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GPU Fan Controller

ESP32-S3 based fan controller for GPUs that lack Linux software fan control (like the AMD Radeon Pro V620).

Architecture

┌─────────────────────────────────────────────────────────────┐
│ LINUX PC                                                    │
│                                                             │
│  ┌──────────────────┐     ┌──────────────────────────────┐ │
│  │ gpu_fan_ctrl.py  │────►│ max junction, both cards     │ │
│  │ (systemd svc)    │     └──────────────────────────────┘ │
│  └────────┬─────────┘                                       │
│           │ USB Serial: "SET 75"                            │
└───────────┼─────────────────────────────────────────────────┘
            │
            ▼
┌─────────────────────────────────────────────────────────────┐
│ ESP32-S3                                                    │
│                                                             │
│  • Receives speed commands (SET 0-100)                      │
│  • Outputs 25kHz PWM on GPIO5 (MOSFET gate)                 │
│  • Tach unused (3-pin fans)                                 │
│  • WATCHDOG: No command in 5s → 100% speed (failsafe)       │
│                                                             │
└───────────┬─────────────────────────────────────────────────┘
            │
            ▼
┌─────────────────────────────────────────────────────────────┐
│ MOSFET-DRIVEN 3-PIN FANS                                    │
│                                                             │
│  • 12V from PSU/header, split to all four fans              │
│  • ESP PWM (GPIO5) drives an N-channel MOSFET gate          │
│  • MOSFET switches the fans' shared ground                  │
└─────────────────────────────────────────────────────────────┘

Wiring

Two GPUs, each with two 30mm 3-pin fans (four fans total), all driven together from a single ESP32 PWM output through a low-side N-channel MOSFET. 3-pin fans have no PWM input pin: speed is controlled by switching their shared 12V rail, not by feeding them a logic signal, so the MOSFET is required.

MOSFET Fan Driver

GPIO5's existing 25kHz PWM drives the MOSFET gate; the MOSFET switches the fans' shared ground. All four fans get 12V in parallel through 3-pin splitters.

                 +12V   (motherboard fan header yellow  —or—  PSU 12V rail)
                   │
     ┌─────────────┼──────────────────────────────────────┐
     │             │                                       │
     │        ┌────┴─────┐                            cathode (striped end)
  3-pin       │  4 fans  │  all + wires → +12V             ┿  1N5819
  splitters   │    in    │                                 │   (flyback diode)
  fan out     │ parallel │  all − wires → node A           │
  the rail    └────┬─────┘                            anode │
     │             │ node A                                 │
     └─────────────┼────────────────────────────────────── ┘
                   │   (node A = every fan's − wire, bundled together)
                   │
                   │ Drain
               ┌───┴───┐
 GPIO5 ─[220Ω]─┤ Gate  │   N-channel LOGIC-LEVEL MOSFET
   (PWM 25k)   └───┬───┘   (AO3400 for this current, or IRLZ44N)
       │           │ Source
     [10kΩ]        │
       │           │
      GND ─────────┴───────────────  COMMON GROUND
                                     (ESP32 GND + PSU/header GND all tied here)

Connection Table

From To Notes
ESP32 GPIO5 220 Ω → MOSFET gate existing PWM output, no firmware change
MOSFET gate 10 kΩ → GND pulldown; keeps fans off while the ESP boots/floats
MOSFET drain node A (all four fan − wires)
MOSFET source common GND
All four fan + wires +12V rail bundled via 3-pin splitters
Flyback diode anode node A (drain) 1N5819
Flyback diode cathode (stripe) +12V clamps inductive kickback
ESP32 GND common GND mandatory — ESP and fan power must share ground

ESP32-S3 Pinout

GPIO Function Notes
5 PWM OUT 25kHz, drives the MOSFET gate through 220 Ω
GND Ground Must be common with the 12V supply ground

Tach (GPIO4) is left unconnected: with low-side switching each fan's ground swings toward 12V during the off-phase, which both corrupts the reading and would push ~12V into a 3.3V pin. RPM reports 0; nothing depends on it.

Wiring Notes

  • Common ground is mandatory. ESP GND, MOSFET source, and the 12V supply ground must all tie together or nothing switches. Powering the fans from a spare PSU 12V connector (rather than a mobo header) keeps the board from doing its own control on that rail. If you use a header, set it to 100%/full in BIOS.
  • Use a logic-level MOSFET (fully on at a 3.3V gate): AO3400 or IRLZ44N. A plain IRF540 will not work.
  • The 10 kΩ gate pulldown holds the fans off during the second or two between power-on and firmware init, then they spin up. Harmless.
  • No firmware change needed. GPIO5's existing 25kHz PWM drives the gate as-is: higher duty = more power = faster, and the lost-link watchdog (100%) now means full airflow on failure. Tach is unused, so RPM always reports 0.

