This is a 61.5mm by 15.24mm ATtiny85 board for controlling two PWM fans using a single potentiometer.  The design principle is to use extra components to make it safer and more robust in use.

Power is supplied from the 12VDC barrel jack (5.5mm by 2.1mm), extending about 5mm further than the board.

The 2023-07-31 version includes suitable pads for a 3.81mm pitch screw terminal (plus an awkwardly placed 5.08mm pitch screw terminal).

12VDC is directly provided for the fans, and regulated to 5V for the ATtiny85 via a ferrite bead using a TO-92 linear regulator.

PWM outputs use Timer1.  For timing purposes, Timer0 can be used.

PWM outputs and tachometer inputs are protected by NX138AK N-channel enhancement mode MOSFETs.  This means that accidentally shorting PWM or TAC to 12V at the fan or fan connector does not damage anything.  (Both signals are inverted on the ATtiny85.)

If the internal pull-ups (20kΩ to 50kΩ) are too weak for the tach inputs, the 0603+0805 pads on the backside (labeled PU) can be used for external pullups.

Adjustment is by remote potentiometer (10k) connected to the JST PH-3 (2.0mm pitch) connector.

There are two 2.54mm diameter mounting holes (at 27.2mm center to center distance), suitable for M2.5 standoffs. UNC #2-56 and #3-48 also fit.

Board top and bottom:

3D renderings of the board:

For the 2023-08-10 version, the rings around the mounting holes were separated from ground.  When mounted with M2.5 or #3-48 hardware, the white rings will remain fully visible around the screw head; I recommend button-head screws for this.