I’ve finally forced myself to layout a small board. It’s been far too long anyway.
It consists of these building blocks: reference voltage, comparator, RC low-pass, gain.
There are better methods for the job, e.g. using a digital one-shot to create pulses independent of the triggering waveform… yes. As usual I just wanted something that works in this special case, something simple, small and cheap.
To make sure the standard firmware can be used without recompiling it, this board uses the same ADC-input (PC5, pin #28) as the voltage-based sensing.
To avoid an electrical short, you must remove R3 (10k) before adding this board!
R3 is in the bottom left quadrant, right above the red capacitor.
Before I forget, you need to add a current-sense resistor for this to work! I’ve previously described adding one here.
Parts and assembly:
Preparing the spacers:
Done with soldering:
A word of warning: this is a serious health hazard, as it involves making adjustments to a device with exposed 240V AC. Use an isolation-transformer and / or let somebody with proper qualifications do it. I shall not be held responsible for any harm that might come your way.
The “trick” is to adjust Vref in such a way that the signal after the comparator looks as shown above. There should not be any significant changes to the signal’s shape when the fan-speed is changed.
The “gain” should be adjusted to result in a stable output voltage of about 2.2V or so. This should translate to a displayed value of about 900 in fan-test mode. The reference voltage for the ADC is 2.5V!