LDE series is an on-board dual in-line packaged AC / DC module power supply, which is widely used in 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 wrapped, you can feel that the front end of the finger has a feeling of relaxation 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, you can know the pulse beating situation.

In order to be easy to carry and manufacture, this machine only uses a CMOS integrated circuit and a triode. The static current is only 10 ~ 20 μ a, and the power consumption is very low, so only two ag10 button batteries are used. 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 four CMOS NAND gates to amplify, reshape and oscillate. The front end of the sensor is made of piezoelectric ceramic chip, which converts the pulsating pressure signal on the finger into electrical signal.

Amplification circuit: because the electric signal sent by the sensor is extremely weak, the amplification circuit with high input impedance must be used. If CMOS inverter is used as analog device, it is a high impedance amplifier with good performance as long as appropriate linear bias is added, as shown in figure 28-1 yf1. A resistor R5 is used to connect the input and output of inverter yf1, which is not only a feedback resistor but also a bias resistor. This makes up the amplifier with negative feedback. The function of capacitor C1 is high frequency bypass to prevent self-excited oscillation.

Shaping circuit: the second stage uses inverter yf2 as shaper. This stage works in the on-off state, so it is not biased. If the output level of yf1 is higher than the on level of yf2, yf2 outputs low level; if it is lower than the on level of yf2, yf2 outputs high level. Because the feedback resistor R5 of the first stage amplifier is selected properly, the output of yf1 just exceeds the on level of yf2. Once the sensor has a weak signal output, after yf1 amplification, yf2 shaping output high level. Yf2 outputs low level in static state.

Audio and LED driver circuit: yf3 and YF4 constitute a controlled multivibrator. When yf2 outputs high level, the oscillator starts to vibrate. The oscillation frequency can be changed by adjusting resistance R1, R2 and capacitance C2. At the same time, yf3 drives BC to emit sound, YF4 connects VT1 to drive led 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. One side of the piezoelectric ceramic plate is welded on the side of the copper ring (three points are evenly welded on the circumference). A thin shielding wire is used as the lead out, the outer skin is connected with the copper ring, and the core wire is welded on the ceramic surface through the notch of the copper ring. Then weld a piece of copper under the copper ring, as shown in the figure.

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

The electronic pulse meter does not have a power switch. When you touch the sensor with your finger, the instrument will make a “creak, creak” sound, and the LED will emit red light, indicating that the instrument is working normally. It’s better to put the sensor on the finger to feel a little pressure, and wait for a moment (because the blood flow of the finger needs to wait for a while before flowing back into the finger), then the instrument will make a sound and display red light with the pulse beating.

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