Heart Rate Monitor Design Solutions

The measurement of heart rate plays a crucial role in assessing an individual’s health status. However, traditional medical practices rely on pulse signals to gauge a patient’s condition, which may be subject to limitations due to factors such as the healthcare provider’s expertise and the patient’s physiological state. To achieve more accurate heart rate diagnostics, the integration of traditional medicine with modern science and technology becomes imperative. Presently, numerous devices for heart rate tracking are available, employing various methods such as pressure sensors, photoelectric sensors, capacitive sensors, and electroacoustic sensors. Nevertheless, these devices exhibit minimal variations in the selection of pulse measurement sites, and there remains a scarcity of instruments capable of achieving precise pulse measurements. In this article, TechSparks aims to present a comprehensive design framework this serves to assist designers in developing a simplified heart rate monitor circuit design that enables non-invasive measurement of heart rate through fingertip assessment.

Table of Contents

Circuit Working Principle

Figure 1 illustrates the heart rate monitor central design component, the AT89C2051 single-chip microcontroller. The patient’s pulse signal is captured by the photoelectric sensor and undergoes a series of signal processing stages, including preamplification, filtering, integration, and comparison circuits. These stages yield a pulse signal that corresponds to the patient’s heartbeat. The pulse signal serves as an interrupt signal, triggering the single-chip microcontroller to calculate the pulse period. Subsequently, the pulse rate per minute, known as the heart rate, is determined. The heart rate value is then displayed on a digital display module. Furthermore, the software component incorporates an upper and lower limit alarm function. If the measured heart rate data falls outside the normal range (greater than 180 beats per minute or less than 45 beats per minute), an alarm is triggered to alert medical professionals to exercise caution and provide appropriate attention.

Schematic diagram of heart rate detection device

Heart Rate Monitor Circuit Design

Sensors and Signal Processing Circuit

The photoelectric pulse sensor is positioned at the fingertip due to the fingertip’s rich arterial content and relatively thin tissue thickness compared to other body parts. This location facilitates the detection of a significant amount of light intensity transmitted through the finger. In this setup, a pair of infrared emitting and receiving probes are placed on opposite sides of the finger. As the arterial blood vessels contract and relax synchronously with the heartbeat, resulting in corresponding changes in blood volume, the infrared receiving probes capture signals that follow the periodic contraction and relaxation of the heart. These diastolic arterial pulsation signals serve as the basis for collecting cardiac pulsation data.

The heart rate sensor employs the use of HRl068C-05Y2 and PT331C infrared paired tubes. Since the physiological signals acquired from the human finger are extremely weak, typically in the microvolt to millivolt range, substantial noise is generated during testing due to body movements and strong power frequency interference. Additionally, the collected pulse signal necessitates amplification through a preamplifier circuit, which demands a circuit with high gain and a high common-mode rejection ratio of at least 80 dB. This requirement entails the use of an integrated operational amplifier with exceptional common-mode rejection ratio and minimal zero drift. It is crucial to select precise resistance parameters for the amplifying circuit. The amplification circuit is constructed using a resistor network and the OP07 operational amplifier. The sensor and preamplifying circuit configuration are depicted in Figure 2.

In the presence of various sources of internal and external noise, as well as 50 Hz power frequency interference, the weak pulse signal tends to be submerged in the interference signal, despite the high common-mode rejection ratio of the circuit. Given that the primary frequency content of the pulse signal is approximately 1 Hz, with stronger energy components extending below 20 Hz, it is crucial to set the upper limit cut-off frequency of the low-pass filter at 40 Hz. To address power frequency interference, a symmetrical double-T RC active notch filter is employed to effectively eliminate it. Subsequently, the pulse signal undergoes shaping through an integral and comparison circuit, resulting in the desired standardized 0-5 V pulse signal for the microcontroller. The filtering, notch circuit, and integral comparison circuit are depicted in Figure 3.

