PWM+fan stop

FANSTOP

An inline hardware adapter that adds true fan stop to 4-wire PWM fans and supply-PWM control to 2- and 3-wire fans.

Some fans keep spinning even when their controller outputs 0% duty cycle. Other fans expose only power and tachometer pins, so they cannot use the standard 4-wire PWM control input at all.

FANSTOP sits between a 12V fan controller and a fan. An ATtiny10 measures the incoming PWM signal and controls a high-side MOSFET in one of two selectable operating modes.

[!CAUTION] FANSTOP can remove power from a cooling fan. Validate the module, firmware thresholds, fan startup behavior, and thermal protection before using it on equipment that can be damaged by loss of cooling.

Features

• True power-off for fans that do not stop at 0% PWM
• Supply-PWM speed control for 2- and 3-wire 12V fans
• Hardware pass-through for PWM and tachometer signals
• Approximately 300ms low-duty debounce to reject short glitches.
• 5-10% hysteresis to prevent repeated switching around the stop threshold
• Static LOW and static HIGH input detection
• Compact two-layer PCB: approximately 24.5 x 11.15 mm
• ATtiny10 firmware written in AVR assembly
• Exposed TPI programming pads compatible with pogo pins or a 2.54 mm 2x3-style fixture
• KiCad 10 hardware sources and FreeCAD/STL programming-fixture files included.

Operating modes

Mode is sampled at startup using solder jumper JP1

4-wire fan-stop mode

This is the default mode.

The firmware accepts PWM periods from 240 to 500 timer ticks. At the 8 MHz MCU clock this corresponds to approximately 16-33.3 kHz, covering the nominal 25 kHz 4-wire PC-fan control signal.

2/3-wire supply-PWM mode

Bridge JP1 before powering the board. FANSTOP measures the incoming duty cycle and generates power PWM through the high-side MOSFET.

The output timing is derived from the measured input:

output frequency = input frequency / 100

For a nominal 25 kHz input, the fan supply is therefore switched at approximately 250 Hz. The tachometer line remains connected directly between input and output.

Because the entire fan supply is switched, fan compatibility, acoustic noise, tathometer quality, and reliable startup at low duty cycles depends on the particular fan.

Connections

Both fan connectors use the standard 4-wire PC-fan pin order:

Connect the controller or motherboard to PWM_IN (J1) and the fan to PWM_OUT (J2). Do not reverse the board direction or connector polarity.

The PWM input is pulled up to 5V on the module and is intended for the usual open-collector/open-drain fan-control interface. Verify compatibility before connecting a push-pull controller.

Electrical notes

• Nominal supply: 12V DC
• Logical supply 5V (generated by an L78L05 regulator)
• Controller: ATtiny10
• High-side switch: A03413 P-channel MOSFET driven through a 2N7002
• The power path uses 0.9 mm traces and was laid out with a 2A design target
• Tachometer and PWM signals are not galvanically isolated
• The board is not intended for 24 V fans; the fitted protection network is designed around a 12 V system

The 2 A figure is a layout target, not a certified continuous-current rating. The practical limit also depends on connectors, assembly quality, ambient temperature, MOSFET dissipation, and fan startup current.

Building the firmware

The firmware is a PlatformIO project and uses a pinned AVR toolchain that contains ATtiny10 support

git clone https://github.com/yvoronkin/FANSTOP.git
cd FANSTOP/firmware
pio run

The resulting firmware image is placed under: .pio/build/attiny10/firmware.hex

Programming

ATtiny10 uses the TPI programming interface. The board exposes five programming pads:

The tools/ directory contains a printable programming gixture in FreeCAD and STL formats.

Project status

FANSTOP is an actively developed hardware project. The firmware builds with PlatformIO, but fan controllers and fans vary considerably; treat electrical, thermal, acoustic, and tachometer compatibility as something to validate for each installation.

Contributing

Bug reports, oscilloscope captures, fan compatibility results, firmware improvements, and hardware revisions are welcome. When reporting a control issue, include the fan model, input PWM frequency and duty cycle, selected mode, and measured output behavior.

License

• Hardware design files: CERN Open Hardware Licence Version 2 — Strongly Reciprocal
• Firmware: GNU General Public License Version 2](./firmware/LICENSE)