Acquiring Photoplethysmography (PPG) Signal Without LED

Photoplethysmography (PPG) sensors should use the least amount of power possible when integrated into wearables as battery life is one of the main concerns in wearable area. In this context, we propose a system which can collect PPG signals under ambient light conditions without turning on any LED of the PPG sensor (NO-LED mode). The system consists of an efficient analog front end, a photodiode and a microprocessor. Results show that PPG signals recorded with the proposed system under different ambient light conditions can be placed in reliable or acceptable category. Moreover, heart rate estimated from NO-LED mode PPG signal shows less than 6% error when compared with heart rate estimated from a typical PPG signal acquired with LED on. Since LED is the most power consuming part of the PPG sensor, the proposed system has potential for increasing the battery life of PPG sensor-based wearables.