Fan Tach Modifier

Allows an Arduino to change the reported speed of a fan by reading the signal from the tachometer wire and creating a new modified output signal. Optionally supports using a potentiometer to set the modification factor between fan speed and output speed.

This is useful to trick servers/routers/switches/etc. into being happy running with slower fans when it expects a much higher RPM to not complain.

The following boards have a preconfigured configuration:

• Digispark Pro (One fan)
• Arduino Uno/Nano (One fan)
• Teensy 2.0 (Two fans)
• Arduino Mega (Four fans)

And easy custom configuration to allow any other board with one 16-bit timer and free hardware interrupt pin per fan configured to be used.

Some Specifics

Uses Timer1/3/4/5 for each fan respectively as these are usually the 16-bit timers if the board has them.

For each fan x (0-3), the following pins need to be configured:

• IN_PIN_x
  • A hardware interrupt pin. Connected to the fan to read its actual RPM.
• OUT_PIN_x
  • Any free non-restricted pin. Connected to the end device to output the modified signal to.
• POT_PIN_x (Optional)
  • An analog input pin. To use a potentiometer to set the modification factor between fan speed and output speed.
  • Can also define the same pin to control multiple fans from the same potentiometer.

Example fan math:

• In (Noctua NF S12A):
  • Fan speed 300-1200 RPM
  • 600-2400 half_revolutions/min
  • 10-40 Hz
  • Input interval received: 100-25 ms
• Out (Emulating: Nidec V12E12BS1B570):
  • Fan speed 1600?-6400 RPM
  • 3200-12800 half_revolutions/min
  • 53.33-213.33 Hz
  • 18.75-4.69 ms
  • Output interval resolution count sent: 4687-1172 counts
• Factor of 5.333, in this case calculated from the max fan speeds.