Biomedical Instrumentation and Measurements
Measurement of pulses and theories related to cardiac activities started developing around 300B.C by the Greeks. Researches on heart rate pulses being used for diagnosis and prognosis purposes were being done. Since then, there has been a continuous development in this field, like the development of a stethoscope for indirect auscultation, the sphygmograph, string galvanometer and even a photoelectric pulse counter or just a stopwatch for measuring the pulse was used.
In the year 1977, Professor Seppo Säynäjäkangas realized the importance and need of using a portable heart rate meter for training. In 1979, Polar developed the wireless heart rate monitor, and in 1982, it came up with the first wearable heart rate monitor for athletes.
In earlier models, the monitoring box consisted of a set of electrodes (or leads) that were attached to the chest. These leads were used to sense the voltage changes due to heart rate changes. Although these models are still available, modern versions consisting of a chest strap transmitter and a wrist receiver were developed. Materials used for the straps have also been changed to give maximum comfort to the patient while monitoring their heart rates. This method was not reliable, accurate or even user friendly as the patient has to be strapped to wires.
The discovery that the muscle contractions involve electrical processes dates back to 18th century. It was in 1887, that the first electrocardiogram was used by Waller, who used the capillary electrometer introduced by Lipmann in 1875, which used mercury filled capillary immersed in dilute sulphuric acid. This was cumbersome to use and inertia of liquid column limited its frequency range.
In 1889, Einthoven used buckets filled with saline solution as electrodes and all the limbs were placed in these buckets to diagnose the electrical signals and the response of the heart to these signals.
The string galvanometer, introduced to the electrocardiogram by Einthoven in 1903, was a significant improvement. These were replaced by devices incorporating electronic amplification, which allowed the use of less sensitive but more rugged devices. But unfortunately, this system is too big and expensive to be used for daily purposes like athletic trainings.
Therefore, the task was to convert the ECG technology into a more portable system. After intensive research and development, the first wireless and ECG accurate heart rate monitor was finally put on the market by the Finnish firm Polar Electro. And it was then , that the heart rate could be measured accurately without any limitations.
To start with the heart rate monitors were almost exclusively of interest to competitive endurance athletes. Athletes quickly recognized that by measuring and monitoring their heart rate during training, they could obtain important information about the training load that enables them to specifically manage their training.
In the years that followed, the development and improvement of the monitors continued, so that every year, heart rate monitors advanced in technical sophistication and performance.
The ECG accurate determination of the duration of each heartbeat using a small, wireless heart rate monitor was an important technological innovation. Today, the electronic measurement of heart rate and heart rate variability of heart rate due to influences of different factors in both recreational and competitive sports is taken for granted and is indispensible in the monitoring of training load intensity.
1.1.2 Heart rate
Heart rate or pulse rate is defined as the number of times that the heart beats per unit time. It is basically the number of ventricular contractions of the heart. Pulse rate can be measured at any point of the body where the arterial pulsations are felt on the body's surface. It is a vital parameter to indicate the heart conditions of a human being. During exercises or any other athletic activities, heart rate measurement is always desirable for achieving optimal results as well as for personal safety.
A heart pumps oxygen-rich blood into the muscles and receives deoxygenated blood from them. As the oxygen demand increases, the heart rate increases proportionally, providing the oxygenated blood to the muscles. Hence, it is an important indication of oxygen supply in different muscles of the body.
The heart beats on an average of 75 beats per minute, in a normal adult, although this figure can differ considerably. For example, as a person stands up, his heart rate increases, whereas it decreases as the person sits down. This range is around 60 to 85 beats per minute. Generally, it is higher in women and decreases with age. In an infant, the heart rate maybe as high as 110 to 160 beats per minute under normal conditions. In a person of age 60, the heart rate can be as low as 60 beats per minute. 
1.1.3 Factors Influencing Heart Rate
Stress, Illness, Over training, Medication, Time of day, Body position (standing, sitting or lying down), Body size, Food and drink (Caffeine), Altitude, Temperature, Hydration levels, Weather conditions, Heart rate drift, Heart rate variability, Age, Emotional stress are few factors that can alter heart rate.
