Arduino Hat Design Inquiry
Worked for 4 mins 2 sec
Part selection is underway. I found a PN532 RFID IC and a 5 V relay with verified 120 VAC-capable contacts; the wide-input buck IC search needs a library fallback because the first pass only found module-style entries.
Worked for 4 mins 32 sec
Main ICs are added. I’m now adding the required passives and will wire power, SPI level shifting, microSD, relay driver, and safe relay contact terminals.
Worked for 6 mins 32 sec
I started the fully integrated Arduino Uno shield schematic and renamed the project to Arduino RFID SD Relay Shield.
Completed:
- Added PN532 RFID IC, microSD socket, relay, relay driver, flyback diode, MP1584 buck, 3.3 V LDO, SPI level shifter, terminal blocks, Arduino header placeholders, and support passives.
- Wired the initial 28 V input, 5 V rail, 3.3 V rail, SPI level-shift path, relay driver, and relay contacts.
- Created:
Important: the schematic is not layout-ready yet. Review still reports unresolved floating pins, mainly because the MP1584 buck support network and PN532 crystal/antenna/matching/reset circuitry need to be completed from the datasheets before moving to PCB layout.
Complete missing circuits
Fix ERC warnings
Improve Arduino headers
Add protection features
Prepare for layout
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Design Notes — Arduino RFID SD Relay Shield
Requirements
- Arduino Uno shield form factor.
- Fully integrated IC/socket design, not plug-in modules.
- 13.56 MHz RFID reader.
- microSD card reader.
- Relay for a 120 VAC signal circuit.
- 28 VDC input bucked to 5 VDC.
Selected Architecture
- RFID: NXP PN532 13.56 MHz NFC/RFID controller, SPI interface to Arduino.
- Storage: microSD socket in SPI mode.
- Logic level strategy: Arduino 5 V SPI/control outputs pass through SN74LVC125A powered from 3.3 V; MISO is returned to Arduino from 3.3 V devices.
- Power: MP1584 buck regulator from 28 V to 5 V, then MIC5504-3.3 LDO for 3.3 V logic/RFID/microSD.
- Relay: 5 V PR2 relay driven by MMBT2222A low-side transistor with flyback diode.
Preliminary Power Budget
Table
| Load | Rail | Estimate |
|---|---|---|
| Arduino Uno from shield 5 V rail | 5 V | 150–250 mA typical allowance |
| PR2-5V relay coil | 5 V | 450 mW / 5 V ≈ 90 mA |
| PN532 + RF front end | 3.3 V | TODO: verify final current from PN532 datasheet/application circuit |
| microSD card | 3.3 V | up to ~100–200 mA transient depending on card |
| SN74LVC125A | 3.3 V | low mA |
Preliminary design target: size the 5 V buck for at least 1 A available output current. At 28 V input and 85% efficiency, 5 V / 1 A output implies about 210 mA input plus margin.
Current Schematic Status
- Main components and initial support passives have been added.
- Power, SPI, relay driver, and relay contact nets are partially wired.
- ERC still reports unresolved floating pins.
Open Design Items Before Layout
- Complete MP1584 datasheet extraction from direct PDF and add the exact feedback divider, bootstrap capacitor, compensation network, frequency setting, catch diode if required, and final inductor/capacitor values.
- Complete PN532 reference design: 27.12 MHz crystal/load capacitors, reset/strap pins, TVDD/PVDD/VBAT/VMID filtering, antenna matching network, and loop antenna layout.
- Replace placeholder Arduino headers with accurate Uno R3 shield headers and pin naming.
- Add test points for VIN_28V, 5V, 3V3, GND, relay drive, SPI_SCK_3V3, SPI_MOSI_3V3, and SD/RFID chip selects.
- Verify 120 VAC creepage/clearance in PCB layout; keep relay contact copper isolated from SELV logic copper.
- Re-run ERC until all violations are resolved or explicitly justified.
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