bGeigieScint is a compact radiation detection board featuring a Silicon Photomultiplier (SiPM) based sensor system with integrated high-voltage generation, analog signal processing, and USB connectivity.
- Microcontroller: ATSAML21G18B (ARM Cortex-M0+, 256KB Flash, 32KB RAM)
- SiPM Interface: Complete analog frontend with adjustable HV bias (30-70V)
- Power Management: Multi-rail power supply (3.3V, 3.0V, Vload, Vbias)
- Connectivity: USB-C interface with ESD protection
- Debug: 10-pin J-LINK SWD interface + 18 test points
- Expansion: Multiple GPIO headers for external interfaces
- Compact: 40mm × 100mm 4-layer PCB
| Component | Part Number | Function |
|---|---|---|
| MCU | ATSAML21G18B-AUT | Main microcontroller (48-pin QFN) |
| USB | USB3.1C16PFSMT | USB-C connector (USB 2.0) |
| LDO 3.3V | XC6209F332MR-G | Main 3.3V regulator |
| LDO 3.0V | TPS7A2030PDBVR | 3.0V rail for analog |
| LDO Vload | LT3014BHVES5 | Low-noise LDO for HV control |
| OpAmp | OPA357AIDBVR | Shaper amplifier |
| Comparator | TLV3201AIDBVT | Fast comparator |
| Latch | MC14043BDR2G | Digital latch for triggering |
- Part Number: AFBR-S4N44P014M (Broadcom)
- Type: NUV-HD Silicon Photomultiplier
- Active Area: 4×4 mm²
- Spectral Range: 350-900 nm (peak sensitivity ~420 nm)
- Datasheet: AFBR-S4N44P014M Documentation
- Material: CsI(Tl) - Cesium Iodide doped with Thallium
- Dimensions: 10 mm × 10 mm × 20 mm
- Emission Peak: ~550 nm (matches SiPM sensitivity)
- Application: Gamma ray detection coupled to SiPM
- Input: 4.5-6V via USB-C or external connector
- Output Rails:
- 3.3V @ up to 200mA (digital)
- 3.0V @ up to 100mA (analog)
- Vload (adjustable)
- Vbias: 30-70V @ low current (SiPM bias)
The board features an integrated boost converter + linear regulator topology for SiPM bias generation:
- Boost Stage: PWM-controlled (L1 10mH inductor, Q3 P-FET, D9 Schottky)
- Regulation: LT3014 ultra-low-noise LDO
- Control: MCU DAC output (PA02_DAC_HV) sets target voltage
- Monitoring: Comparator feedback (PB10_HV_COMP_OUT)
- Output: Adjustable 30-70V Vbias for SiPM
bGeigieScint/
├── bGeigieScint/
│ ├── bGeigieScint.kicad_pro # KiCad project file
│ ├── bGeigieScint.kicad_sch # Schematic (organized in functional blocks)
│ ├── bGeigieScint.kicad_pcb # PCB layout (4-layer, 40×100mm)
│ ├── bGeigieScint-backups/ # Automatic backups
│ ├── firmware/ # Firmware source code
│ │ └── PomeloCore/ # Main firmware project
│ ├── bootloader/ # Bootloader files
│ │ └── uf2-samd21/ # UF2 bootloader for SAMD21
│ ├── v1_2/ # Version 1.2 reference files
│ │ ├── schematic/ # V1.2 schematic PDF
│ │ ├── firmware/ # V1.2 firmware
│ │ └── bootloader/ # V1.2 bootloader
│ ├── Images/ # Project images and photos
│ └── Misc Docs/ # Additional documentation
├── BOM.csv # Bill of Materials
├── Python scripts (*.py) # KiCad automation scripts
└── README.md # This file
The schematic is organized into 7 functional blocks:
- USB & POWER INPUT: USB-C interface, power connectors, reset button
- POWER MANAGEMENT: LDOs, inductors, power distribution
- MCU CORE: Microcontroller, crystal, decoupling
- ANALOG FRONTEND: SiPM signal chain, opamps, comparators
- LOGIC & CONTROL: Digital logic, latches, level shifters
- EXPANSION I/O: Pin headers for external connections
- DEBUG & TEST: J-LINK interface, test points
1: VCC 2: SWDIO
3: GND 4: SWCLK
5: GND 6: SWO
7: NC 8: NC
9: GND 10: RESET
- H2, H3: 7-pin GPIO expansion
