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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Safety Analysis — Arduino RFID SD Relay Shield
Status
Draft — schematic incomplete, layout not started.
Applicable Safety Concern
The board switches a 120 VAC signal circuit through relay contacts while low-voltage Arduino/RFID/microSD circuitry remains SELV. The relay contact side must be treated as hazardous until the external circuit is proven current-limited and isolated.
Hazard Summary
Table
| Hazard | Source | Mitigation | Status |
|---|---|---|---|
| Electric shock | 120 VAC relay contacts at J_RELAY | Maintain creepage/clearance, isolate contact copper, use rated terminal block and relay | Pending layout verification |
| Fire/overcurrent | Fault on relay contact circuit | External fuse/current limit required unless added on PCB | Open |
| Low-voltage damage | 28 V input transient/reverse polarity | Add input protection as needed before manufacture | Open |
| Relay coil transient | K1 coil | D_FLY flyback diode added | Implemented |
Isolation Analysis
Table
| Boundary | Primary Side | Secondary Side | Required Action |
|---|---|---|---|
| Relay contacts to 5 V/3.3 V logic | J_RELAY, K1 COM/NO/NC | Arduino headers, PN532, microSD, power rails | Enforce PCB creepage/clearance and avoid copper pours under relay isolation gap |
Open Items
- Confirm whether the 120 VAC circuit is truly signal-only/current-limited or can deliver mains branch current.
- If it can deliver hazardous current, add a fuse or require external fuse protection.
- Verify terminal block voltage/current rating and relay contact rating against actual load.
- Add PCB layout keepouts/slots if required to meet creepage/clearance.
- Do not export manufacturing files until ERC and safety spacing checks are clean.
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