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Project Specification
Project Overview
Status: Draft schematic and auto-routed PCB complete; mechanical HAT outline still pending.
Raspberry Pi 5 HAT board that accepts 12V DC input, protects it with a 5A fuse and Schottky reverse-polarity diode, regulates to 5V for the Raspberry Pi 5, and exposes headers for an MPU6050 IMU, a NEO-8M GPS module, and a GPIO-driven status LED.
Intended Use
Prototype/learning HAT for powering a Raspberry Pi 5 from a 12V DC supply while connecting common I2C and UART modules.
What the Device Should Do
- Accept 12V DC through a 2-pin screw terminal.
- Protect the input path with a 5A fuse and reverse-polarity Schottky diode.
- Generate a regulated 5V rail for Raspberry Pi 5 power pins.
- Provide standard Raspberry Pi 40-pin female GPIO header connectivity.
- Expose I2C pins for an MPU6050 module.
- Expose UART pins for a NEO-8M GPS module.
- Drive a status LED from a spare GPIO through a current-limiting resistor.
Main Features
- 12V protected power input.
- 5V buck-regulated Pi supply.
- Raspberry Pi HAT-compatible 2x20 GPIO header footprint.
- 4-pin MPU6050 header: 3.3V, GND, SDA, SCL.
- 4-pin NEO-8M GPS header: 3.3V, GND, TX, RX.
- GPIO17 status LED.
System Architecture
Diagram
Hardware Subsystems
- Power input/protection: J1 KF128-5.08-2P-AA screw terminal, F1 Littelfuse RGEF500 5A resettable PTC, D1 SS56 5A/60V Schottky diode.
- Regulation: U1 Murata UHE-5/5000-Q12-C module, 9-36V input, 5V/5A output.
- Raspberry Pi interface: J2 2x20 Raspberry Pi 5 pin socket.
- Sensor headers: J3 1x4 MPU6050 I2C header and J4 1x4 NEO-8M UART header.
- User indication: D2 red LED and R1 330Ω current-limiting resistor on GPIO17.
Interfaces and Connections
Table
| Interface | Schematic Connection |
|---|---|
| 12V input | J1 pin 1 -> F1 -> D1 -> U1 +INPUT; J1 pin 2 -> GND |
| Pi 5V | U1 +OUTPUT -> J2 physical pins 2 and 4 |
| Pi GND | U1 -OUTPUT and input return -> all J2 GND pins |
| MPU6050 | J3 pin 1=3.3V from J2 3.3V pins, pin 2=GND, pin 3=SDA to J2 GPIO2/physical 3, pin 4=SCL to J2 GPIO3/physical 5 |
| NEO-8M GPS | J4 pin 1=3.3V from J2 3.3V pins, pin 2=GND, pin 3=GPS TX to J2 GPIO15/RXD physical 10, pin 4=GPS RX from J2 GPIO14/TXD physical 8 |
| Status LED | J2 GPIO17/physical 11 -> R1 330Ω -> D2 anode; D2 cathode -> GND |
Power and Runtime Expectations
External 12V DC supply required. No battery or charging support is included.
Power Tree and Power Budget
Assumption: Raspberry Pi 5 may require up to 5A at 5V for full-load operation and peripherals. U1 is rated 5V/5A, so the 5V rail target is 25W. At 90% efficiency, 25W output reflects to about 2.31A from a 12V input before margin. F1 is a 5A hold-current PTC with approximately 8.5A trip current, so it is sized above the expected 12V-side operating current.
Manufacturing and Assembly Expectations
Default assembly style is SMD/passive production style with through-hole headers and screw terminal where mechanically appropriate. PCB placement/routing exists, but the final Raspberry Pi HAT mechanical outline and mounting-hole review are still pending before manufacturing export.
Firmware-Relevant Hardware Requirements
- Enable I2C on Raspberry Pi for MPU6050 on GPIO2/GPIO3.
- Enable UART on Raspberry Pi for NEO-8M GPS on GPIO14/GPIO15.
- Configure GPIO17 as output for the status LED.
Physical Design Expectations
Board should be laid out as a Raspberry Pi HAT in a later layout phase, including correct 40-pin header placement and HAT outline/mounting if requested.
Important Design Decisions
- Used GPIO17 / physical pin 11 for the status LED.
- Used Pi 5V header pins directly from the buck output.
- Moved J3/J4 module header power to the Pi 3.3V GPIO header pins so I2C/UART module logic remains safe for the Raspberry Pi's 3.3V-only GPIO.
- Used a resettable 5A PTC fuse rather than a one-time cartridge fuse.
- U1 ON/OFF_CONTROL and TRIM are intentionally left open: datasheet says standard positive-logic ON/OFF is enabled when open; TRIM is open when unused.
Assumptions and Risks
- Buck converter is an external module footprint, not a discrete buck IC design.
- MPU6050 and NEO-8M headers are powered from the Raspberry Pi 3.3V pins to keep attached module I/O at Pi-safe 3.3V levels. Confirm attached modules operate correctly from 3.3V supply.
- Raspberry Pi HAT EEPROM ID circuit is not included because it was not requested.
- The Schottky series diode may dissipate significant heat at high current; layout should provide adequate copper area or consider an ideal-diode MOSFET approach if efficiency/temperature is critical.
Verification Results
- ERC checks requested for floating pins, power pins, pin-to-pin issues, undriven nets, single-pin nets, no-connects, passive ratings, and parts availability returned no open warnings/errors after wiring.
- Connectivity verified for 12V input chain, 5V rail, 3.3V module header rail, GND rail, I2C, UART, and status LED nets.
Change Notes
- Initial schematic created with selected library components and verified connectivity.
- Changed J3/J4 VCC from 5V to the Raspberry Pi 3.3V GPIO header pins for safer Pi I/O logic levels.
Project Overview
Intended Use
What the Device Should Do
Main Features
System Architecture
Hardware Subsystems
Interfaces and Connections
Power and Runtime Expectations
Power Tree and Power Budget
Manufacturing and Assembly Expectations
Firmware-Relevant Hardware Requirements
Physical Design Expectations
Important Design Decisions
Assumptions and Risks
Verification Results
Change Notes
