Pulse measurement sensor. How does a heart rate monitor work in a sports watch? What running indicators can you get from a heart rate monitor?

In this article, you will learn about several details that you need to pay attention to when designing photoplethysmograph sensors.

Introduction

In the previous article, you got acquainted with the design of a sensor that measures a heart rate monitor. Today I will share some developments that may be useful when choosing the element base of a plethysmograph and developing its electrical circuit. They will help improve the quality of the useful signal, which is primarily influenced by the following factors:

lack of artifacts;
the presence of a pronounced pulse wave at the registration point;
design of the sensing element.

An artifact is a change in the signal shape that is not related to the useful component, but is spectral and amplitude similar to it.

There are several sources of artifacts:

the movements of a person using a photoplethysmograph relative to a light source, natural or artificial, for example, the movement of the shadow of the sun during sports;
movement of a light source relative to a person or a change in the brightness of this source. For example, flickering fluorescent lamps;
movements of parts of the body not related to the pulse causing movements of the photoplethysmograph or points of the body in the place where the sensitive element is installed. For example, movements of the forearm bones that occur when moving fingers, movements of the head bones associated with speech and facial expressions.

In addition to artifacts, the quality of pulse measurement depends on the severity of the pulse wave. In the same person, the pulse can be very good and very bad. For example, I observed changes in heart rate many times during a three-hour computer psycho-physiological test. The pulsogram was measured from the earlobe. The signal deteriorated over time. This could happen quite quickly - in half an hour, and is presumably due to the fact that the ear clip impairs blood flow, as well as the forced immobility of the subject.

A similar situation is observed when measuring the pulse from the phalanx of the finger. A change in room temperature or a slight change in a person’s posture and the resulting shift of the recording point by a small distance can lead to a decrease in the signal level or even its disappearance.

When measuring pulse from the temple, the problem of lack of signals becomes more acute. The area of the temple is larger than the area of the finger, it is more difficult to find the point where the pulse is better manifested, and the user is more likely to put the sensor on incorrectly.

Multichannel sensing elements

To solve the described problem, a principle common in technology can be applied - duplication, which in this case implies the use of a sensor with several sensitive elements. A schematic diagram that implements this idea is shown in the following figure.

I foresee skeptical thoughts from readers about parallel-connected LEDs. Please do not judge strictly, since this is a prototype that should not have been used for a long time.

LEDs and phototransistors on printed circuit board arranged in pairs. The size of the board is chosen to cover the entire temple area, this allows the signal amplification and filtering circuit to be located there. The board may contain holes for attaching to a tape. Appearance A sensor with nine sensitive elements is shown in the following figure.

A similar solution can be applied to measure heart rate from a finger or wrist. Below is a diagram of a sensor consisting of four phototransistors and one LED.

The emitters of phototransistors may not be connected, and then the signals from each of them are measured independently; in this case, a special multichannel measuring device. Multi-channel execution can also be useful for eliminating artifacts. If an artifact occurs only in the area of one photocell, it is recorded and is not taken into account in the overall measurement picture. However, using such a scheme is not always convenient, as it leads to an increase in size. It's a completely different matter if you connect photosensitive elements in parallel. In this case, only one measuring channel is required. The following figure shows a prototype of such a sensor. It works according to the “reflection” scheme. The LED is located in the center, and phototransistors are located at the edges. The sensor can be used to record a pulsogram from the phalanx of a finger or wrist. The printed circuit board is laid out so as to be able to connect phototransistors in multi-channel or single-channel options.

Compounding

For better fixation of photocells, the surface of the printed circuit board can be filled with a compound. For filling, a special mold is made, which you also see in the picture. To prevent the compound from sticking to the mold, it is better to make it from fluoroplastic. If the mold is made of another material, for example metal, then before pouring the compound it should be lubricated with a special compound. If this composition is not available, regular Vaseline will do. You should also be careful when choosing a compound, since an incorrectly selected composition can deform the elements during curing.

In addition to fixation, the compound acts as a light filter. Epoxy compounds with dyes are suitable for this purpose. For example, the Epoxycon compound produced by St. Petersburg State Technical Institute can be used.

An alternative to compounds can be solid filters. They fit closely to the printed circuit board, and for LEDs and phototransistors, grooves are made using a milling cutter or laser. The following illustration shows a sensor with elements covered by a milled plate.

The presence of a light filter allows you to minimize artifacts created by external light sources. The following image shows the optical compounds before and after curing.

Features of choosing phototransistors and LEDs

To record the pulse wave, photosensitive elements are used - photodiodes or phototransistors. This article deals only with phototransistors. Because at the time I started working in this direction, I already had on hand several dozen different transistor sensors (clips, clothespins and fingertips), and also had developed circuits technical solutions. The use of diodes is no worse and is widely used in various applications, for example in the common medical sensors of the Nellcor standard.