1.2 Cardiac Cycle
The heart can be considered as a hydraulic pump which is divided into two major pumps, i.e, the right heart pump and the left heart pump. The right heart pumps the blood to the lungs for oxygenation whereas the left heart pumps the blood to the different parts of the body. Also, each of these two pumps consists of a pulsatile two chamber pump consisting of an upper atria and a lower ventricle. The flow of blood from the right heart to the lungs is called as pulmonary circulation, whereas that of the left heart to the body parts is the systemic circulation.
It is evident that the systemic circulation requires stronger contraction by the left heart as the pressure required is more than that of the right heart. The pumping action is achieved by the contraction of cardiac muscles. Also, the atrias are considered as weaker pump as it moves the blood into the ventricles. The ventricles, in turn , are stronger pumps which gives the driving force which helps in flow of blood from left ventricles to the rest of the body and also for the blood to move into the heart.
The operation of heart consists of many phases. The first phase involved the filling of atrias. Blood enters the right atria from the vena cavas, i.e., from the upper extremities through the superior vena cava and that of the lower extremities through the inferior vena cava. In addition to these two veins, the blood is emptied in the right atria through the coronary sinus. The coronary sinus contains the blood which is circulated within the cardiac chambers via the coronary loop.
Now as the atrium is being filled completely, it gets ready to contract. Once the right atria is completely filled, the contraction takes place. During this, the blood enters the right ventricle through the tricuspid valve. Finally, when the ventricles are filled completely and the ventricular pressure far exceeds the atrial pressure, then the tricuspid valve closes and the pulmonary valves open. The blood then rushes into the pulmonary artery, which takes it to the lungs for oxygenation.
The pulmonary artery bifurcates into many small arterioles, which are further divided into small capillaries. The red blood cells are filled with oxygen and the carbon dioxide is eliminated. On the other side of the lung, capillaries feed into tiny veins or venules. These venules join in order to form large veins and these recombine to transfer the oxygenated blood into the heart through pulmonary vein.
The oxygenated blood now enters the left atria, and similar to the functioning of the right heart, as the pressure increases in the atria and it is completely filled, the bicuspid valves open to allow the blood to enter into the left ventricle. As the pressure exceeds the atrial pressure, these bicuspid valves close and semilunar valves open to allow the blood enter the aorta from where the blood is passed on the rest of the body parts. This is the most important phase which involves the blood to flow to the different organs and muscles of the body.
The cardiac cycle basically consists of two stages, namely, the diastolic stage and the systolic stage. The diastolic stage is the period of relaxation or the filling of chambers. On the contrary, the period of contraction or ejection of blood is called the systolic stage.
As soon as the blood moves out of the ventricles, there is a brief period of muscular relaxation, pressure drop in the chambers, closure of outlet valves and opening of inlet valves to allow the blood enter the heart again and initiate a new cycle.
Blood continually flows into the atrias from the vena cavaes and the pulmonary veins, and almost 75% of the blood flows into the ventricles before the atrial contraction. During the contractions, just 25% of the blood is pushed into the ventricles.
When the heart rate increases, the duration of both systolic and diastolic stage as well as the duration of each cycle decreases. The decrease in systolic stage is lower than that of the diastolic stage. At the normal heart rate of 72 beats per minute, the period of contraction is 0.40 of the entire cycle. At three times the normal heart rate, this period is about 0.65 of the entire cycle. 
1.2.1 Signal Generation
The heart is endowed with a specialized electrogenic system for generating impulses to cause rhythmical contraction of the heart muscle and conducting these impulses rapidly throughout the heart. The sino-atrial node or the pacemaker of heart, is considered vital for the rhythmic contraction and relaxation of the heart muscles. The sino atrial node generates an electrical impulse which is passed on to the cardiac chambers, the right atria being the first to receive (due to its close proximity to the sino-atrial node). Due to this excitation signal, the atria contracts and simulated the atrio-ventricular node after a brief period. This signal is received by the ventricles after a short delay, through the bundle of HIS. The A-V node also initiates an impulse into the bundle branches that connect the Purkinje fibers to the myocardium. This contraction in myocardium gives the force needed by the blood to flow through the circulatory system.
