Electro-Optical Heart Rate Detection PCB

Photoplethysmography analog front end running from LED driver and photodiode through to a clean digital pulse, laid out in KiCad and fabricated through JLCPCB.

Overview

A photoplethysmography (PPG) front end: an LED illumination driver plus a photodiode, feeding an analog chain that ends in a clean digital pulse. The design target was 180 BPM detection.

Signal chain

LTC6078 transimpedance amplifier — roughly 2.1 MΩ feedback with 10 nF
4.7 µF AC coupling
Second gain stage
Non-inverting Schmitt trigger comparator
BJT monostable multivibrator and edge differentiator

Comparator chatter

Chatter on the comparator output was traced to an insufficient hysteresis band. Raising R8 to 100 kΩ produced a 96 mV band, measured with oscilloscope cursors. Stable switching was verified at 2.174 Hz.

Oscilloscope: PPG signal (cyan) and monostable output (yellow)

Oscilloscope: monostable output and LED driver pulses

Simulation

Bench behaviour was validated against LTSpice within ±10 ms.

Layout and fabrication

Schematic and layout were done in KiCad and the boards fabricated through JLCPCB.

PCB layout render