When choosing phototransistors and LEDs, you should first pay attention to the following characteristics:

wavelength (maximum spectral characteristic) [nm];
half-brightness angle for LEDs and coverage angle for phototransistors [deg];
radiation intensity [mW/sr] for LEDs and sensitivity for phototransistors [mA/(mW/cm2)];
rated current of phototransistor and LED [mA];
phototransistor dark current [mA];
the presence of lenses and filters built into the body.

The wavelengths that are most strongly absorbed by the blood are best for measuring pulse. These are waves corresponding to the green color 530 nm. Red and infrared bands are also used. I highly recommend it with a classification of methods for measuring pulse, where you will also learn about the absorption spectrum of hemoglobin.

When choosing photocells, you should pay attention to the presence of lenses and filters that allow you to achieve the desired half-brightness angle and coverage, and, therefore, be less sensitive to radiation from other sources. Built-in filters allow you to work only in the selected spectral range. If you choose a high-angle half-brightness LED and a high-angle phototransistor, the light will pass past the surface of the skin. This will lead to a deterioration in the measuring range and the light flux modulated by the pulse wave will have virtually no effect on the output signal measuring circuit. This situation is illustrated in the following figure

Angle a2 is acceptable, but angle a1 is too large to use an LED with such an angle in a pulse measurement device. This example applies to the case of measuring pulse “by reflection”. Choosing an LED with a large half-brightness angle in devices operating “through transmission” will lead to the fact that a large radiation power will pass past the photodetector. This is undesirable, especially on mobile devices.

You should also pay attention to the LED's emission intensity, measured in milliwatts per steradian [mW/sr]. In documents for LEDs, it is usually indicated at currents of 20, 100 and 1000 mA. To save energy, it is better to choose LEDs that have a higher characteristic for the same current consumption. You should pay attention to the value of the photoelectric current of the phototransistor; the higher its value, the better. The last two characteristics are related. As a result, the level of the minimum expected signal must be at least several times higher than the expected noise level in the measuring device.

LEDs and phototransistors are often sold in pairs, suitable friend to each other constructively and according to spectral characteristics. The table shows the characteristics of several pairs of LEDs and phototransistors. The pairs in lines 2 and 3 are not suitable for use in heart rate monitors due to the large angle and low radiation power. Pairs 1, 4 and 5 are suitable, with the first pair being the best fit. This has been confirmed by tests. All other things being equal, the best pulsogram signal was taken when using the first pair. It should be noted that if an opaque barrier is placed between the LED and the phototransistor, then the angle of radiation and sensitivity will not have such a strong influence on the quality of pulse measurement.

Conclusion. Three in one

Instead of a conclusion, I will mention a wonderful integral solution, which was given by habra user valexey in the comments to the previous article. We are talking about the Si1143 device manufactured by Silicon Labs. Inside it has two photodiodes - red and IR, a control unit for three LEDs, a built-in amplification and filtering circuit, an ADC and an I2C serial interface module. I won’t describe other details, since I haven’t had time to try it yet. Judging by the description, this device is well suited for various measurements related to heart rate monitoring.

P.S.

The repository has been replenished with drawings of sensitive elements and intermediate amplifiers.

Reading time: 21 minutes

A heart rate monitor is a measuring device that measures your heart rate. It is also called a heart rate monitor.

Heart rate monitor is used to monitor heart function, analyze loads, determine heart rate zones and go beyond these zones. Sold on the sports paraphernalia market large number various models for heart rate monitoring. Let's figure out what a heart rate monitor is for, what its advantages and benefits are, how to choose one, and also consider the most popular models of heart rate monitors on the market.

Heart rate monitor: what is it for and what are the advantages

If you need information about how your heart works during exercise, then a device like a heart rate monitor is a must-have. During training, the heart rate monitor helps maintain the desired heart rate, measures the number of calories burned and monitors heart function and workload. Most often, a heart rate monitor is used during interval and cardio training, but it will also come in handy during strength training. In addition, the heart rate monitor can be used during daytime activities to monitor heart function.

Who might need a heart rate monitor?

For those who do cardio training to lose weight or develop endurance.
For those who do high-intensity interval training (HIIT).
For those who have heart problems and need to control their heart rate.
For those who want to control the number of calories burned during exercise.
And also for those who want to regularly improve their results without harm to their health.

Why is it even necessary to measure your heart rate during exercise? Depending on your pulse or heart rate (abbreviated HR), your body will use different sources of energy. Based on this, there are several load zones that determine the effectiveness of your workout:

The indicated percentage is taken from the maximum heart rate value. To calculate it, we use the formula: Maximum heart rate = 220 – age.

Accordingly, in order for the body to use fatty acids as a source, it is enough to keep the pulse in the zone of 60-70% of the maximum heart rate. For example, if your age is 30 years old, then the following calculations will be used to calculate the possible range of your heart rate:

Lower threshold = (220-30)*0.6=114
Upper threshold = (220-30)*0.7=133

With such a pulse (114-133 beats per minute) you can practice for a long time, maintaining a continuous pace. In this case, the exercise will be aerobic, that is, using oxygen. Such cardio workouts help burn fat and train the heart.