The sinus node controls the heart beat as its rhythmical discharge is greater than any part of the body. Basically , the heart rate is controlled by the pace of electrical signal generation by the sinus node.
The heart is supplied with parasympathetic and sympathetic nerves which have a direct effect in the control of heart rate. The parasympathetic nerves are spread throughout the S.A node and the A.V node and to the lesser extent to the atrial muscle and very little directly to the ventricular muscle. Conversely, the sympathetic nerves are distributed to the entire muscular structure of the heart.
Simulation of the parasympathetic decreases the rhythm of the sinus node and it decreases the excitability of the A.V node junction fibers between the atrial musculature and the A.V node, which results in slow transmission of cardiac signals into the ventricles. Weak to moderate vagal stimulation slows the rate of heart pumping often to as little as one half normal. But strong stimulation of the vagi can stop completely the rhythmical excitation by the sinus node or block completely transmission of the cardiac impulse through the A.V junction. The rhythmical impulses are no longer transmitted into the ventricles.
Sympathetic nerves, on being stimulated, cause the completely opposite effect to that caused by the vagal nerves. It increases the rate of sinus nodal discharge and also increases the excitability level in all portions of the heart (Fig 1.2). 
1.3 Abnormal Heart Rates
Tachycardia is defined as abnormally fast heart rate, usually more than 100 beats per minute (Fig 1.3) . The general causes of tachycardia are increased body temperature, stimulation of heart by the sympathetic nerves and toxic conditions of the heart. 
It's interesting to know that the heart rate increases by about 10 beats per minute for each degree Fahrenheit increase in body temperature upto around 105 degree Fahrenheit. Beyond this the heart rate starts decreasing due to countinous weakening of the heart muscle due to fever. The rate of metabolism of sinus node increases due to fever, which in turn increases its excitability and rate of rhythm.
Tachycardia may also be caused due to excessive blood loss by a patient or simple weakening of the myocardium. 
Bradycardia is defined as abnormally slow heart, usually as low as 60 beats per minute (Fig. 1.4).
Generally, an athlete's heart is considered to be stronger than that of a normal person, due to its ability to pump a large stroke of volume output per beat even during periods of rest. The excessive quantities of blood pumped into the arterial tree with each beat initiate feedback circulatory reflexes or other effects to cause bradycardia when the athlete is at rest. 
Also, stimulation of vagus nerve by any factor results in bradycardia. In patients suffering from carotid sinus syndrome an arteriosclerotic process in the carotid sinus region of the carotid artery causes excessive sensitivity of the pressure receptors located in the arterial wall. As a result, mild pressure on the neck elicits a strong baroreceptor reflex, causing extreme bradycardia. Sometimes, this reflex is strong enough to stop the heart for about 5 to 10 seconds
1.3.3 Sinus Arrhythmia
Arryhthmias are abnormalities in heart rate and rhythm. Primarily, slow or fast arrhythmias, they can be mild with no serious effects. A few can also be serious causing dizziness, lightheadedness or fainting. Cardiac arrhythmia or dysrhythmia is a term for any of a large and heterogeneous group of conditions in which there is abnormal electrical activity in the heart. In this, the heart beat may be too fast or too slow, and may be regular or irregular. It results from circulatory reflexes that alter the sympathetic and parasympathetic nerve signals to heart sinus node. 
A few symptoms of arrhythmia include medical emergencies resulting in cardiac arrest and sudden death, abnormal awareness of heart beat or palpitations. A few arrhythmias might not give any warnings or symptoms but develop into life threatening stroke. Some arrhythmias are very minor and can be regarded as normal variants.
The term sinus arrhythmia refers to a normal phenomenon of mild acceleration and slowing of the heart rate that occurs with breathing in and out. It has pronounced effects in children, and decreases with age.