USB Connection

Connect ESP32-S3 to internal motherboard USB 2.0 header using a USB-C to USB 2.0 header cable:

  1. Plug USB-C end into ESP32-S3 COM port
  2. Connect header end to motherboard (e.g., JUSB2)
  3. Route cable inside case

Fan Fan-Out (Splitters)

All four 3-pin fans run off one switched rail, so they wire in parallel:

  1. Bundle every fan + (12V) wire to the +12V rail (3-pin splitters, one per GPU or a single 4-way, whatever fits the routing).
  2. Bundle every fan − (ground) wire together into "node A" and run that to the MOSFET drain.
  3. Leave every fan tach wire disconnected (see the pinout note above).
  4. Tie ESP32 GND, MOSFET source, and the 12V supply ground together.

Building the Firmware

Prerequisites

# Install ESP-IDF (if not already)
# https://docs.espressif.com/projects/esp-idf/en/latest/esp32s3/get-started/

# Set up environment
. ~/esp/esp-idf/export.sh

Build & Flash

cd /path/to/gpu_switch

# Configure (first time)
idf.py set-target esp32s3

# Build
idf.py build

# Flash (adjust port if needed)
idf.py -p /dev/ttyACM0 flash

# Monitor (for debugging)
idf.py -p /dev/ttyACM0 monitor

ESP32 Serial Protocol

Commands (case-insensitive):

Command Description Response
SET <0-100> Set fan speed % OK: <value>
GET Get current speed SPEED: <value>
RPM Get fan RPM RPM: <value>
STATUS Full status Speed, RPM, watchdog state
PING Connectivity check PONG
WD Watchdog status Timeout, last command time

Watchdog

If no SET or PING command received within 5 seconds, ESP32 automatically sets fan to 100% as a safety measure.

Linux Service Installation

cd linux_service
sudo ./install.sh

Manual Testing

# Test serial connection
echo "PING" > /dev/ttyACM0
cat /dev/ttyACM0  # Should show PONG

# Test service manually
sudo python3 /opt/gpu_fan_ctrl/gpu_fan_ctrl.py -v

# Check GPU temp sensor
cat /sys/class/drm/card*/device/hwmon/hwmon*/temp1_input

Service Management

# Enable on boot
sudo systemctl enable gpu-fan-ctrl

# Start/stop
sudo systemctl start gpu-fan-ctrl
sudo systemctl stop gpu-fan-ctrl

# Check status
sudo systemctl status gpu-fan-ctrl

# View logs
journalctl -u gpu-fan-ctrl -f

Configuration

Edit /etc/gpu_fan_ctrl/config.yaml:

serial_port: "/dev/ttyACM0"
poll_interval: 2.0

# Fan curve: [temp_C, speed_%]
# Step function at 35C, then linear ramp to 100% at 75C
fan_curve:
  - [35, 0]      # 35C and below -> fan off
  - [35.1, 25]   # Just above 35C -> step to 25%
  - [75, 100]    # 75C -> full blast

hysteresis_c: 2.0      # Temp change threshold
min_speed_change: 3    # Speed change threshold

The service reads junction temperature (hottest spot on GPU die) - preferred for LLM/compute workloads where sustained high utilization causes heat buildup.

Troubleshooting

ESP32 not detected

# Check USB devices
lsusb | grep -i espressif

# Check serial port
ls -la /dev/ttyACM*

# Check permissions (add user to dialout group)
sudo usermod -a -G dialout $USER
# Then logout/login

GPU temp sensor not found

# Check for amdgpu driver
lsmod | grep amdgpu

# Find hwmon devices
ls /sys/class/hwmon/*/name
cat /sys/class/hwmon/*/name

# Try rocm-smi if available
rocm-smi --showtemp

Fan not spinning

  1. Check 12V power from motherboard header
  2. Verify BIOS fan header is set to 100% or DC mode
  3. Test PWM signal with oscilloscope/multimeter
  4. Try SET 100 command manually

Parts List

  • ESP32-S3 DevKit with USB-C (~$8)
  • USB-C to USB 2.0 header cable (~$8)
  • 4× 30mm 3-pin fans (2 per GPU)
  • Logic-level N-channel MOSFET (AO3400 or IRLZ44N)
  • 220 Ω gate resistor + 10 kΩ gate pulldown
  • 1N5819 (or 1N400x) flyback diode
  • 3-pin fan splitters to fan out the 12V rail
  • Jumper wires / Dupont connectors
  • (Optional) 3D printed enclosure to mount inside case

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ESP32-S3 based PWM fan controller for AMD GPUs lacking Linux software fan control

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