If you are doing high-intensity interval training (for example, training according to the Tabata protocol), then at the peak moments your heart rate should be in the anaerobic zone, i.e. 80-90% of maximum heart rate:

Lower threshold = (220-30)*0.8=152
Upper threshold = (220-30)*0.9=171

The heart rate monitor helps you monitor your heart rate and keep it in the zone that meets your requirements. If your heart rate monitor model allows it, you can set the heart rate zones you are interested in, and you will be notified when your heart rate leaves the specified zone.

Benefits of a heart rate monitor:

A heart rate monitor protects your heart from overload during exercise because you monitor your heart rate.
You will exercise in the heart rate zone you need - for fat burning or endurance, depending on your goals, and therefore train more effectively.
With a heart rate monitor it is easy to track your progress, analyze the level of load and its perception by the body.
You will know exactly how many calories you burned during your workout.
You can use the heart rate monitor during your normal daily activities to assess your body's performance or monitor your stress levels.
The heart rate monitor is indispensable when running or walking fast on the street, when there are no other sources for determining the level of exercise.

Many cardio machines already have a built-in heart rate monitor. But firstly, such heart rate monitors show inaccurate data, which is better not to rely on. Secondly, to record data you need to hold the handles while running or walking, which is not always convenient. Therefore, if you want to receive the most accurate data on heart rate and calories, it is better to purchase a heart rate monitor.

You can also use manual heart rate monitoring. To do this, you need to stop and count the beats, recording the resulting values. However, additional manipulations during training are not always convenient, and the obtained values will have a strong error. In addition, constant stopping lowers your heart rate, which disrupts the rhythm of the activity. This is why a heart rate monitor is indispensable: it will record data instantly throughout the entire workout.

Main functions of the heart rate monitor:

Heart rate (HR) monitoring
Setting your heart rate zone
Notification of heart rate zone changes by sound or vibration
Calculation of average and maximum heart rate
Calorie counter
Time and date display
Stopwatch, timer

Some heart rate monitors have additional functions: GPS navigation, alarm clock, pedometer, training history, automatic calculation of training zones, fitness test, heart rate calculation for a single lap (useful for runners), synchronization with applications and computer. The more functions a device is equipped with, the more expensive it is.

Types of heart rate monitors

Heart rate monitors can be divided into 2 large groups: breastplates(using a chest strap) and carpals. Heart rate monitor with chest strap used O more popular among practitioners, but thanks to new technologies, models have appeared that allow you to accurately measure your pulse without a chest sensor.

A chest heart rate monitor is a sensor with electrodes that is worn under the chest and transmits data to a receiver watch or mobile application. There are two types of chest heart rate monitor models, which differ in configuration:

Heart rate monitor without watch receiver. In this case, the data is transferred to the smartphone via Bluetooth Smart technology. The sensor is synchronized with special applications on the smartphone, where all the necessary information about heart rate and calories burned is automatically stored. This is convenient for training analysis, since the application stores the entire data history. Most often, heart rate monitors are synchronized with applications on operating systems Android and iOS.
Heart rate monitor with watch receiver. In this case, the sensor sends data to the receiver watch, where it is processed and you can see it on the screen. Such models are more expensive, but also more convenient. You do not need to additionally use a smartphone; all information will be displayed on the watch. For example, it is more convenient to use such heart rate monitors outdoors.

If you purchase a heart rate monitor with a watch, then also pay attention to the type of data transmission. There are two types of data transfer from the chest strap to the watch:

Analog (uncoded) type of data transmission. May be subject to radio interference. It is considered less accurate, but if there is an error, it is very small. An analog heart rate monitor can synchronize with cardio equipment, picking up data about heart rate from your belt. But if in close proximity If someone is using a heart rate monitor with the same type of data transmission from you (within a meter), for example, in a group training, then interference may occur.
Digital (encoded) type of data transmission. A more expensive and accurate type of data transmission, not subject to interference. However, a digital heart rate monitor cannot be synchronized with exercise equipment.

Both analog and digital heart rate monitors are quite accurate, so The type of data transfer does not play a key role when choosing a heart rate monitor. There is no point in overpaying additionally for digital data transmission.

Wrist heart rate monitors

The convenience of wrist heart rate monitors is that you don't have to wear a chest strap with the sensor. To measure the data, you only need a watch that is worn on your wrist. However, this version of heart rate monitors also has a number of features and disadvantages, so despite the apparent convenience, wrist heart rate monitors are still less popular.

