Tinyfan

tinyfan

Temperature-controlled fan driver for a PC power supply. An ATtiny85 reads an NTC thermistor and drives a standard 4-pin PWM fan from the PSU's own +5 V and +12 V rails. Two-layer 26.55 × 18.05 mm board, all SMD parts on the top side, designed in KiCad 10.

How it works

Connectors:

PB2 and PB4 are unused.

Firmware

firmware/tinyfan/tinyfan.ino is an Arduino sketch for ATTinyCore, built with arduino-cli (no IDE needed):

cd firmware
make deps                 # once: install the ATTinyCore board package
make                      # compile -> build/tinyfan.ino.hex
make fuses PROGRAMMER=usbasp   # once per chip: 8 MHz internal RC, BOD 4.3 V
make flash PROGRAMMER=usbasp   # upload over ISP

What it does, once a second:

1. Reads the NTC on ADC3 (16-sample average), converts with the Beta equation (10 kΩ @ 25 °C, B = 3435). Open or shorted sensor forces 100 %.
2. PI controller around a 55 °C setpoint: duty = 20 % + 3·err + 0.05·∫err, clamped to 20–100 %, integral clamped for anti-windup. Above 70 °C the fan goes to 100 % immediately.
3. Writes the duty to Timer1 PWM on PB1 at ~24 kHz. Q1 inverts it, so the register value is TOP − duty; at 100 % the output is detached and held low so Q1 is off and the fan's PWM line is released.

A 2 s watchdog restarts the chip if the loop stalls; the fan runs at 100 % whenever PB1 is not actively driven (reset, boot, dead MCU).

The board has no ISP header. Program the ATtiny85 before soldering (SOIC-8 adapter) or with a SOIC-8 test clip. In-circuit, MISO (PB1) is loaded by R3 and Q1's base, which may make clip programming unreliable.

Layout

tinyfan.kicad_pro / .kicad_sch / .kicad_pcb   KiCad 10 project
gerbers/      Gerber X2 (Protel extensions) + Excellon drill, as sent to the fab
production/   bom.csv (JLCPCB columns), bom_pcbway.csv (hand-edited, with MPNs),
              tinyfan-all-pos.csv (JLCPCB CPL format, SMD parts only)
lib/          digispark_attiny85.kicad_sym — early Digispark symbol, not used by
              the current schematic (it uses the stock MCU_Microchip_ATtiny lib)
firmware/     ATtiny85 sketch + Makefile (arduino-cli, ATTinyCore)
Makefile      regenerates everything above with kicad-cli

Regenerating outputs

Requires kicad-cli from KiCad 10 on PATH (override with KICAD_CLI=...).

make            # ERC + DRC, gerbers, drill, BOM, pos file
make zip        # build/tinyfan-gerbers.zip for the fab
make pdf        # build/tinyfan-schematic.pdf

ERC is clean. DRC reports 15 silk_edge_clearance warnings (J1 and J2 silkscreen outlines cross the board edge); make drc prints them but only errors fail the build.

BOM

Working on it with Claude Code

The project is set up for the kicad plugin (file-based KiCad MCP server, no running KiCad needed):

claude plugin marketplace add ProductOfAmerica/mcp-server-kicad
claude plugin install kicad