1. Pulse detection : the circuit implemented uses a photodiode and an light emitting diode in order to obtain the heart pulse signals. Voltage created will be as low as 0 mV to 10mV.
2. Pulse digitization: the circuit implemented here, uses an IC 555 or better known as a monostable multivibrator in order to obtain a digitized square pulse. Around 20dB of SNR will be present in the signal.
3. Count : in this, an assembly of a mod-10 counter is used along with a BCD to seven segment decoder, in order to obtain the pulse counted till 200.
4. Display : for this, FND or seven segment digital display are used.
5. Accuracy: the design is expected to have accurate readings with no more than +/- 10% error. The accuracy of current optical heart monitoring devices is typically 10-15%.
2.1 DESIGN TECHNIQUES
2.1.1 Traditional Method
Heart rate can be measured at different sensitive point in a body. The most common palpitation points are at the wrist and the neck. The wrist consists of a radial artery whereas the neck has the carotid artery. The pulse can be easily detected at these points. Two fingers can be placed on the either side of the neck and number of beats can be counted. To measure the radial pulse, index and middle fingers can be placed on the opposite wrist, around half an inch on the inside of the joint, in line with the index finger. Heart rate can be calculated by counting the number of heart beats for 10 seconds and multiplying by 6 or even counting for one minute. This method is totally unreliable but gives the patient a rough idea of their pulse rates.
2.1.2 Using a Stethoscope
An electronic heart rate detector can also be designed using the stethoscope principle. In this a transducer is used for producing an electrical signal in response to heart sounds. A bandpass filter for filtering the electrical signals can be used. Then a pulse generator can be added which helps converting heart sounds from the filtered electrical signal. Then a counter to count these inputs is where these pulses are fed into. 
The transducer has to be placed on the patient's chest which can be very uneasy.
As the electric pulse pass through the different chambers of heart, a small amount of current also spreads from the heart to different tissues around the heart. Also, a small amount of current spreads through the entire body. If electrodes are placed on the skin on the opposite sides of heart, electrical potentials generated by this current can be recorded. This recording is known as an electrocardiogram. In other words, ECG is a graphic recording display of the time variant voltages produced by the myocardium during the cardiac cycle.
An electrocardiogram of a normal, healthy person consists of a P wave, a QRS complex and a T wave. The P wave occurs due to the atrial depolarization before the actual atrial contraction begins. The QRS complex is caused by potentials generated due to the ventricular depolarization just before the ventricles contract. Thus, both P wave and QRS complex are depolarization waves. The T wave occurs due to the potentials generated as the ventricular repolarization takes place. Some normal values for amplitudes and durations of important ECG parameters are as follows :
P wave: 0.25mV
R wave: 1.60 mV
Q wave: 25 % of R wave
T wave: 0.1 to 0.5 mV
P-R interval: 0.12 to 0.20 sec
Q-T interval: 0.35 to 0.44 sec
S-T interval: 0.05 to 0.15 sec
P wave interval: 0.11 sec
QRS interval: 0.09 sec
The rate of heartbeat can be determined from the electrocardiogram as the heart rate is the reciprocal of time between two successive beats. The normal interval between two QRS complexes in an adult is around 0.83 seconds which implies a heartbeat of 72 beats/minute.
An electrocardiogram is obtained by connecting following leads across different points in the body.
Three bipolar limb leads: Lead I, Lead II, Lead III are connected across the left arm, right arm and the left leg.
Chest leads (Precordial leads) : V1, V2, V3, V4, V5 and V6 are connected across the heart and left leg as a reference point.
Augmented Unipolar Limb Leads: two limbs are connected to the negative terminals and another limb is connected to the left leg used as reference.
Now using these leads, an electrocardiogram is generated and used as an important tool to calculate heart rate and diagnose cardiac problems like myocardial infarction, etc.
2.1.4 USING A PIEZOELECTRIC CRYSTAL
In this system, a piezoelectric crystal, which is capable of generating power as well detecting signal, is used. The pulsed ultrasonic energy is emitted and the reflected signal is received by the same crystal as soon as the output power is switched off. The distance between the source and the object decides on the time required to receive the signal.