There are two types of wrist heart rate monitors, which differ in the principle of heart rate monitoring:

Pulse is measured upon contact of fingers and sensor on the front side of the device. You simply place the heart rate monitor on your wrist, touch it, and the device gives you your heart rate readings. The disadvantage of such monitoring is that you will measure your pulse not for a certain period of time, but on demand, only after contact of your fingers and electrodes on the body. This heart rate monitor is more suitable for tourism, mountaineering, or for those who, due to health restrictions, are forced to periodically monitor their heart rate zone.
Pulse is measured via tracking behind blood vessels. The principle of operation of such heart rate monitors is as follows: you put the bracelet on your hand, the LEDs illuminate the skin, optical sensor measures the narrowing of blood vessels and the sensor displays the obtained values on the watch screen. But the disadvantages of such devices are also obvious. For data accuracy, the belt must be tightly tightened on the wrist, which is not always convenient during training. Additionally, heavy sweating or rainy weather may interfere with the sensor's performance.

Of course, a watch is a more common piece of equipment than a chest strap. Therefore, if you feel uncomfortable wearing a belt under your chest, we recommend purchasing a second version of a wrist heart rate monitor. But discomfort and inconvenience are perhaps the only argument in favor of a wrist heart rate monitor. Most trainees still opt for a heart rate monitor with a chest strap because of the convenience and accuracy of the data.

Prices for a heart rate monitor are determined by the following parameters:

Manufacturing company
Heart rate monitor type: chest or wrist
Contents: is there a watch receiver, replaceable straps, cases, etc.
Data transmission type: analog or digital
Moisture protection
Belt, its width, quality, ease of fastening
Quality of the watch receiver case
Availability of additional functions

Heart rate monitors: a selection of the best models

We offer you a selection of heart rate monitor models with brief description, prices and pictures. Based on this review, you can choose the right heart rate monitor for yourself. Prices are indicated according to Yandex Market data as of September 2017 and may differ from the cost of the heart rate monitor in your store.

Sigma heart rate monitors

Popular models of Sigma heart rate monitors are developed by a Taiwanese manufacturer. Among heart rate monitors, Sigma is considered one of the market leaders; their models are almost ideal in terms of price and quality ratio. They mainly offer heart rate monitor models with a chest strap and a watch:

Sigma PC 3.11: the most primitive model with a basic heart rate counting function. There is no calorie counting.
Sigma PC 10.11: optimal model with all the necessary basic functions, including calculation of average and maximum heart rate, calorie counter, beep when the target heart rate zone is violated.
Sigma PC 15.11: This model is suitable for running enthusiasts, as it adds functions such as lap counter, average and maximum heart rate per lap, number of calories burned per lap, lap time.
Sigma PC 22.13: This heart rate monitor uses digital data transmission, so the price is a little more expensive. The model is offered in several body colors. Standard functions: calculation of average and maximum heart rate, calorie counter, zone indicator, sound signal when the target heart rate zone is violated.
Sigma PC 26.14: model similar to the previous one, but with the addition of new functions. For example, this device has a lap counter, an automated function for calculating the target zone, memory for 7 training sessions, totals per week.

Polar heart rate monitors

Polar is one of the most famous brands in the heart rate monitor market. Polar produces high-quality devices, but their prices are much higher. You can purchase a chest strap with a sensor that will transmit data to your smartphone, or a set of strap and watch receiver for easier data tracking.

Chest straps with sensor:

Polar H1: GymLink communication interface, Android and iOS support, moisture protection.
Polar H7: GymLink and Blutooth Smart communication interfaces, Android and iOS support, moisture protection.
Polar H10: a new generation of heart rate sensors, replacing the H7, one of the popular heart rate monitor models.

Chest heart rate monitor with watch included:

Polar A300: in addition to the standard functions, this device also has many additional features: pedometer, sleep monitoring, reminder function, goal setting, accelerometer. It is also possible to connect to a smartphone via Bluetooth.
Polar FT60: this model includes a calorie counter function, as well as a number of auxiliary, but very convenient and useful functions, such as: alarm clock, second time zone, indicator low level battery charge, locking buttons from accidental pressing.
Polar M430: Another very multifunctional gadget, waterproof, with GPS navigation and backlight. Added notification function about incoming calls, received messages and notifications from applications for social networks GPS.

Beurer heart rate monitors

This brand offers models of heart rate monitors with a chest strap and models in which you need to touch the sensor of the device to measure data. For training, we recommend choosing heart rate monitors with a chest strap; it is more convenient and practical.

Beurer PM25: a simple and convenient model, there are all the important functions, for example, a built-in calendar, clock, alarm clock, stopwatch, calorie counter, alert when leaving the training zone.
Beurer PM45: The set of functions is similar to the PM25 models, but adds interchangeable straps, a bike mount, and a storage case.
Beurer PM15: This is a wrist-based heart rate monitor with a touch sensor, the device monitors heart rates, alerts you when you go beyond the training zone, but does not count calories. Price: 3200 rubles.

