Small project #9 — PC-fan tacho-signal spoofer

Some time ago I got an email. Yes it still happens.

Someone was having a problem with computer / server fans. These things can get insanely loud, think vacuum cleaner on maximum power. Some of the 40x40mm models spin up to 6000 rpm. Absolutely not an issue if servers are rack-mounted in a server-room, but if you like to keep one of those machines in your personal space, that is a big no-no. So said person replaced these fans with slowly-spinning and silent PC-fans (80×80 or bigger, I don’t really know). But, the server’s BIOS / fan-monitoring wasn’t happy. It thought the fans were failing and forcefully shut the server down.

I was asked if I could build a little something that would measure the tacho-signal of the slow-spin-fans and scale that up to values that match the signal from the previous fans, fooling the hardware-monitoring, making it believe everything was ‘as usual’.

I did. A nice weekend project. It works with my 3-wire PC-fans. I built the prototype, wrote (well, re-purposed actually) some code and sent a reply to the person mentioned above.

No answer… weird… maybe it got lost in the spam-folder.

Anyhow, here it is for the world to enjoy or laugh about:


The potentiometer didn’t make it onto the schematic. It is used to adjust the frequency-scaling.

  • Code: [1][2]
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2 Responses to Small project #9 — PC-fan tacho-signal spoofer

  1. Peter says:

    Hi, this is what I was looking for..thanks.
    I’d like to get some more info: what is that chip in the heart of the design?
    Where should I put the potentiometer exectly and what is its spec (kOhms)?
    Thanks in advance.

    • robert says:

      It is a micro-controller: ATtiny85 (overkill, but I had it in a box). The potentiometer is connected to one of its analog-to-digital converter inputs, the value doesn’t matter much: 10k should be fine.

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