Doppler shift, which is dependent on the speed of moving object, uses only a single piezoelectric crystal, although the detector circuit works only after the transmitter power is switched off. To measure the heart rate, action of a blood vessel wall is detected. It is in the case of superficial veins that this system needs a high speed during switching power as the distance of object and source is very less. Also the movement of a person, results in unnecessary reflections and noise.
Nonetheless, it's a low cost and low power consumption technique. Additions can be done in order to obtain a better response. For example, two piezoelectric elements can be used to achieve continuous response. They can be placed at an angle to the direction of the flow on opposite sides, and either one can be used as a transmitter or receiver. The flow rate is directly proportional to the Doppler shift operating frequency. The Doppler shift due to the pulse rate application is different from the shifts due to muscular movements or tissue movement. Blood flow frequency shift is higher than that of muscular movement shift. Therefore, these signals can be easily filtered out, even if they are of larger amplitudes, using a high pass filter. The ultrasound method is hence, better than that of infrared or EKG technologies.
2.1.5 Electronic Sphygmomanometer
A sphygmomanometer works on the principle that when the cuff placed on the upper arm is inflated, arterial blood can flow past the cuff only when the arterial pressure exceeds the pressure in the cuff. Moreover, when the cuff is inflated to a pressure that partially occludes the brachial artery, the blood spurts out through tiny opening of the artery, and hence, generates turbulence. This turbulence can be detected in terms of the Korotkoff sounds that are generated. Now this sphygmomanometer cuff, when connected to a microphone and attached to the digital circuit, can count the number of pulses and display them. This device also measures the systolic and diastolic pressures by detecting the first and last korotkoff sounds heard. 
Photodiode (dark chamber arrangement), Light emitting diode (LED), IC 74LS90 (mod 10 counter) , IC 7474 (D flip flop), IC 7447 (BCD to 7 Segment decoder), FND, IC 7408(AND gate) , IC 7432 (OR gate), resistors, connecting wires.
The design used in this optical heart rate meter consists of an LED and a photodiode. The pulse rate is measured through the changes in the blood flow at the index finger. Each pulse of blood from the heart to the finger would increase the density of the blood (as it is rich in oxygen) in the finger tissue and would decrease the light power received by the photo-sensor. The photosensor will not pick up purely AC signal since there are some DC components received from other non-pulsatile tisues and ambient light levels. This varying light signal would be converted into voltage by a photodiode.
3.1.2 Brief explanation
To successfully simulate this project, design will be done in stages. The first stage will be design a circuit that will receive the pulse rate. Next, the signal must be digitized so that it can be counted. Once these two stages are designed, they will be tied together to show a working design through simulation. If this is done successfully, a counter will be used to count the pulse rate and display it on a three digit LED display. Finally, each stage will be put together to form a working project.
Entire arrangement includes the following parts:
1. The black box
2. Circuit of the signal receiving circuit
3. Monostable circuit
4. The counter and display circuit
The black box is used in order to not let any external light interfere in the signals received and transmitted by the photodiode and photoelectric transmitter, respectively. This would also be helpful in noise reduction caused by the interfering signals and hence, would result in a better response.
The assembly of the infrared LED and the photodiode is placed in this black box and connected to a transimpedance operational amplifier of a high gain. The gain is higher as the signal obtained by the photodiode is weak in nature.
Now, as the signal is detected in the Cathode Ray tube Oscilloscope, the output of this circuit is connected to the monostable circuit. The IC 555 is operated in the monostable mode to produce square pulses. The resistor and capacitors, connected to the pins of the IC, define the width of the pulse.
These pulses are then sent to the counter circuit. These pulses are counted till the input signal exists and the result is displayed on the FNDs. It's important to understand that the counter designed in this experiment, can count till 200 and then it is reset automatically.