Suunto heart rate monitors

Another well-known company in the sports equipment market, which produces a series of sports watches with the ability to measure heart rate. Suunto offers chest straps and chest straps included with your watch:

Suunto Comfort Belt: Chest strap suitable for all T-series sports watches and computers that can be used as a heart rate monitor.
Suunto Smart Belt: Chest strap with Bluetooth Smart technology. Compatible with Suunto's Movescount app.
Suunto M2: a chest strap with a watch that has all the basic functions, including heart rate control, calorie counting, automatic selection of the desired heart rate zone.
Suunto M5: This heart rate monitor comes with additional features to help you determine the optimal training regimen based on your individual performance, as well as get reliable information about your speed and distance during your running workouts.

Sanitas heart rate monitors

Sanitas does not have many models, but they are notable for their low prices, so we also mention them.

Sanitas SPM22 and SPM25: Heart rate monitor with chest strap that includes all the basic functions and is perfect for regular use.
Sanitas SPM10: You don't need a chest strap to measure your heart rate with this model. You simply place the device on your wrist and touch the sensor on the front of the device with your finger. This device is suitable for people who do not want to wear a chest belt or, for example, for tourism.

Other models

Nexx HRM-02. Budget option a chest strap with a sensor, which is suitable for those who are not ready to seriously spend money on fitness gadgets. The device has built-in Bluetooth Smart and is compatible with almost all mobile applications, supporting the function of transmitting data from wireless heart rate monitor. Counts heart rate and calories burned.
Torneo H103. Chest strap with watch receiver. Equipped with all the basic functions: heart rate calculation, calorie counter, setting heart rate zones, measuring time in the target zone, stopwatch, calendar and alarm clock, water resistance.
Wahoo TICKR. Another option for a chest heart rate monitor that transmits information via Bluetooth to a smartphone. In addition to heart rate, such characteristics as steps taken and calories burned are recorded.

Which heart rate monitor to choose:

If you want to buy a heart rate monitor with an optimal price-quality ratio, then buy the Sigma or Beurer models.
If you want to purchase the most reliable and accurate device, then buy Polar or Suunto models.
If you want to purchase the simplest and most inexpensive option for a heart rate monitor, you should pay attention to the models offered on the Aliexpress website (review below).

Heart rate monitors: a selection of the best models on Aliexpress

We offer you a selection of heart rate monitors that can be purchased on Aliexpress at an affordable price. All heart rate monitors have similar functions and are in approximately the same price range, so we suggest you focus on customer reviews, the average product rating and the total number of orders for this product.

Chest strap without watch

If you purchase a chest strap without a watch, your heart rate data will be sent to an app on your smartphone. Chest sensors Compatible with all devices with Bluetooth Smart (4.0) and ANT. The presented sensors are quite accurate in measuring heart rate.

We suggest you pay attention to the following chest sensors:

To monitor your heart rate, all cardio equipment is equipped with pulse sensors. Everything is standard treadmills are equipped with wired sensors that have a simple design but a high measurement error.

Wireless sensors are the most accurate pulse measurement devices, the error of which does not exceed +/- 1 beat.

Pulse is the number of dilations of the artery at the moment of blood ejection by the heart per unit of time. It should be noted that pulse and heart rate (HR) are not the same thing, although for physical healthy person their values will indeed be the same. Heart rate characterizes the work of the lower parts of the heart (ventricles) per unit of time (minute) and can differ significantly from the pulse rate. This phenomenon can be observed when the heart rhythm is disturbed (arrhythmia).

Norms of pulse values

Each person is individual and heart rate values can vary significantly for different people. The factor influencing heart rate is physical fitness, the degree of fitness of the heart and the body as a whole. The body is a complex system in which the heart solves the problem of transporting oxygen to all tissues and organs.

As a rule, the heart of trained athletes at rest contracts much less frequently than the heart of the average person.

The range of 60-90 beats per minute is considered the norm for a healthy person. At pulse values below 60 beats per minute, bradycardia occurs, at rapid values above 90 beats, tachycardia occurs. You need to know that in a newborn baby, an increased heart rate of up to 140 beats per minute is considered normal, and a woman’s pulse is 5-10 beats higher than a man’s.

Heart rate values increase rapidly when physical activity, during emotional outbursts (anger, fear, excitement), depends on the statistical position of the body (standing, sitting), increases after eating or after using certain medications.

Table 1 - Average heart rate values for a healthy person.

Age	Heart rate per minute
Newborn	135-140
6 months	130-135
1 year	120-125
2 years	110-115
3 years	105-110
4 years	100-105
5 years	93-100
7 years	90-95
8 years	80-85
9 years old	80-85
10 years	78-85
11 years old	78-84
12 years old	75-82
13 years old	72-80
14 years old	72-78
15 years	70-76
16 years old	68-72

Why is it necessary to monitor your heart rate on a treadmill?

To make your workouts as effective as possible, you need to monitor your heart rate. The efficiency zone is calculated based on the values of the maximum heart rate (MHR). For men, MHR = 220 – age, for women this value is MHR = 226 – age.

