Traditional Chinese Electronic pulse meter: Circuit Design, Parts List & Workshop Prototype Guide

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Electronic pulse meterThe electronic pulse meter takes out the "finger pulse" signal from the finger and is very convenient to use. When the front end of the finger is slightly pressed, you can feel the front end of the finger relaxing under the action of blood pressure. This signal is extracted with a sensor, converted into an electrical signal, and then displayed in the form of light and sound, so that you can know the pulse beat.In order to facilitate frequent portability and production, this machine only uses a CMOS integrated circuit and a triode. The quiescent current is only 10~20μA and consumes very little, so the whole machine only uses two AG10 button batteries.The electrical schematic diagram of the electronic pulse meter is shown in Figure 28-1. It consists of sensor (B), amplifier, shaping circuit and drive circuit. The entire circuit consists of four CMOS NAND gates to form an amplification, shaping and oscillation circuit. The sensor made of piezoelectric ceramics at the front end converts the pulsating pressure signal on the finger into an electrical signal.Amplification circuit:Since the electrical signal sent by the sensor is extremely weak, an amplification circuit with high input impedance must be used. If the CMOS inverter is used as an analog device, as long as appropriate linear bias is added, it will be a high-impedance amplifier with good performance, such as YF1 in Figure 28-1. Use a resistor R5 to connect the input terminal and the output terminal of the inverter YF1, which is both a feedback resistor and a bias resistor. This constitutes an amplifier with negative feedback. The function of capacitor C1 is a high-frequency bypass to prevent self-oscillation.Shaping circuit:The second stage uses inverter YF2 as a shaper. This stage works in a switching state, so no bias is added. If the output level of YF1 is higher than the turn-on level of YF2, YF2 outputs low level; if it is lower than the turn-on level of YF2, YF2 outputs high level. Since the value of the feedback resistor R5 of the first-stage amplifier circuit is properly selected, the output of YF1 just exceeds the turn-on level of YF2. Once the sensor has a weak signal output, it will be amplified by YF1 and then shaped by YF2 to output a high level. YF2 outputs low level when static. alt="43.jpg (33497 bytes)">Figure 28-1Audio and light-emitting tube drive circuit:YF3 and YF4 form a controlled multivibrator. When YF2 outputs high level, the oscillator starts to vibrate. By adjusting resistors R1, R2 and capacitor C2, the oscillation frequency can be changed. At the same time, YF3 drives the piezoelectric ceramic piece BC to emit sound, and YF4 is connected to the transistor VT1 to drive the light-emitting diode to emit light.In addition to the circuit part of the electronic pulse meter, the quality of the sensor production will directly affect the sensitivity of the instrument, so the production of the sensor is a very important part. As shown in Figure 28-2(a), the ceramic side of the piezoelectric ceramic sheet is welded to the copper ring (three points are evenly welded on the circumference). A thin shielded wire is used as the lead wire, the outer skin is connected to the copper ring, and the core wire passes through the gap in the copper ring and is welded to the ceramic surface. Then weld a copper piece under the copper ring, as shown in Figure 28-2(b) after welding. Finally, based on the thickness of ordinary people's fingers, a section of metal watch with a spring is cut out and connected with the sensor to form a circle so that it can be put on the finger during use, as shown in Figure 28-2(c). The model of the piezoelectric ceramic sheet used in the sensor is HTD-20, but when choosing a piezoelectric ceramic sheet, the following requirements should be paid attention to: Connect the silver-plated layer on the ceramic surface to the Y-axis input end of the oscilloscope and the ground end of the metal substrate. Touch one side of the metal substrate with your hand and observe that the output of the silver-plated side should be a positive pulse before it can be used. …

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