PPG Heart Rate Sensor

Overview

Designed and prototyped a photoplethysmography (PPG) heart-rate sensor using an infrared emitter and photodiode to measure blood volume changes in tissue. The system converts extremely small photodiode currents into a filtered analog waveform representing the cardiac cycle, which is then digitized using a Schmitt-trigger comparator to produce heart-rate pulses.

Problem

• Blood flow variations produce extremely small optical signals that generate photodiode currents on the order of nanoamps to microamps.
• The measured signal contains a large DC component caused by constant tissue reflection, while the useful pulse signal is a small AC variation riding on top.
• Motion, ambient light, and circuit noise can easily corrupt the measurement, requiring careful analog signal conditioning.

Solution

I designed a multi-stage analog signal chain that converts the photodiode current into a clean pulse waveform suitable for digital processing.

• Transimpedance amplifier to convert photodiode current into voltage
• Bandpass filtering to isolate the physiological heart-rate band (~0.2–12 Hz)
• Gain stages to amplify the small AC component
• Schmitt trigger comparator to generate clean digital pulses

Signal Chain

The circuit consists of three main analog stages followed by a digital pulse output.

• Stage 1 – Transimpedance Amplifier
  Converts photodiode current (~1 µA DC with ~0.1 µA AC variation) into a measurable voltage using an op-amp with a high feedback resistance.
• Stage 2 – Bandpass Filtering and Gain
  A high-pass filter removes the DC offset caused by constant tissue reflection, while a low-pass filter suppresses high-frequency noise. This creates a bandpass response tuned to the physiological heart rate range.
• Stage 3 – Comparator (Schmitt Trigger)
  The analog waveform is converted into a digital pulse using a hysteretic comparator, ensuring robust switching even in the presence of noise.

Impact

The final system produces a clean pulse signal corresponding to each heartbeat. This architecture demonstrates how extremely small biosignals can be extracted using careful analog circuit design, signal conditioning, and threshold detection. The project reinforced concepts in transimpedance amplification, filter design, and analog-to-digital signal conversion.

Technical Details

• IR emitter wavelength: 940 nm
• Photodiode current: ~1 µA DC with ~10–100 nA AC variation
• Transimpedance gain: multi-megohm feedback resistance
• Bandpass response: approximately 0.2 Hz – 12 Hz
• Analog modeling and verification performed in LTSpice using Bode plots and transient simulations