Data Collection

The change of the blood volume with heartbeat causes the small changes in the current of the photodetector and results in a series of periodic pulses. A plot of these periodic pulses versus time is what we called a PPG signal. In this project, we use two different ways to detect the PPG signal, the transmission type and the reflective type as shown in the figure below. For transmission type, we use two LEDs (infrared LED + red LED) and two photodiodes to detect both heart rate and blood oxygen level by putting a finger between LEDs and photodiodes. For the reflective type, we use a red LED to detect only heart rate by attaching a finger to the sensor, which is TCRT1000.

Chip-level Structure

The PPG signal contains both DC and AC components. The DC component relates to the tissues and the average blood volume, so it does not contain any useful information. Before sending the PPG signal into our chip, we use a high pass filter to filter out the DC component. The AC component is mainly caused by pulsatile changes in arterial blood volume, which is synchronous with the heartbeat. So, the AC component contains useful information for our system. However, as the magnitude of AC component is very small, the chip must include high-gain, low-noise amplifiers in order to amplify the useful information before we extract it out and process it. We also decide to include an on-chip comparator, whose square-wave output makes it convenient for a downstream microcontroller to process and analyze. The overall circuit block diagram is shown in the figure below.

As shown in figure above, the core circuit in chip consists of three stages. The signal from the sensor or photodiode passes through two stages of amplification, before being converted to digital signals by the comparator. The square waveforms will then be fed into an Arduino microcontroller, which will process the information and display the user’s heart rate and blood oxygen level on a LCD screen.

Signal Processing & Display

In our design, we use the Arduino Uno board to power on the chip and do the signal processing. We have an LCD screen to display both heart-rate and oxygen saturation (SP02).