pulse by the sensor. The sensor output is processed by suitable
electronic circuits to obtain a visible indication (digital display or graph).
PHOTOPLETHYSMORGRAPHY
4. APPARATUS REQUIRED
SNO APPARATUS QUANTITY
1 LOW POWERED LED 1
2 LCD DISPLAY 1
3 MICRO CONTROLLER 8051 1
4 PC B BOARD 1
5 TRANSISTORS 3
6 RESISTORS AS PER THE
REQUIREMENT
7 CAPACITORS AS PER THE
REQUIREMENT
8 OP AMP LM324 3
6. WORKING
LTH1550-01 photo interrupter forms the photoplethysmographic
sensor here. LTH1550-01 is simply a IR diode – photo transistor
pair in single package. The front side of the IR diode and photo
transistor are exposed and the remaining parts are well isolated.
When the finger tip is placed over the sensor the volumetric pulsing
of the blood volume inside the finger tip due to heart beat varies the
intensity of the reflected beam and this variation in intensity is
according to the heart beat.
When more light falls on the photo transistor it conducts more, its
collector current increases and so its collector voltage decreases.
When less light falls on the photo transistor it conducts less, its
collector current decreases and so its collector voltage decreases.
This variation in the collector voltage will be proportional to the
heart rate. Any way this voltage variation is so feeble and additional
signal conditioning stages are necessary to convert it into a
microcontroller recognizable form.
7. The next part of the circuit consists of a two active low pass
filters using opamp LM324. The LM324 is a quad opamp
that can be operated from a single rail supply. Resistor R23,
R17 and capacitor C5 sets the gain and cut off frequency of
the first filter. With the given component values, gain will be
11 and cut off frequency will be 2.5Hz. The gain and cut off
frequency are determined using the following equations.
Voltage gain Av =1 + (R17 / R23)
Cut off frequency Fc= 1/(2π *R17*C5)
The second low pass filter also have same gain and cut off
frequency. The two low pass filters form a very critical part
of the circuit as any noise or false signals passing to the
microcontroller stage will produce disastrous results. The
output of the filter stage will be a voltage level fluctuating
between 0 and 0.35 volts and this fluctuation is converted
into a 0 to 5V swing using the comparator based on the
third opamp (IC1c). The reference voltage of the comparator
is Sset to 0.3V.
8. When ever the output voltage of the
filter stage goes above 0.3V, the
output of the comparator goes to zero
and whenever the output voltage of
the filter stage goes below 0.3V, the
output of the comparator goes to
positive saturation. The result will be
a neat pulse fluctuating between 0
and 5V at a rate equal to the heart
rate. This pulse is fed to the
microcontroller for counting.
11. ABOUT THE PROGRAM
For the counting purpose both the timers of 8051 (Timer0
and Timer1) are used. Timer 1 is configured as an 8 bit
auto reload counter for registering the number of
incoming zero going pulses and Timer0 is configured as a
16 bit timer which generate the necessary 1 second time
span for the Timer1 to count.For counting the number of
beats Timer0 and Timer1 are used. Timer1 is set as an 8
bit auto reload counter for counting the the number of
pulses (indicating the heart beat) and Timer0 is set as a
16 bit timer which generates a 65536uS delay. When
looped 230 times it will produce a 15 second time span
(230 x 65536uS =15S) for the Timer 1 to count. The
number of counts obtained in 15 seconds is multiplied by
4 to obtain the heart rate in beats per minute.
12. The Timer 0 which generates the 1 second time span
is configured in Mode 1 (16 bit timer). So the
maximum it can count is 2^16 and it is 65536. In
8051 the crystal frequency is divided by 12 using an
internal frequency divider network before applying it
as a clock for the timer. That means the timer will
increment by one for every 1/12th of the crystal
frequency. For an 8051 based system clocked by a
12MHz crystal, the time taken for one timer
increment will be 1µS (ie; 1/12MHz). So the
maximum time delay that can be obtained using one
session of the timer will be 65536µS. Go through this
article Delay using 8051 timer for a better grasp.
