YOUYUE 858D+ hotair station — BLDC Fan-speed mod

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.

Empty spot:


Parts and assembly:



Preparing the spacers:



Test fit:


Done with soldering:





Making adjustments:

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.

IMGP13214[x: 1ms/div – y: 50mV/div]

IMGP13216[x: 0.5ms/div – y: 50mV/div]

IMGP13215[x: 1ms/div – y: 1V/div]

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!


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24 Responses to YOUYUE 858D+ hotair station — BLDC Fan-speed mod

  1. Peter says:

    Ordered 3 fan mod pcbs thank-you! Was putting together mouser parts list, c2 on pdf is marked electrolytic? I was so impressed with what you have accomplished in rehabilitating this “Full of Hot Air” Station, I want to hab one. Cheaply of course, so I will be skipping the breakout board, since you did all the hard work… and mounting your fan pcb elsewhere, maybe post-ups in the housing and run wires. I might have to rebadge mine mwm 858d+luxe in your honor, madwormmod, stylize the mwm to look like a pulse train, add your smileyled somewhere…it’ll be cool man! Of course, anything would be cooler than youyue.

  2. Serhii says:

    Tried to run station with this mod and always have a warning at start “FUN SPEED”.
    RV1 = 71mV
    RV2 = 2.4V
    Firmware 1.43 atmega168
    What I do wrong?

    • Serhii says:

      At the schematic R1 is 1M but on your pcb 4.7M. How is it critical?

      • robert says:

        Did you enable the mod in the source code? I’ve added a lot of comments in there.

        I think I didn’t have 1M and adjusted the capacitor for the same RC time.

  3. Serhii says:

    How I can enable the mod?

  4. Marcus says:

    do you have a parts list?

    • robert says:

      With part numbers? No. I used ‘jelly-bean’ components.

      * LM2904: SOIC-8
      * capacitors / resistors: 0805
      * potentiometers: “Bourns TC33X”-compatible footprint

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