Design of a Heart Rate Monitoring System Using Photoplethysmography

Abstract

The photoplethysmogram(PPG) is a non-invasive alternating signal related to the pulsatile volume in tissue and the waveform can be obtained throughpulse oximeters. Pulse oximeters measures the arterial blood oxygen saturation through optical sensors and detectors.There are various techniques available for heart rate monitoring but most of them are invasive or complex to use so a simple easy to use method like PPG was developed. PPG is a strongly reliable device and one can depend on it since it enhances ease of construction, low manufacturing cost, and can be used by any non-professional at any point of time for a quick cardiac diagnosis. Though PPG is similar in appearance to the invasive oscillating component it is actually noninvasive and is most commonly used in hospitals.This paper focusses on the design of a low cost heart rate monitoring device and the calculation of heart rate by processing the signal through Arduino or microcontroller. Later the calculated heart rate was sent to a mobile device through GSM module. Photoplethysmography can be used in both transmission and reflection mode but in this project we have settled for the reflectance mode Photoplethysmography(PPG) since the results were more satisfactory. The device manufacturing process can be divided into basically three phases. First is the design of sensor biasing circuit for data acquisition through fingertip or earlobe using the optical sensors and detectors of required wavelength. Second step is the design and implementation of signal conditioning circuit for obtaining the PPG signal in the required form that is the pulsating ac part. Finally, the signal is digitized using a suitable microcontroller or Arduino and pulse rate is calculated for display on computer as well as a mobile device. The signal conditioning circuit was designed by selecting various resistors and capacitors and the circuit was simulated in Proteus before designing it in the real-time. There are various advantages of this method. Firstly, the method implied is non-invasive and do not require any professional for diagnosis so can be used as a personal health monitoring system. In addition,it doesn’t require complex circuitry and avoids use of electrodes and gels which can be prone to infection, thus is user friendly. The disadvantage is that PPG signals are prone to motion artefacts and can lead to false recording of heart rate if used when the subject is under motion.