Briefly analyze the working principle of the medical electronic pulse meter circuit

LDE series is an onboard dual in-line package AC/DC module power supply, which is widely used in many fields such as industry, power, instrumentation, communication and intelligent buildings.

The Electronic pulse meter takes out the “finger pulse” signal from the finger, which is very convenient to use. When the front end of the finger is slightly compressed, it can be felt that the front end of the finger has a slack feeling under the action of blood pressure. This signal is extracted by the sensor, converted into an electrical signal, and then displayed in the form of light and sound. Know the pulse beat.

In order to facilitate frequent carrying and production, this machine only uses a CMOS integrated circuit and a triode. The quiescent current is only 10-20μA, and the power consumption is small, so the whole machine only uses two AG10 type button batteries. The electrical schematic diagram of the electronic pulse meter is shown in the figure. It consists of sensor (B), amplifier, shaping circuit and driving circuit. The whole circuit consists of 4 CMOS NAND gates to amplify, shape and oscillate. The front-end sensor made of piezoelectric ceramic sheets converts the pulsating pressure signal on the finger into an electrical signal.

Amplifying circuit: Since the electrical signal sent by the sensor is extremely weak, an amplifying circuit with high input impedance must be used. If the CMOS inverter is used as an analog device, as long as the appropriate linear bias is added, it is a high-impedance amplifier with good performance, such as YF1 in Figure 28-1. Connect the input and output of the inverter YF1 with a resistor R5, which is both a feedback resistor and a bias resistor. This constitutes an amplifier with negative feedback. The function of capacitor C1 is to bypass the high frequency to prevent self-oscillation.

Shaping circuit: The second stage uses the inverter YF2 as the shaper. This stage works in the switching state, so no bias is added. If the output level of YF1 is higher than the turn-on level of YF2, YF2 outputs a low level; if it is lower than the turn-on level of YF2, YF2 outputs a 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, after the amplification of YF1, YF2 will output a high level after shaping. YF2 outputs low level in static state.

Briefly analyze the working principle of the medical electronic pulse meter circuit

Audio and LED drive circuit: YF3 and YF4 form a controlled multivibrator. When YF2 outputs a high level, the oscillator starts to vibrate. By adjusting the resistors R1, R2 and capacitor C2, the oscillation frequency can be changed. At the same time, YF3 drives the piezoelectric ceramic sheet 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 will directly affect the sensitivity of the instrument, so the production of the sensor is a very important part. The ceramic side of the piezoelectric ceramic sheet is welded to the copper ring side (three points are welded evenly 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 is welded through the copper ring gap. on a ceramic surface. Then weld a copper sheet under the copper ring, as shown in the figure after welding.

Finally, according to the thickness of ordinary people’s fingers, cut out a piece of metal watch with a spring and connect it with the sensor to form a circle, so that it can be put on the finger during use. The model of the piezoelectric ceramic sheet used in the sensor is HTD-20, but the following requirements should be paid attention to when selecting the piezoelectric ceramic sheet: connect the silver-plated ceramic surface to the Y-axis input end of the oscilloscope, the ground end of the metal substrate, and press the metal substrate by hand. On the one hand, observe that the output of the silver-plated side should be a positive pulse before it can be used.

The electronic pulse meter does not have a power switch. When using the sensor, touch the sensor with your finger first, and the instrument will make a “squeak, squeak” sound, and the light-emitting tube emits red light, indicating that the instrument is working normally. It is advisable to wear the sensor on your finger to feel a little pressure, and wait for a while (because it will take a while for the blood to flow back into the finger after the finger is compressed), and the instrument will emit a sound and Display red light with the beating of the pulse.

Author: Yoyokuo