Table of contents
2. The principle of pulse measurement
2.1 Pulse signal processing overall circuit
2.2 Signal Acquisition Circuit
foreword
The design of the pulse measuring instrument is to detect the changes in the blood of the heartbeat, generate different reflection signals, use the sensor to convert the pulse beating into an electrical signal, and then amplify, shape and filter, and finally send it to the single-chip microcomputer for processing. This article introduces The hardware circuit design of pulse measurement using the photoelectric pair tube composed of 5mm infrared receiving tube and transmitting tube.
1. Pulse measurement method
The photoelectric detection method uses a photoelectric sensor to detect the difference in transmittance or reflectance of light when the blood flows in the human body and converts it into an electrical signal. There are two ways of this method: one is transmissive and the other is reflective .
The opposite-beam type is to place a transmitting tube and a receiving tube on both sides of a ring of appropriate size. When measuring, extend the human finger into the ring, and the photocurrent of the photoelectric receiving tube will change due to the change of blood flow in the finger. It also changes accordingly.
The reflective type is that the photoelectric emission and photoelectric reception point to the same direction. When the blood flow in the human body changes, its reflectivity to light also changes, thereby detecting the heart rate. This time, only the hardware circuit design of the transmission-type photoelectric tube measuring pulse composed of a 5mm infrared receiving tube and a transmitting tube is introduced. The physical picture of the 5mm infrared receiving tube and transmitting tube is shown in Figure 1-1 below:
Figure 1-1 Physical picture of 5mm infrared receiving tube and transmitting tube
2. The principle of pulse measurement
2.1 Pulse signal processing overall circuit
As shown in Figure 2-1, the original pulse signal signal processing circuit mainly consists of three parts: low-pass filter, signal amplification, and waveform shaping. Signal amplification and waveform shaping are realized by dual operational amplifiers LM358.
Figure 2-1 Overall signal processing circuit
2.2 Signal Acquisition Circuit
The pulse signal acquisition circuit is shown in Figure 2-2. U2 and D2 are infrared emitting and receiving devices respectively. Since the larger the current in the infrared emitting diode, the smaller the emitting angle, the greater the emission intensity, so the resistance to R5 The selection of values is more demanding. The choice of 220Ω for R5 is also based on the consideration of the infrared light sensitivity of the infrared receiving transistor. R5 is too large, the current through the infrared emitting diode is too small, and the infrared receiving transistor cannot distinguish the signal when there is a heartbeat and no heartbeat. Conversely, if R5 is too small, the current passing through is too large, and the infrared receiving transistor cannot accurately distinguish the signal when there is a heartbeat or no heartbeat.
Because the pulse signal output by the photoelectric sensor is a very weak signal, and the frequency is very low (such as 0.78Hz for 50 beats/min, 3.33Hz for 200 beats/min), and it is accompanied by various noise interference, so the signal must be After R7, C4 low-pass filter, remove high-frequency interference.
Figure 2-2 Signal Acquisition Circuit
2.3 Signal amplifier circuit
As shown in Figure 2-3, the ratio of the resistance of RT1 to R14 is the magnification of the amplifier. After calculation, the theoretical value of the amplifier is 200 times. factor, the magnification is only about 20 times. In the figure, C5 is the coupling capacitor, which is used to block DC and communicate with AC. The reason why a 1uF capacitor is used is to allow all signals to pass through.
Figure 2-3 Signal amplification circuit
2.4 Waveform shaping circuit
A voltage comparator is a commonly used integrated circuit. It can be used in alarm circuit, automatic control circuit, measurement technology, and can also be used in V/F conversion circuit, A/D conversion circuit, high-speed sampling circuit, power supply voltage monitoring circuit, oscillator and voltage-controlled oscillator circuit, zero-crossing detection circuit etc. Here we mainly introduce its basic concept, working principle and typical working circuit. The voltage comparator compares two analog voltages and determines which voltage is higher. When the received signal voltage is less than this value, it displays 0V. When it is greater than this When the value is 5V, this forms a square wave of 0 and 5V. As shown in Figure 2-4, the pulse signal after low-pass amplification is shaped into a square wave of 0 and 5V. The high level of the pulse is synchronized with the heartbeat, and is indicated by the flashing of the red light-emitting diode RED1, that is, the light-emitting diode acts as a Heartbeat status display, the light-emitting diode lights up once every time the heart beats.
Figure 2-4 Voltage Comparator
The pulse signal after filtering, amplifying and shaping is sent to the single-chip microcomputer for processing and the processed result is displayed on the display unit to complete the design of the pulse measurement system.
Summarize
This article introduces the circuit design in detail. The photoelectric pair tube composed of 5mm infrared receiving tube and transmitting tube is used to measure the hardware circuit design of the pulse, including filtering, amplification and shaping. If there are any deficiencies, please point out in the comment area and let us communicate together.