Precision Photodiode Analog Front-End (AFE)

📌 Project Overview

This repository contains the hardware design files and LTspice simulations for a high-precision, low-noise Photodiode Signal Conditioning Board.

Designed as a modular Analog Front-End (AFE), this board converts ambient light intensity into a clean, stable analog voltage (0–3.3V) suitable for direct sampling by microcontrollers (e.g., ESP32, STM32, Arduino) using a Transimpedance Amplifier (TIA) architecture.

✨ Key Features

• Transimpedance Amplifier (TIA): Converts microscopic photodiode currents into a measurable voltage with a calculated gain of 68kΩ.
• Optimized Stability: Includes a precisely calculated 20pF feedback capacitor to compensate for the photodiode's junction capacitance and prevent op-amp oscillation.
• ADC Shock Absorption: Features an RC low-pass output filter (100Ω + 10nF) to eliminate high-frequency noise and stabilize the signal against MCU ADC sampling glitches.
• Ultra-Low Leakage: Utilizes the MCP6001 CMOS operational amplifier with an input bias current of just 1pA to ensure the sensor signal is not degraded.
• Plug-and-Play Interface: Standard 3-pin connector (VCC, GND, OUT) for seamless integration into embedded systems.

🛠️ Hardware Specifications

Component	Part / Value	Description
Photodiode	BPW34S	Broadband silicon photodiode. High photocurrent (~50µA at 1000 Lux), low dark current (2nA).
Op-Amp	MCP6001	Single-supply, Rail-to-Rail Output (RRO), CMOS input (1pA Bias Current).
Gain Resistor ($R_f$)	68 kΩ	Sets the voltage output range (Max 3.3V at ~50µA input).
Compensation Cap ($C_f$)	20 pF	Prevents ringing/oscillation caused by the photodiode's 72pF junction capacitance.
Supply Voltage	3.3V	Single-supply operation. Decoupled via 0.1µF capacitor.

📐 Circuit Architecture & Theory

1. The Sensor Stage

The BPW34S photodiode is operated in photovoltaic/zero-bias mode. This configuration minimizes dark current leakage, resulting in the highest possible precision and lowest noise floor for ambient light detection.

2. The Transimpedance Stage

The core of the board is the TIA. The gain is set by the feedback resistor ($R_f$). Using Ohm's Law ($V = I \times R$), a peak ambient room light producing $~50 \mu A$ of current is multiplied by $68 k\Omega$ to produce a $~3.4V$ output, perfectly utilizing the 3.3V ADC range of modern microcontrollers.

3. Stability & Compensation

Photodiodes with large active areas (like the BPW34S) have high junction capacitance ($C_j \approx 72pF$). Without compensation, this capacitance creates a phase shift in the op-amp's feedback loop, causing severe ringing and instability. A 20pF feedback capacitor ($C_f$) was mathematically calculated and verified via SPICE simulation to critically damp the circuit and provide a smooth transient response.

📂 Repository Structure

• /kicad - KiCad 9.0 project files (Schematic, PCB Layout, Footprints).
• /Simulation - LTspice .asc files demonstrating Transient and AC (Frequency Response) analysis.
• /Docs - Datasheets for the BPW34S and MCP6001.
• /Manufacturing - Gerber files, Drill files, and BOM ready for PCB manufacturing (e.g., JLCPCB).
• /Media - Project media and screenshots.

🚀 Getting Started

1. Clone this repository: git clone https://github.com/shyver/Precision-Light-Sensor-AFE.git
2. Open the .pro or .kicad_pro file in KiCad.
3. To view the simulations, open the .asc files located in the /Simulation folder using LTspice.

📝 License

This project is open-source and available under the MIT License.