How to make a simple pulse meter

A pulse meter is a medical device used to measure the rate, strength, frequency, and rhythm of the pulse to monitor the health of a patient. Advanced pulse meter can store pulse data to a computer or cloud for further analysis and diagnosis, while simple pulse meter use electronic sensors to measure the pulse. In this article, TechSparks will explore how to make a simple version of the electronic pulse meter.

Step 1: Diagram of Electronic Pulse Meter

To make the electronic pulse meter more portable and easier to use, it has been designed to use only a CMOS integrated circuit and a triode. The quiescent current of the device is between 10-20μA, which ensures that the power consumption is small. As a result, the entire device operates on only two AG10 button batteries.

As shown in the schematic diagram it includes comprises a sensor (B), amplifier, shaping circuit, and driving circuit. The circuit consists of 4 CMOS NAND gates that are used to form the amplification, shaping, and oscillation circuits. The sensor at the front-end is made of piezoelectric ceramics that convert the pulsating pressure signal on the finger into an electrical signal. This design is optimized for low power consumption, compactness, and simplicity, making it an ideal tool for monitoring the pulse of patients.

Schematic diagram of a simple pulsometer

Step 2: Circuit Design of Pulsometer

The amplifying circuit of the pulsometer is designed to boost the weak electrical signal from the sensor. A high input impedance amplifying circuit is used to ensure optimal performance. In the circuit diagram shown in Figure 1, the inverter YF1 is used as the first-stage amplifier. By adding an appropriate linear bias, the circuit is designed to achieve high impedance with excellent performance. A feedback resistor is connected between the input and output terminals of the inverter YF1 to form an amplifier with negative feedback. Capacitor C1 is used to prevent self-oscillation as a high-frequency bypass.

The shaping circuit uses the inverter YF2 as a shaper. As this stage is in a working condition, no bias is added. The feedback resistor R5 in the first-stage amplifying circuit is properly selected so that the output of YF1 just exceeds the turn-on level of YF2. Once the sensor receives the electrical signal, it is amplified when passing through YF1 and output high level after being shaped by YF2. YF2 outputs a low level when in static condition.

The sound and LED drive circuit comprises YF3 and YF4, which are combined to form a controlled multivibrator. When YF2 outputs a high level, the oscillator starts to vibrate. 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. The oscillation frequency can be adjusted by adjusting resistors R1, R2, and capacitor C2.

Step 3: Pulse Meter Sensor Production

When making a homemade pulsometer, it is crucial to pay extra attention to the quality of the sensor, as it greatly affects the sensitivity of the device. The key component is the piezoelectric ceramic sheet, which converts the pulsating pressure signal on the finger into an electrical signal. The HTD-20 model is used in this project, but it is essential to select a piezoelectric ceramic chip that meets the following requirements:

.Connect the silver-plated ceramic surface to the Y-axis input of the oscilloscope.

.The ground terminal of the metal substrate must be connected properly.

.Touch and press one side of the metal substrate by hand, and observe that the output of the silver-plated side should be a positive pulse before use.

After selecting the appropriate piezoelectric ceramic chip, the following steps can be taken to assemble the sensor:

  1. Weld the side of the piezoelectric ceramic sheet with ceramics to the side of the copper ring.
  2. Use a thin shielded wire as the lead wire.
  3. The outer skin should be connected to the copper ring.
  4. The core wire should pass through the gap of the copper ring and be welded on the ceramic surface.
  5. According to the thickness of the finger, cut a piece of metal watch spring and connect it with the sensor to form a circle.

The picture below shows the result after welding. Attention to detail is essential when assembling the sensor, as any mistake can lead to a decrease in sensitivity and overall device performance.

Soldering diagram of a simple pulsometer

Step 4: Measuring with a homemade pulse meter

The pulse meter is designed to detect the “finger pulse” signal, making it a convenient and user-friendly device. When the sensor is applied to the fingertip, it detects the pulsatile pressure changes caused by blood flow. These changes in pressure are converted into electrical signals by the sensor, which are then amplified and shaped by the electronic circuitry. The resulting signal is then presented in the form of both audio and visual feedback, enabling the user to determine their pulse rate.

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