Conventionally, target zones can be divided into four ranges:

Zone of general health load (gentle mode): 50-60% of MHR. This zone is recommended for novice users and people leading a sedentary lifestyle.
Moderate load zone (general mode): 60-70% of MHR. Suitable for most workouts that are aimed at effective fat burning.
High load zone (advanced mode): 70-80% of MHR. Recommended for experienced people with a trained heart, the target zone is designed to strengthen the cardiovascular system.
Anaerobic load zone (short-term extreme mode): 80-90% of MHR. Recommended for athletes working on individual programs in the presence of a coach.

Types of heart rate sensors for treadmills

Wired heart rate sensors

The first attempts to measure pulse electrically appeared at the beginning of the 20th century. In 1902, Willem Einthoven obtained the first electrical cardiac signal using a string galvanometer. Its weight measuring instrument was 270 kg, but the principle of measurement has reached our times. Heart rate measurements are based on a lead system (Einthoven triangle), which records the moment of electrical excitation of the ventricles.

Galvanometer used to measure heart rate in 1902

Modern treadmills are equipped with wired heart rate sensors. The operating principle of such sensors is simple: two electrodes located on the handrails measure the potential difference, and the information is transmitted via wires to the analog-to-digital converter (ADC) of the console. There the information is processed and displayed on the screen.

The disadvantage of such a system is the high measurement error (20-30%), inconvenience of use and speed of display of real values.

It often turns out that only after 30-40 seconds of holding the sensors can one judge the true heart rate values.

There are wired heart rate sensors on the console handrails

Wireless heart rate sensors

Wireless heart rate sensors have a simple design and a number of advantages compared to wired devices:

The most accurate heart rate measurements. Wireless sensors error +/- 1 beat per minute
Ease of use. The heart rate sensor is attached to the heart area using a special belt. Using two electrodes, the potential difference is recorded. It is recommended to moisten the electrodes with water for good contact. Next, an analog or digital signal is transmitted over the radio channel, which arrives at the console receiver and is displayed on the screen.
Possibility of using cardio-dependent programs.

Wireless heart rate sensors have more accurate heart rate measurements. Wireless sensors error +/- 1 beat per minute

The disadvantages of this method are insignificant:

The need to use a battery in the sensor. With daily training, the charge will last for 1 year.
Inconvenience of using a cardio belt during long-term training.

The most popular wireless heart rate sensors

Used to measure pulse wireless sensors, operating in the frequency range 5 kHz. Sensors can be coded (used in fitness rooms) or uncoded (intended for home use).

The leading leader in the heart rate monitor market is the company Polar. Along with it, you can find heart rate monitors on sale brands Sigma, Beurer, Oregon, Garmin, Suunto. The most budget models have a small set of functions and start at 500 rubles. In the average price range of 3,000 rubles you can find high-quality and comfortable heart rate monitors. Expensive models intended for intensive and professional use often have a coded signal and sell for around 20,000 rubles.

Many treadmill models come with a wireless cardio belt, mainly from Polar, operating at a frequency of 5.4 kHz.

How can I find out if I can use a wireless heart rate sensor on my exercise machine?

Before purchasing a treadmill, you should check whether this model has a telemetric heart rate receiver. Such technical information can be obtained on the official website of the seller, or in the operating instructions for the simulator.

Connecting a heart rate sensor

If the heart rate sensor is turned on for the first time, you need to install a battery, which is also included in the kit. Next, the surface of the cardiac sensor in contact with the body is wetted with water and the cardiac belt is fixed on the chest. After turning on the simulator, automatic matching of devices occurs.

Due to numerous requests from readers of our blog, in addition to materials on self-assembly of an electrocardiograph, we publish everything you need to assemble a heart rate monitor. We will measure heart rate using the optical “reflection” method. An LED and a photodetector mounted in the device body are used as a sensor. You can make your own sensor of any other design (for example, a “transmission” sensor from a clothespin). We present to your attention the first public (in fact, the eighth experimental) version of the "Pulse Lite" device.

Dear radio amateurs, please note that photoplethysmograph - a complex device, in which you can make a lot of mistakes during assembly, and it won’t start with “two kicks.” If you are going to assemble a device from what you have on hand, replacing the parts and ratings shown on the circuit diagram, keep in mind that most likely the device will not work. Even the home cardiograph "ECG Lite" is much less picky in this regard. You shouldn’t then blame the developers for wasted time, textolite and radio components. If you need a heart rate monitor consisting of a couple of amplifiers, an LED and a photodetector, use other circuits.

First difficulties

A few words about why a photoplethysmograph is much more complex than a cardiograph from a circuit design point of view.

Recall that an electrocardiograph records electrical potentials induced by the electrical activity of the heart muscle on the body. These same bipotentials do not differ greatly from person to person, and normally the signal amplitude (from the limbs) is 1 ± 0.2 mV.

The pulsograph records signals using the optical method - a photodetector records changes in the intensity of light (the source is an LED) passed through the finger (or scattered by it - for a “reflection” sensor) caused by the pumping work of our heart - a periodic increase in blood supply to the tissues.

