Tuning Pulse Sensor

Context

The Pulse Sensor (PPG) has been widely used in research projects due to its availability and (relatively) low cost. However, it is not uncommon to see saturated signals when performing the acquisitions at the finger, particularly around the systolic peak. Additionally, attempts to acquire PPG from other body areas, such as the ankle, often result in insufficient signal amplitudes.

Solution

To address these issues, I identified and modified a single circuit component responsible for signal gain. Through this adjustment, the signal saturation can be eliminated for finger acquisitions and we can possibly increase signal amplitude when acquiring from other body parts. Here's a video showing how the circuit can be easily modified using a soldering iron.

Roadmap:

Identified the non-inverting OpAmp in the circuit (publicly available) and resistors R5 and R6 as resposible for the circuit's gain.
Simulated the PPG sensor in LTSpice, using a sine-wave with 1 Hz (~60 bpm) and tested different values of R6.
Removed R6 (originally 3.3M) from the Pulse Sensor and connected the circuit to a resistor box, tuning the sensor on-the-fly. A new value of R6 = 1.3 M seemed to resolve signal saturation for finger acquisitions.

Comments

It could be possible to make a Pulse Sensor with gain control by incorporating a digital potentiometer (or multiplexer connected to fixed resistors) into this part of the circuit.

Here's an example of the PPG signal before and after adjusting the circuit's gain (image extracted form this paper).