3.2 Design and Working
3.1.1 Working of photoelectric source or transmitter
In this experiment an infrared phototransmitter is used to obtain better response. An LED basically comprises of semiconducting material doped, with impurities to create a p-n junction. In this, the current flows easily from the p-side to the n-side and never in the opposite direction. Electrons and holes, which are the charge carries, flow into the junction from electrodes with different voltages. When an electron encounters a hole, it releases energy in the form of photons or light energy. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light. Generally, GaAs or AlGaAs are used for making of an LED.
The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. The infrared LED used here has a wavelength of above 760 nm and applied voltage is around 2V. Also a resistor of around 1 kilo ohms is connected to the negative terminal of the LED in order to protect the diode from any sort of damage.
So, according to Ohm's law.
Or, I= V/R
Therefore, the current flowing in the circuit is,
I=2V/ 1*103Ohms = 2*10-3 = 2 mA, which is the appropriate amount of current required to drive the circuit.
3.1.2 Working of Photoelectric receiver or a p-n photodiode.
When a photon is absorbed in a semiconductor, the hole electron pair is formed and swept across the junction by an electric field developed across depletion region. A photocurrent due to separated electron hole pairs result. The photocurrent flowing in the external circuit is proportional to the illumination. The photo generated current is passed through a feedback resistance. Strong reverse bias creates a strong electric field in the junction which increases the drift velocity of the carriers, thereby reducing transit time. Strong reverse bias increases width of depletion layer, thereby reducing junction capacitance and improving response time. Increased width of depletion layer leads to larger photosensitive area. This makes it easier to collect more light.
The input current for operational amplifier is very very small. Hence, the photo diode current which is in range of few microamperes is passed through a feedback resistance 100 kilo-ohms for the op amp. The gain of the transimpedance amplifier would be equivalent to the feedback resistance value. (Circuit diagram No 3.1.)
3.1.3 Monostable circuit
A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator circuit in which the duration of the pulse is determined by the R-C network, connected externally to the 555 timer. For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capacitor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accomplished by external triggering. The schematic of a 555 timer in monostable mode of operation is shown in figure. (Circuit diagram No 3.2)
The monostable multivibrator has primarily, one stable and another quasi stable or semi stable state. It can remain in the stable state indefinitely and can be triggered into the quasi stable state for a predetermined period and returns to the stable state.
To implement a monostable circuit, external R and C are connected, between the main supply and pin 6. When the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and capacitor- C is shortedto ground. The flip flop is initially reset so that the Q output is low and Q' is high, driving the discharge transistor into saturation and discharging the timing capacitor C.
At t=0, the negative trigger pulse is applied making the output C2 high , the trigger input falls below +1/3 VCC, and setting the flip flop to 1 and Q' output to zero, turning the transistor to an off state. The capacitor C starts charging towards Vcc, through R. however, as soon as the voltage across it reaches Vut=(2/3)Vcc, the output of the comparator C1 becomes high thereby resetting the flip flop. The output of the flip flop will remain zero until a new trigger pulse appears.
The width of the pulse T is the time interval during which the monostable multivibrator is in the quasi stable state. Denoting t=0 as the instant when the trigger pulse is applied, the capacitor voltage can be expressed as
Vc(t)= Vcc - Vcce-t/(RC)
Vc(t)= Vut= (2/3) Vcc
2/3 Vcc= Vcc-Vcce-T/(RC)
T= RCln3= 1.1RC
Thus the pulse width is determined by the external component values. Thus the output returns back to stable state from quasi-stable state. The output of the Monostable Multivibrator remains low until a trigger pulse is again applied.
The pulse width of the circuit varies in this design in milliseconds. Its approximately around 2 milliseconds, with the values of R= 100 ohms and that's of the C= 10 microfarads.
T= 1.1*200*10*10-6 seconds
T= 0.002 seconds = 2 milliseconds.