It would seem nothing complicated, if not for two main "BUT". Blood supply, elasticity of blood vessels, pressure and, most importantly, the thickness of the skin in humans differ extremely. This leads to the fact that the level of constant illumination of the photodetector (which is affected by our skin and the size of our fingers) and the level of the variable component (pressure, blood vessels, state of blood supply in the extremities, etc.) differ hundreds of times between different people.

To create a pulsograph, you need signal-forming circuits (driver) of the light source, complex infra-low-frequency amplifiers (ECG - a higher-frequency signal), circuits that suppress interference from constant illumination of third-party sources; as well as clever automatic gain control circuits.
For fun, you can compare the prices of professional cardiographs and pulse oximeters (the latter are much more expensive).
I hope we scared you enough 🙂 so that the desire to assemble a photoplethysmograph yourself disappears. Isn't it missing? Then read on.

Device characteristics

If you did everything correctly - without errors in the board and circuit changes and without defective parts, then at the end you will receive a device that will delight you with the following features:

registers a pulse wave with a sensor consisting of an LED and a photodetector (the sensor can be made for transmission or reflection);
transmits the signal to the PC via USB, and PC software can do a lot:
calculates instantaneous heart rate;
Performs pulse wave contour analysis and heart rate variability analysis;
records a photoplethysmogram of any duration to a file;
performs automated diagnostics (the diagnosis database is customizable);
prints research results.

Limitations of this computerized pulsograph:

does not work with Nellcor clothespins or ear clips from Aliexpress!
does not work with the latest version of Pulse Lite Control!
does not measure oxygenation!

I repeat once again: the circuit, board and firmware of the heart rate monitor is the first well-debugged version of the “Pulse Lite” photoplethysmograph, so it doesn’t work with the Nellcor clothespin, and it doesn’t work with the latest version of the software either. We do not plan to “open” the latest version of the Pulse Lite pulsograph.

Everything for self-production

Download the circuit diagram and everything you need to make a board at home using LUT (in pdf format) from this link. The archive contains, in addition to the circuit diagrams, ready for printing (note that you don’t need to mirror anything, print without scaling, i.e. 1:1!) the top and bottom sides of the board, a map of vias (top and bottom views), a location map elements.

Tricks when constructing circuit solutions

The author of these lines assumes that you have already downloaded and seen electrical diagram photoplethysmograph. If you read further, it means that the desire to make a device has not yet disappeared, and this cannot but rejoice :) Only to such persistent readers will we reveal the main secrets of the creation of our device. So to circuit diagram photoplethysmograph has become more understandable, we will clarify the most important technical solutions and the reasons that prompted them to be introduced into our device.

One of the problems of photoplethysmography has already been voiced by us - the sensitivity of the device to illumination from third-party sources, the influence of which is very difficult to exclude with such an obvious use of filter circuits, because the useful signal lies in the same frequency range as low-frequency interference (from fractions to tens of Hertz) . To amplify the useful signal (photoplethysmogram), it was decided to use the principle of modulation - demodulation, which is as follows:

We transfer the useful signal to the high frequency region. To do this, the LED is powered not by direct current, but by alternating current, with a frequency of 5 kHz. In this way, a high frequency carrier signal is formed. When passing through the finger, the intensity of the light (pulsating at a frequency of 5 kHz) changes due to periodic fluctuations in blood supply. Consequently, the photodetector receives an RF signal modulated in amplitude by the useful photoplethysmogram signal.
Next, it is quite safe and relatively simple to filter low-frequency interference caused by external illumination, since the spectrum of the useful signal lies in the HF range (5 kHz).
We amplify the RF signal with classic amplifiers using cheap op-amps.
We perform amplitude detection to extract the useful low-frequency signal (envelope).
We filter and enhance the low frequency signal.

Problem No. 2 (different blood supply, skin thickness, etc.) was solved by implementing automatic adjustment of the gain of the high-frequency and low-frequency amplification stages.

As a matter of fact, these are all the tricks that, on the one hand, complicated the scheme to the point of disgrace, on the other hand, made it possible to create a photoplethysmograph that stably records the pulse wave not only from the patient who developed it, but from everyone who wants it, and which is built on the basis inexpensive electronic components available in every self-respecting radio parts store.

We explain the circuit design

Now let's move on to the details. The photoplethysmograph receives power from the PC via USB cable. Galvanic isolation The device is not implemented with a PC, since there is no electrical contact with the patient when recording the pulse. Boost pulse power converter based on the NCP1406 boost controller, the output of which is connected to a voltage doubler with midpoint, connected to the common GND wire, provides bipolar power supply ± 4V for the amplification path, oscillator and LED driver. The controller is powered separately from the entire analog part by a 3.3V linear stabilizer NCP1117ST33T3G, since for the device to operate from a PC via USB (the device operates as an HID-compatible device), the D+ and D- levels on the controller lines should not exceed 3.3V. You can, of course, install 3.3V zener diodes on the D+ and D- lines, relieving excess voltage, but this leads to unnecessary consumption, and in itself, decoupling the power supply circuits of the analog and digital parts is always a plus.

