MICRO CONTROLLER BASED HEART RATE MONITORING SYSTEM, MINI PROJECT FOR EC OR ELECTRONICS AND BIOMEDICAL ENGINEERING

1. MICROCONTROLLER BASED HEART RATE MONITORING SYSYTEM
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.
PHOTOPLETHYSMOGRAPHY
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).
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2. CIRCUIT DIAGRAM
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3. WORKING OF THE HEART RATE MONITOR
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.
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 set to 0.3V. 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.
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4. 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.
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.
SETTING UP THE CIRCUIT
When power is switched ON, the indicator LED D4 will glow an continues
in that state. Now place your finger tip over the sensor and adjust preset
R14 so that the LED D4 starts blinking. After you got the LED blinking,
reset the power and wait for 15 seconds. The display will show your heart
rate in beats per minute.
LCD VERSION OF THE HEART RATE MONITOR.
A simple LCD version of the heart rate monitor is shown below. This is just
a modification of the above circuit.LCD displays are very popular now a
most of the embedded system designers prefer them over multiplexed
seven segment LED displays. Using LCD displays you can display text,
custom characters, graphics and a lot of other stuff and it is a great
advantage over the LED counterparts. JHD162 is the LCD display used
here. It is a 16X2 LCD display based on the HD44780 driver IC. Go through
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5. the following link for knowing more about JHD162 and its interfacing to the
8051 microcontroller. Interfacing LCD display to 8051. The circuit diagram
of the LCD version of the heart rate monitor is shown below.
Data/command input pin DB0 to DB7 of the display is interfaced to Port0
of the microcontroller. Resistor network R17 is used for pulling up
thePort0. Port0 needs external pull up for proper functioning. Preset
resistor R1 is used for adjusting the contrast of the display. R2 limits the
current through the back light LED. Other parts of the circuit are similar to
theLEDversion
LCD VERSION CIRCUIT
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