Filter, notch circuit and integral comparison circuit

SCM Control and Display Circuit

In Figure 4, the control and display circuit of the single-chip microcomputer utilizes a dynamic display mode. The seven-segment code of the digital tube is inputted through P1.0-P1.6 of the P1 port on the single-chip microcomputer. The selection signals for the four digital tubes are assigned to P3.0, P3.1, P3.2, and P3.3. The heart rate pulse generated by the photoelectric sensor is directly connected to pin 9 (T1 end) of the 89C2051 single-chip microcomputer as an interrupt signal. Timing is achieved through T0, while counting is performed by T1. P1.7 outputs the upper and lower limit alarm signals for the heart rate, which activate the alarm circuit driven by a diode. If the heart rate falls below the lower limit of 45 beats per minute, a continuous beep is emitted. Similarly, when the heart rate exceeds the upper limit of 180 beats per minute, a short audible alarm is triggered.

SCM Control and Display Circuit

Software Design

The system software’s flow chart is presented in Figure 5. The heart rate’s thousands, hundreds, tens, and ones digits, intended for display, are stored in memory locations 41H, 42H, 43H, and 44H within the 89C2051 microcontroller. A dynamic scanning technique is employed to sequentially display the thousands, hundreds, tens, and ones digits every 5 ms. Upon a rising edge at the 9th pin of the microcontroller, the T1 pin increments by one, while T0 performs timing for 50 ms. This cycle is repeated 1200 times, with the T1 count representing the heart rate value. Subsequently, the program returns to the main routine to continue executing the display program.

Circuit Debugging and Noise Analysis

Circuit debugging plays a crucial role in filtering and amplifying the input pulse signal, directly impacting the accuracy of data acquisition. Through testing, it has been determined that the pulse signal suffers from significant noise interference, underscoring the importance of designing a reliable preamplifier circuit. The utilization of the DFl405 digital synthetic signal generator allows for the simulation of high-frequency pulse signals, enabling effective attenuation of high-frequency components (approximately 106 Hz) in the signal processing circuit. Furthermore, the signal can be appropriately amplified according to the design requirements when the signal frequency is within a moderate range. The integration of a 50 Hz notch filter has proven highly effective in suppressing power frequency interference. Shaping the pulse signal through integration and comparison circuits yields the desired pulse signal required by the microcontroller. Thorough system debugging has ensured that the system meets the intended design specifications.

During the measurement process, the pulse signal captured by the sensor is inherently weak and susceptible to external disturbances. Consequently, it is imperative to analyze potential sources of interference affecting the pulse sensor. The primary sources of interference in photoelectric pulse sensors include ambient light, electromagnetic fields, and motion-related noise. To mitigate the impact of ambient light on pulse signal measurements while maintaining sensor usability, a finger-sleeve packaging design is adopted. The finger-sleeve packaging employs an opaque material with appropriate coloring to minimize the influence of external ambient light. Electrical signals containing pulse information, obtained through photoelectric conversion, tend to be weak and vulnerable to external electromagnetic signals. Therefore, proper shielding measures should be implemented within the hardware circuit. Since the pulse signal exhibits slow variations, it is particularly susceptible to power frequency interference. The incorporation of a notch filter effectively resolves this issue. By ensuring tighter contact between the cuff and finger during the measurement process, relative motion between the two is reduced, thereby minimizing motion-related noise.

Summarize

The core technology of the simple heart rate monitor circuit revolves around sensor production and the amplification of the weak pulse signal. Through practical design and manufacturing, the results validate the rationale and feasibility of the design, demonstrating the possibility of non-invasive detection of finger pulse using a scientifically engineered transmissive sensor. However, further research is required in the realm of small signal amplification technology. In comparison to other heart rate detectors, this device boasts compact size, lightweight construction, cost-effectiveness, user-friendliness, and precise measurements, thus presenting promising prospects for application.

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