- H5: 7-pin expansion with labeled signals
- MEAS1: 2-pin measurement header
- P1, P2: 2×2 power connectors (mirrored on Pomelo Physics board)
| Signal | Description |
|---|---|
| SiPM_A | SiPM analog input |
| Vbias | SiPM bias voltage output |
| SHAPER_OUT | Shaped pulse output |
| PA02_DAC_HV | HV control DAC output |
| PA10_PWM | Boost converter PWM |
| PB03_PEAKDET_EN | Peak detector enable |
| PA23_TRG_OUT | Trigger output |
Major improvements over V1.2:
✅ Added (47 components):
- USB-C connector (USBC1)
- Reset button (BUT1)
- J-LINK debug header
- 18 test points (TP1-TP18)
- Additional transistors (Q4, Q5) and diodes (D6, D8, D10)
- 15 new resistors for better signal conditioning
✅ Changed:
- C44: 1uF → 4.7uF
- C45: 4.7uF → 1uF
- H2, H3: 6-pin → 7-pin headers (expanded)
✅ Enhanced:
- Better debugging capability
- Improved power filtering
- More flexible I/O expansion
- Initial production version
- PDF schematic available in
v1_2/schematic/
- KiCad 9.0 or later
- Basic understanding of radiation detection and SiPM operation
- JTAG/SWD programmer (e.g., J-Link, ST-Link)
- Clone this repository
- Open
bGeigieScint/bGeigieScint.kicad_proin KiCad - The schematic shows functional blocks with dashed rectangles
- The PCB has components organized by function
PCB Specifications:
- Layers: 4 (signal/GND/VCC/signal)
- Dimensions: 40mm × 100mm
- Thickness: 1.6mm
- Copper: 35μm (1oz) on all layers
- Min Track/Space: 0.15mm / 0.15mm (for 0805 components)
- Finish: HASL or ENIG recommended
- Solder Mask: Green (top and bottom)
Assembly Notes:
- Minimum component size: 0805 for passives
- QFN-48 MCU requires reflow soldering
- LDOs are SOT-23-5 packages
- Test points for easy debugging
- Layer 1 (In1.Cu): GND plane - solid pour for low noise
- Layer 2 (In2.Cu): VCC plane - split for 3.3V/3.0V rails
- Keep SiPM analog traces short and shielded
- HV traces (Vbias) should be isolated from digital signals
- Crystal traces should be short with GND guard ring
Components are organized into functional blocks:
- Power supply on left side
- MCU in center
- Analog section on right
- Digital logic bottom-right
- Radiation detection and measurement
- Scintillation counting
- Environmental monitoring
- Educational radiation physics
- DIY Geiger counter replacement with better sensitivity
Original Designer: Mihai Cuciuc (M. Cuciuc) Original Repository: github.com/mihaicuciuc/pomelo Organization: Safecast Date: 2024-2025
Safecast appreciates the great work that Mihai has done and made publicly available. This project builds upon the original Pomelo design.
- Hardware Design (schematics, PCB layouts, design files): CC0 1.0 Universal
- Firmware and Software: MIT License
This project is completely open source and free to use, modify, and distribute.
Contributions are welcome! Please:
- Fork the repository
- Create a feature branch
- Make your changes
- Submit a pull request
For major changes, please open an issue first to discuss what you would like to change.
- Project Page: Pomelo on Crowd Supply
- Safecast: https://safecast.org
- Report issues through the repository's issue tracker
This board generates high voltage (up to 70V) for SiPM biasing. Handle with care:
- Do not touch HV traces while powered
- Use proper ESD protection
- Follow safe electronics handling practices
- Designed for educational and research purposes
Made with ❤️ for the radiation detection community