A generator based on the TL072 op-amp chip (stage DA1:A) generates a sinusoidal signal, the LED power driver (DA1:B) provides electric current through an LED whose strength is proportional to the output voltage of the generator. Together, the oscillator and driver provide 5 kHz pulsating output from the X1 LED with minimal high harmonics. LED power supply rectangular pulses leads to significant distortion of the useful signal by higher harmonics after detection, which is why we power the LED with a sine wave.

The photodiode is turned on in photovoltaic cell mode (without external reverse voltage), R29 is a load resistor that allows you to increase the speed of the sensor when turned on in this way. Capacitors C29 and C36 allow you to remove the DC component of the signal, which is caused by extraneous light. After the first RF amplification stage, a resistive divider controlled by a microcontroller is installed (on a digital potentiometer MCP41010 controlled via the SPI interface).
Since the MCP41010 power supply is unipolar (+4V), we shift the RF signal to half the power supply (R35-R37). After attenuating the signal by a divider (with the attenuation level set by the ATMega controller), the constant bias is removed by capacitor C31, and the RF signal is fed to the input of an RF amplifier with frequency-selective circuits in feedback(with maximum gain at 5 kHz) and then to the amplitude detector VD7-R28-C28 to extract the useful PPG (demodulation) signal.

The level of signal attenuation by a resistive divider in the RF path is selected based on the value of the constant component measured by the controller’s ADC at the output of the ADC_AMP detector.

After amplitude detection, the useful signal is sent to an op-amp repeater, which serves to match resistances, and a low-frequency amplifier using a composite transistor VT1-VT2. The Darlington circuit allows you to obtain a minimum level of infra-low-frequency noise with high amplification of the low-frequency signal. After the low-frequency amplification stage, the signal is fed to the MCP41010 digital potentiometer and the last amplification stage DA2:A. The level of signal attenuation with a potentiometer is selected based on the signal swing measured at the ADC_IN controller ADC input.

The digital part of the photoplethysmograph is built on the basis of a microcontroller of the AVR ATMega48 family. The controller automatically adjusts the gain of high-frequency and low-frequency stages, measures signals on ADC channels (the constant component of the PPG after demodulation ADC_AMP and amplified signal pulsogram ADC_IN).

The result is that the photoplethysmograph circuit is far from trivial. There are no unnecessary parts or electrical connections. If you are going to use our heart rate monitor firmware and our PC program, do not change anything in the circuit. If you only need ideas, but are planning to implement your own device with your own software, go ahead and experiment for your health!

Microcontroller Programming

The controller is programmed via the X3 in-circuit programming connector via the SPI interface using a programmer STK-500, ucGoZillla, USBtiny or others. To flash the controller, you will also need the Atmel AVR Studio environment, which can be downloaded from the official Microchip website.

When programming the microcontroller, set the settings according to the screenshots below (pay attention to this point so as not to turn the controller into a “brick”).

What is possible

Use the diagram (or its parts) in any of your projects (including commercial ones).
Build a computer photoplethysmograph for yourself and your loved ones, for scientific experiments and other good purposes.
Write in the comments on the website about problems or successes in assembling the device.
Report in the comments any ambiguities, inaccuracies, or incompleteness of materials for assembling the photoplethysmograph.
Report possible errors in the materials for assembling the pulsograph in the comments on the website.
Suggest in the comments more reasonable technical solutions for the problems of pulse wave registration.
Share information about the assembly of the device on thematic blogs and forums with a link to the original source.
Leave a link to our website as a thank you to the authors of the project.

What not to do

Ask for source codes for firmware and PC programs :)
Require us to write additional materials of any content on the topic of a computer photoplethysmograph (technical specifications, business plan, diploma, product passport, etc.).
Ask to post open materials on assembly latest version computer photoplethysmograph "Pulse Lite".
Change the pulsograph circuit at your own discretion, and then scold the developers for a non-working result.
Criticize circuit solutions without weighty arguments and reasonable proposals.

On the Internet you can easily find simpler and cheaper heart rate sensor circuits. Our device is not for those who just want to “pass the evening with a soldering iron and play with heart rate.” Here we have published a diagram of our eighth prototype of a photoplethysmograph, so we can say with confidence that this device will allow you to register a pulse wave with a minimum noise level in the vast majority of people. You don't have to turn the trimmer knobs to see the pulse on the screen. Based on the shape of the pulse wave, you can calculate the stiffness and reflection indices, and not just the instantaneous heart rate (especially since the program will do everything for you). This device is not a Chinese toy, with unfinished software and buggy firmware, and not a craft made wall-mounted from the "Old Paradise". This is a full-fledged computer photoplethysmograph, which can become a reliable assistant in matters of objective monitoring of your health.

Thank you for your attention to our developments and good luck in assembling your home pulsograph!

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