Pulse Monitor

Heart rate measurement indicates the soundness of the human cardiovascular system. This project demonstrates a technique to measure the heart rate by sensing the change in blood volume in a finger artery while the heart is pumping the blood. It consists of an infrared LED that transmits an IR signal through the fingertip of the subject, a part of which is reflected by the blood cells. The reflected signal is detected by a photo diode sensor. 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. 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 heart rate is displayed on a 3 digit seven segment display. To count the heart beats, here we use a small light source on one side of the finger (thumb) and observe the change in light intensity on the other side. The blood flow causes variation in light intensity reaching the light dependent resistor (LDR), which results in change in signal strength due to change in the resistance of the LDR.

CIRCUIT DESCRIPTION-

the circuit of microcontroller-based heart-rate meter. The setup uses a 6V electric bulb for light illumination of flesh on the thumb behindthe nail and the LDR as detector of change in the light intensity due to the flow of blood. The photo-current is converted into voltage and amplified by operational amplifier IC LM358 (IC1). The detected signal is given to the non-inverting input (pin 3) and its output is fed to another non-inverting input (pin 5) for squaring and amplification. Output pin 7 provides detected heartbeats to pin 12 of the microcontroller. Preset VR1 is used for sensitivity and preset VR2 for trigger level settings. Microcontroller IC AT89C2051 (IC2) is at the heart of the circuit. It is