![]() ![]() A 1 uF capacitor at the input of each stage is required to block the dc component in the signal. The gain of each filter stage is set to 101, giving the total amplification of about 10000. The filtering is necessary to block any higher frequency noises present in the signal. It operates at a single power supply and provides rail-to-rail output swing. The operational amplifier IC used in this circuit is MCP602, a dual OpAmp chip from Microchip. This means the maximum measurable heart rate is about 150 bpm. The signal conditioning circuit consists of two identical active low pass filters with a cut-off frequency of about 2.5 Hz. The pulses can be later counted by the microcontroller to determine the heart rate. ![]() With a proper signal conditioning, this little change in the amplitude of the reflected light can be converted into a pulse. So, each heart beat slightly alters the amount of reflected infrared light that can be detected by the photodiode. The intensity of reflected light depends upon the blood volume inside the fingertip. The IR diode transmits an infrared light into the fingertip (placed over the sensor unit), and the photodiode senses the portion of the light that is reflected back. The sensor unit consists of an infrared light-emitting-diode (IR LED) and a photodiode, placed side by side as shown below. ![]() This project describes a microcontroller based heart rate measuement system that uses optical sensors to measure the alteration in blood volume at fingertip with each heart beat. You can count the number of pulses within a certain interval (say 15 sec), and easily determine the heart rate in bpm. The most common places are wrist and neck. You can measure heart rate at any spot on the body where you can feel a pulse with your fingers. The resting heart rate is directly related to the health and fitness of a person, and hence is important to know. In adults, a normal heart beats about 60 to 100 times a minute during resting condition. Heart rate is the number of heartbeats per unit of time and is usually expressed in beats per minute (bpm). Heart rate measuring device using PIC16F628A Theory The microcontroller used in this project is PIC16F628A. The heart rate is displayed on a 3 digit seven segment LED display. Therefore, a two-stage, high gain, active low pass filter is designed using two Operational Amplifiers (OpAmps) to filter and amplify the signal to appropriate voltage level so that the pulses can be counted by a microcontroller. The changing blood volume with heartbeat results in a train of pulses at the output of the photo diode, the magnitude of which is too small to be detected directly by a microcontroller. The reflected signal is detected by a photo diode sensor. A part of this infrared light is reflected by the blood cells. It consists of an infrared LED that transmits an IR signal through the fingertip of the subject. This project demonstrates a technique to measure the heart rate by sensing the variation of the blood volume inside a finger artery, which is caused by the pumping action of the heart. Heart rate measurement indicates the soundness of the human cardiovascular system. ![]()
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