PSM HeinzmannKretz

Semester Project: Photoelectric Sensor Array

As part of the Sensor Technology and Mechatronics Project Lab, an photoelectric sensor array (light barrier array) was designed to detect beam interruptions simultaneously using a single analog signal line.

The project focused on the development of the analog electronics, the circuit design and simulation, the design of the PCB, and the implementation of the firmware for digital signal processing and evaluation.

Project Structure

project/
├── docs                   # Technical documentations
│   ├── datasheets
│   └── manuals
├── firmware               # STM32CubeMX Project
├── pcb 
│   ├── .kicad_xxx
│   └── postpr             # Manufactoring outputs (Gerber, Drill...)
├── README.md
├── report                 # Project report (LaTeX source) + Compiled PDF
├── sandbox                # Test env (ignored)
└── sim                    # LTspice & Python simulations 

Overview

Each light barrier consists of an LED–photodiode pair driven by pulse-width-modulated (PWM) signals at unique frequencies ($3\text{ kHz}$, $5\text{ kHz}$, $7\text{ kHz}$, and $9\text{ kHz}$). The optical signals are received by BPW34 photodiodes operated in reverse-bias (photoconductive) mode to maximize response speed and bandwidth. The photodiode currents are converted to voltages via individual Transimpedance Amplifiers (TIA), additively combined in an inverting summing amplifier stage, filtered using a 2nd-order Sallen-Key low-pass filter ($f_c \approx 15.6\text{ kHz}$), and digitized by an STM32 microcontroller ADC. Digital signal processing is performed via Fast Fourier Transform (FFT) to simultaneously evaluate all four channels.

As a practical application, a digital logic game was implemented in firmware where users enter a randomized 6-digit passcode (generated via True Random Number Generator - TRNG) by selectively breaking the light barriers (each representing a binary bit from 1 to 15).

Architecture

1. Analog Front-End

• Photodiode Bias: BPW34 photodiodes are operated in reverse-bias mode to lower junction capacitance ($C_j \propto 1/\sqrt{U_R}$), improving bandwidth and response time.
• Transimpedance Amplifier (TIA): Converts photodiode currents into voltages ($U_{\text{out}} = U_{\text{ref}} - I_{\text{ph}} \cdot R_f$). Feedback capacitance $C_f$ is added in parallel with feedback resistor $R_f$ to compensate for loop instability caused by photodiode junction capacitance $C_j$, achieving a Butterworth response ($\zeta = 0.707$) without resonance peaking.
• Summing Stage & Filtering: An active inverting summer merges the four TIA outputs into a single composite signal. A Sallen-Key 2nd-order low-pass filter ($f_c = 15.6\text{ kHz}$) suppresses higher-order PWM harmonics before ADC sampling.
• Custom PCB: Adapter PCB for the STM32F767 Eval-Bard designed in KiCad

2. Firmware & Digital Signal Processing

• Platform: STM32 microcontroller (ARM Cortex-M7 / STM32F7 series).
• Sampling & DMA: Timer-triggered 12-bit ADC running at $f_s = 20\text{ kHz}$ with Circular Direct Memory Access (DMA) transferring 512 samples per frame directly to RAM without CPU overhead.
• FFT Spectral Analysis: Computes 512-point real FFT using ARM CMSIS-DSP (arm_rfft_fast_f32), providing a frequency bin resolution of $\Delta f = f_s / N \approx 39.06\text{ Hz}$. Energy across 5 adjacent bins surrounding each target frequency ($3\text{ kHz}$, $5\text{ kHz}$, $7\text{ kHz}$, $9\text{ kHz}$) is integrated to detect channel presence/absence robustly against spectral leakage.
• State Machine Game App: Manages sampling timing ($100\text{ ms}$ system tick), TRNG passcode generation, UART terminal user interface, binary inputs via sensor array, and pass/fail evaluation via onboard status LEDs.

Useful Commands

Format auto-generated Makefile:

cd firmware/
dos2unix Makefile

Generate compile_commands.json for LSP (from Makefile)

cd firmware/
bear -- make

Requirements

• STM32CubeMX

• GNU Arm Embedded Toolchain

  • Debian

  sudo apt install gcc-arm-none-eabi

  • Fedora

  sudo dnf install arm-none-eabi-gcc arm-none-eabi-newlib arm-none-eabi-binutils-cs 

• STLINK Tools