1. Introduction Pulse measurement is a measurement that detects the presence or absence of a pulse. When a pulse is present, light is blocked; when there is no pulse, light is easily transmitted. The sensors used are infrared receiving diodes and infrared emitting diodes. Pulse measurement for sports measurements generally uses two methods: finger pulse and ear pulse. Each method has its advantages and disadvantages. Finger pulse measurement is more convenient and simple, but because fingers have many sweat glands, the finger clip may become contaminated over time, potentially reducing measurement sensitivity. Ear pulse measurement is cleaner, the sensor operates in a less polluted environment, and is easier to maintain. However, because ear pulses are weak, especially during seasonal changes, the measured signal is significantly affected by ambient temperature, leading to inaccurate measurement results. 2. Pulse Signal Acquisition The pulse signal acquisition circuit is shown in Figure 1. IC1A is connected as a unity-gain buffer to generate a 2.5V reference voltage. The infrared receiving diode generates electrical energy under infrared light illumination; a single diode can generate 0.4V voltage and 0.5mA current. Both the BPW83 infrared receiving diode and the IR333 infrared emitting diode operate at a wavelength of 940 nm. In the finger clip, the infrared receiving diode and the infrared emitting diode are placed opposite each other to obtain optimal pointing characteristics. The larger the current in the infrared emitting diode, the smaller the emission angle, and the greater the emission intensity. In Figure 1, RO is chosen to be 100 Ω based on the sensitivity of the infrared receiving diode to infrared light. If R0 is too large, the current through the infrared emitting diode is too small, and the BPW83 infrared receiving diode cannot distinguish between signals with and without a pulse. Conversely, if R0 is too small, the current through it is too large, and the infrared receiving diode also cannot accurately distinguish between signals with and without a pulse. When the infrared light emitted by the infrared emitting diode directly illuminates the infrared receiving diode, the potential at the inverting input terminal of IC1B is greater than the potential at the non-inverting input terminal, and Vi is "0". When the finger is in the measurement position, two situations will occur: one is the pulseless period. Although the finger blocks the infrared light emitted by the infrared emitting diode, the presence of a dark current in the infrared receiving diode still causes the Vi potential to be slightly lower than 2.5 V due to a 1 μA dark current. Secondly, there is the pulse period. When there is a pulsating pulse, the blood vessels reduce the light transmittance of the finger, decreasing the dark current in the infrared receiving diode and causing the Vi potential to rise. Therefore, the so-called pulse signal pickup is actually obtained by amplifying the slight changes in dark current in the infrared receiving diode during pulse and pulseless periods using IC1B. The picked-up signal is a voltage signal of approximately 2 μV. 3. Signal Amplification A low-pass amplifier was designed based on the calculation that the human pulse can reach a maximum of 240 beats per minute after exercise. It consists of IC2A and C04, as shown in Figure 2. The cutoff frequency is determined by R07, C04, R08, and C05, and the amplification factor is determined by the ratio of R08 and R06. Based on the transfer function of the second-order low-pass filter, considering a human pulse with a maximum frequency of 4 Hz, the low-frequency characteristics are satisfactory. It should be noted that the above analysis is made under the condition of neglecting C03. If C03 is considered, then: Therefore, C03 has no effect on the frequency characteristics; its function is only DC blocking. The two-stage amplifier/comparator is shown in Figure 3. Rpll is used to adjust the system's amplification factor, and C06 is used to prevent amplifier self-oscillation. Using a two-stage amplifier, the zero-point drift is not very significant, around 0.1 V. Therefore, the comparator's threshold voltage is designed to be 0.25 V to ensure the filtering of interference signals. The advantage of using a comparator is that it can effectively overcome the influence caused by zero-point drift and improve measurement accuracy. 4 Waveform Shaping The waveform shaping circuit is shown in Figure 4. IC3A is a CD4528 type monostable multivibrator with an effective pulse width of 0.05 s. Its width is determined by R22 and C20. IC3B also forms a monostable multivibrator with a pulse width of 240ms. D2, D1, and T3 form a NOR gate; the signal amplifier output is high only when both C and E are low. The purpose of this circuit design is to output a narrow pulse at the output terminal, and to ensure that no signal interferes with the output within the time determined by R13 and C07. The charging time of R23 and C21 determines the width of the counting pulse, which is generally not desired to be too wide. The waveform shaping timing is shown in Figure 5. 5 Conclusion When this amplifier is used in a cluster pulse meter, it is essential to pay attention to the mutual influence between different signal channels. It is recommended to separate the power supplies of each amplifier. In addition, a switching circuit is required for the measurement channel. When the finger clip is suspended, this switching circuit turns off the monostable circuit, cuts off the signal path, and prevents erratic counting. Several years of production practice have proven that this amplification processing circuit is stable and reliable. Below are some insights gained by the author in the design. Two-stage amplification is better than three-stage amplification; the zero-point drift of some three-stage amplification circuit boards is large enough to reach full amplitude, making the measurement inaccurate. The gain of each single-stage amplifier should ideally not exceed 30 to avoid self-oscillation. The high-frequency cutoff frequency of this signal amplifier is determined by C05, C04, R07, R08, and R06. C05 and C04 are typically polypropylene or polycarbonate capacitors, while R07, R08, and R06 are typically metal film five-ring resistors. IClA, R02, and R03 form a voltage follower with a design value of 2.5 V. The accuracy is determined by R02 and R03, and metal film five-ring resistors are preferred. The leakage current of the DC blocking capacitor C03 should be low; a tantalum electrolytic capacitor is preferable. IClA and IC1B should be operational amplifiers with low bias current and low input offset voltage. Considering cost-effectiveness, the author used TLC2264 and TLC2262.