3.1.4 Pulse Counter and Display
In the counter circuit (Circuit diagram No 3.3), three mod 10 counters (IC 74LS90) are used for a 3 digit decimal counter. And autoreset RC network acts as a reset counter during power up via the master reset inputs. Before the count begins, the D flip flops (IC 7474) have their Q compliment output held as high, disabling the clock from reaching the first counter's clock input. When the push button switch is closed, this input goes low, enabling the first counter to count. The BCD outputs of each counter are fed through separate BCD to seven segment decoder ICs which lead to led display. The far left counter's output represents the count's LSB, while the far right counter's output represents the MSB. As shown, the last counter's output is wired so that as 200 is reached, an AND gate is enabled, causing the three output OR gate to disable the clock (stop count). To reset the counter, the manual reset switch is momentarily closed.
Initially, all counters are cleared to the 0 state. The decimal display is 000. As input pulses arrive, the BCD unit counter advances one count per pulse. After 9 pulse, the hundred and tens BCD counters are still at 0, and the units counter is at 9 (binary 1001). The decimal display reads 009. On the tenth input pulse the unit counter recycles to 0, causing its FND output to go from 1 to 0. The decimal readout is 010.
As additional pulses occur, the units counter advances one count per pulse, and each time the units counter recycles to 0, it advances the tens counter one count. After 99 input pulses have occurred, the tens counter is at 9, as the units counter. The decimal readout is 099.
On the hundredth input pulse, the units counter recycles to 0, which in turn causes the ten counter to recycle to 0. The FND output of tens counter makes a 1-to-0 transition, which acts as the clock input for the hundreds counter and causes it to advance one count. The decimal readout is 100. 
It can be concluded that the heart rate can be measured by the blood flow in the index finger. The graph (Fig 5.1 ) shows the expected output from the first circuit. It can be clearly understood that the peaks represent heart beats or heart pulses, as blood rushes in the index finger. Subsequently, this signal is fed into a digitizing circuit or a monostable circuit, which converts these random signal into square pulses (Fig 5.2). These pulses are then counted by a counter circuit. The result is displayed on the digital display FND. Furthermore, these results obtained can be verified using traditional methods like a stethoscope or a modern digital pulse rate meter.
The design of an optical heart rate meter is presented in this report. Here, we have also understood the basic working of a human heart, the different design techniques like that of an ECG was studied for their application in measurement of heart rate and also the advantages of an optical heart rate meter.
The expected results and the graphs are shown. It can also be understood that there might be some shortcomings and variations.
The sensors are noncontact, able to perform their sensing or measurement functions without the need for physical contact with specimens such as blood and can be conveniently used outside the body.
The device acts like an indicator for any heart abnormalities. For example, SIDS, Tachycardia etc, can be detected if the heart rate is aberrant.
The heart monitor would also have an impact on athletes. Once athletes start monitoring their heart rates, they'll probably discover the answers to most of their fitness questions.
With the aid of the heart monitor, athletes can learn to exercise at an ideal heart rate for their training and health maintenance. Monitoring the heart rate would help decrease over-training that can lead to injuries and months of rehabilitation.
Stress can also be monitored using the heart rate monitor. Person at work could frequently check his or her pulse rate to reduce the onset of stress. In summation, the heart monitor can help monitor anyone's health at his or her leisure.
4.2 PROBLEMS ENCOUNTERED IN MEASURING HEART RATE
4.2.1 VARIABILITY OF THE DATA
Few of the variables that can be measured in the human body are truly deterministic variable. Infact, such variable can be considered as stochastic processes, which are variable related to other variables in a non deterministic way. Physiological variables, such as HEART Rate, can never be viewed as strictly deterministic value, measurements taken under fixed set of conditions at one time, will not necessarily be same as measurements made under same conditions at another time. 
Artifacts or external noise signals refers to any component of a signal that is extraneous to the variable represented by the signal. Thus, random noise generated within the measuring instrument, optical interferences and all unwanted variations in a signal are considered artifacts. Due to the movement of a person, there can be changes in the result obtained at the end of heart rate measurement. 
4.2.3 Technical Precautions
Care should be taken so that the resistors don't get overheated. Proper voltage should be applied so that the diode is not damaged. Dark chamber should be maintained, to avoid any interference signal or extraneous light signals. Any sort of movement should be avoided to obtain perfect signals.
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