AutoFan

A custom CircuitPython controller for an AC Infinity Cloudline S6 axial fan that can automatically turn on based on a current sensor. Useful for external exhaust fans on laser systems — the fan engages automatically whenever the laser starts lasing.

The controller supports two modes:

• Auto — the fan runs at full speed whenever the current sensor detects the monitored load is on, and shuts off when it turns off.
• Manual — up/down buttons step through the 8 fan speeds by hand.

An 8×8 LED matrix shows the current speed.

Hardware

The AutoFan is deliberately built from off-the-shelf breakout boards rather than a from-scratch custom design. Leaning on existing, well-documented modules (microcontroller, current sensor, LED display, buck converter, and IO expander) kept the proof-of-concept cheap and fast to bring up — no custom silicon or low-level board bring-up, just wiring together known-good parts and focusing the effort on the firmware. The Eagle PCB in design/eagle/ simply carries and connects these same modules for a tidier, more permanent build.

The controller runs on an Adafruit Trinket M0 (SAMD21). Full bill of materials (reference designators match the schematic):

Bill of materials

Ref	Part	Qty	Purpose
U2	Adafruit Trinket M0	1	Microcontroller (SAMD21)
—	Modern Device current sensor	1	Non-contact load detection
U3	Adafruit 8x8 LED matrix (HT16K33)	1	Speed display
U1	Adafruit MPM3610 3.3V buck converter	1	10 V → 3.3 V power
IC1	Microchip MCP23008	1	IO expander for buttons / switch
T1	2N2222 (P2N2222) NPN transistor	1	Drives the fan's 10 V PWM line
VR1	10 kΩ trim potentiometer	1	Sensor threshold adjust (A3)
R2, R3	2.2 kΩ resistor	2	I2C SDA/SCL pull-ups
R4, R5, R6	1 kΩ resistor	3	Button/switch current limiting
R1	470 Ω resistor	1	Transistor base resistor
C2	0.1 µF capacitor	1	MCP23008 decoupling
S1, S2	Momentary push button	2	Up / down speed
S3	SPDT switch	1	Manual / Auto mode select
—	4-pin "molex" ATX HDD connector	1	Fan connection (10 V / GND / PWM)

The fan's PWM control wire is pulled up to 10 V by the fan itself; T1 pulls it low under control of the Trinket's D4 output to generate the speed signal.

See design/eagle/ for the PCB and design/enclosure/ for the 3D-printable case.

Schematic

The editable schematic is in design/eagle/Autofan.sch (Autodesk EAGLE).

Current sensing

Load detection uses the Modern Device current sensor, which senses the magnetic field around a conductor rather than making any electrical connection to it. In practice you simply zip-tie the sensor alongside the load's AC power cable — no cutting, splicing, or breaking into the mains wiring. As the AC current through the cable rises and falls, the sensor picks up the changing field and outputs a proportional analog voltage that the Trinket reads on A0. Because it never touches the conductor, the sensor stays galvanically isolated from mains, which keeps the low-voltage controller safe.

The firmware compares this reading against a threshold (set by the trim pot on A3); when the monitored load draws current, AutoFan switches the fan on. See Calibration for how to dial in the threshold.

Pinout

Derived from src/code.py:

Signal	Connection
Fan PWM output	D4
Current sensor input	A0 (analog)
Threshold trim pot	A3 (analog)
LED matrix + IO expander	I2C (SCL / SDA, 400 kHz)
Down button	MCP23008 pin 0 (active low)
Up button	MCP23008 pin 1 (active low)
Mode switch (Manual/Auto)	MCP23008 pin 2

Firmware

The controller runs CircuitPython.

Dependencies

The Adafruit libraries under src/lib/ are bundled for convenience and come from the Adafruit CircuitPython Bundle (MIT licensed):

• adafruit_bus_device
• adafruit_debouncer
• adafruit_ht16k33
• adafruit_mcp230xx

Their copyright notices and the MIT license text are retained in src/lib/THIRD_PARTY_LICENSES.txt.

Install

1. Install CircuitPython on the Trinket M0 (see the Adafruit guide).
2. Copy the contents of src/ to the board's CIRCUITPY drive:
   • code.py → the drive root
   • the lib/ folder → the drive root
3. The controller starts automatically. Open the serial console to see status messages.

Calibration

Use the scripts in src/test/ to read the sensor voltage and to regenerate the PWM speed table. See that folder's readme for details.

Testing

TODO: Provide details for test rig to verify the AutoFan is working.

Repository layout

src/
  code.py            main controller firmware
  lib/               bundled Adafruit CircuitPython libraries
  test/              sensor / PWM calibration scripts
design/
  eagle/             PCB schematic and board (Autodesk EAGLE)
  enclosure/         3D-printable case and button/switch caps

Reverse-engineered OEM controller data

Details captured from the original AC Infinity controller, used to match the fan's expected PWM signal.

The stock controller has one button that cycles through 8 fan speeds and 8 LEDs that indicate the current speed. It connects to the fan via a 4-pin "molex" ATX HDD power connector.

The fan connection provides three lines:

1. 10V DC power
2. GND
3. PWM fan-speed control wire (pulled up to 10V by the fan)

Fan speed is controlled by grounding and opening the 10V control wire to create a PWM signal.

Current draw

• Controller logic: ~9.2 mA at rest, ~18.2 mA at full speed (all LEDs on).
• PWM signal line: 1 mA when grounded (left grounded to turn the fan off).

PWM timing

Based on a 245 µs cycle time — almost 4 kHz, likely the intended target frequency but slightly off due to drift.

PWM "high" times:

Speed	High time	Notes
0	0 µs	0% PWM (off)
1	~82 µs
2	~100 µs
3	~120 µs
4	~140 µs
5	~162 µs
6	~185 µs
7	~210 µs
8	~245 µs	100% PWM (full)

The stock controller transitions instantly between speeds 0 and 1, and smoothly transitions between the other speeds over ~2 seconds.

TODO

• Add digital trim for current sensor. (the manual trim pot is a bit of a pain)
• Add some hysteresis to handle flickering at the sensor threshold. Favor staying "on".

License

This project is dual-licensed to fit the two kinds of content it contains:

• Source code (src/) — MIT License.
• Hardware and design files (design/, including the Eagle PCB and 3D enclosure models) — Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0).

The bundled Adafruit libraries under src/lib/ remain under their own upstream MIT licenses; see src/lib/THIRD_PARTY_LICENSES.txt for their copyright notices and license text.