Rpi3FanHatPlus

Raspberry Pi 5 HAT+ that drives the three fans of a case with a Microchip EMC2303 fan controller and breaks out the Pi's UART and a PPS input for a GNSS module (J2).

Features

EMC2303 I²C fan controller on the Pi's I²C1 bus (address 0x2E, ALERT on GPIO5).

Fan 1 / Fan 2 (J5, J6): standard 4-pin PWM fans with tachometer, e.g. Noctua NF-A4x10 PWM (12 V).

Fan 3 (J4): small 2-wire 12 V fan (up to 100 mA) that cools the GNSS module, switched on the low side by an AO3400A MOSFET with an SS14 flyback diode.

GNSS connector (J2): connects a GNSS module (a Unicore UM982 in this build) to the Raspberry Pi: UART0 (GPIO14/15), PPS on GPIO18, 5 V and GND.

12 V input (J3) for the fans, protected by a 0.5 A resettable fuse.

HAT+ ID EEPROM (AT24CS64) as required by the HAT+ specification.

4-layer board, 65 × 56.5 mm HAT+ outline, M3 mounting holes. Designed to sit on top of a PoE HAT.

Connectors

RefFunctionPinoutJ1Raspberry Pi 40-pin GPIO (Samtec HLE-120-02-F-DV-BE-A, bottom-entry SMT socket)Standard Raspberry Pi headerJ2GNSS module to Raspberry Pi (2×6, 2.54 mm, right angle)1: PPS → GPIO18 · 3: +5 V · 5: GPIO14 TXD (Pi → GNSS RX) · 7: GPIO15 RXD (GNSS TX → Pi) · 9: GND · others not connectedJ312 V input (1×2, 2.54 mm)1: +12 V · 2: GND — not keyed, check polarityJ4Fan 3, 2-wire (1×2, 2.54 mm)1: +12 V · 2: Fan − (switched)J5, J6Fan 1, Fan 2, 4-pin PWM (Molex 47053)1: GND · 2: +12 V · 3: Tach · 4: PWM

GNSS signals on J2 are 3.3 V logic: the module's UART and PPS must not exceed 3.3 V.

Raspberry Pi resources

SignalGPIONotesI²C SDA / SCLGPIO2 / GPIO3EMC2303 at 0x2EFan controller ALERTGPIO5Active low, 4.7 kΩ pull-upUART TXD / RXDGPIO14 / GPIO15GNSS module (J2)GNSS PPSGPIO18HAT+ ID EEPROMGPIO0 / GPIO1

Example config.txt lines:

dtparam=i2c_arm=on

enable_uart=1

dtoverlay=pps-gpio,gpiopin=18

Fan controller notes

PWM1/PWM2 reach the 4-pin fans through non-inverting 2N7002 stages: the EMC2303's default PWM polarity and open-drain output are correct.

The fans stop at 0 % duty. At power-up the EMC2303 watchdog drives all fans to full speed after 4 s until the host writes a fan setting.

Fan 3 has no tachometer: run it in direct-drive mode (no RPM control loop).

Manufacturing

The HATPlus/fab/ folder contains:

Rpi3FanHatPlus-gerbers.zip: Gerber (4 copper layers, masks, paste, silkscreen, outline) and Excellon drill files.

HAT+-BOM-PCBWay.csv: bill of materials with manufacturer part numbers and LCSC codes.

HAT+-CPL-PCBWay.csv: component placement file, in millimetres, top side only.

The files are prepared for PCB fabrication and assembly (PCBA) at PCBWay. Board: 4 layers, 1.6 mm, ENIG recommended, minimum track/space 0.15 mm, minimum drill 0.4 mm.

Design files and tools

KiCad 10 project in HATPlus/ (HAT+.kicad_pro); project footprint libraries in HATPlus/libs/ and libs/.

The schematic review, design changes, footprints and fabrication outputs were done with Konnect, an AI-assisted PCB design tool for KiCad 10, driven by Anthropic's Claude through Claude Code.

Credits

Based on the Raspberry Pi HAT+ KiCad template by Helge Wurst (CERN-OHL-S v2).