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Company Logo TE Connectivity

  • Heart Rate Measuring Device

  • Created: Aug 24, 2015

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The goal of this reference design is to provide a low cost digital heart rate measuring device. The heart rate is one of the factors that determine the health condition of a person. The resting heart rate of a person is normally between 60 to 100 beats per minute. People who get a lot of physical activities or taking a drug such as beta blocker tends to have a lower heart rate while less active person or obese might have a higher resting heart rate but not more than 100 beats per minute. In this design, the heart rate of a person is measured at the tip of the person’s finger.

The whole design is composed of a sensor, a signal conditioner, a microcontroller and a display. The circuit in this reference design uses two diodes (infrared emitter diode and a photodiode) to determine the heart rate of a person by just placing the person’s finger at the top of the diodes placed close to each other. The concept of this type of measurement is to measure the variation in the volume of the blood at the fingertip in each beat of the heart. Once the start button is pressed, the RA3 pin of the microcontroller will start transmitting pulses for 15 seconds to the base terminal of Q1 so that the infrared emitter diode will start firing infrared waves to the fingertip. The photodiode senses the portion of the waves reflected back by the blood inside the fingertip. The intensity of the reflected waves is dependent on the volume of the blood that it hit. The heartbeat controls the blood volume inside the fingertip. So, the heartbeat is responsible for the change in the amount of the reflected waves detected by the photodiode.

The infrared waves sensed by the photodiode will be conditioned by the signal conditioner to shape it into a pulse. The signal conditioner is made up of two amplifying high pass filters with a voltage gain of 101 and a cut-off frequency of 2.34Hz. The purpose of the filter is to remove the DC components of the signal. The shaped pulses from the signal conditioner output will be counted by the microcontroller at the T0CKI input pin. The heart rate would be four times the number of pulses counted by the microcontroller. The result will be displayed on the LCD screen driven by the microcontroller. The whole circuit can be operated with a 5V DC USB port power supply through the VCC pin of the TE Connectivity 1-292303-6 USB port.