Sh*t! But you are alive.
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You never answered that question.
Please ask, what to do concerning electronics if you’re unsure! A mistake could kill you!!!
Especially with something like this!!! Working with Line Voltage (110 V / 230 V / >50 V)
Never touch stuff above AC 50V or DC 120V, if you have not the proper training!
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I fear, you did this
Before connecting the “Tacho” signal to your MCU you should have measured the voltage with a multimeter. I expect, you would have measuered something below 5V, but above 3,3V or something below 12V, but above 5V (depends to your fan). Anyway, whatever you would have measured, this will kill your MCU, if connected like above.
I’ll keep my fingers crossed!
hcet14