Fan Stroboscope

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Stroboscopic effects from case fans — FrankisGER

Disclaimer: As usual, if you try it, you take all of the credit and all of the blame. It’s all your responsibility, not ours.

Stroboscopic effects can cause seizures in those people prone to have them, so if you have friends/associates in that category, please be considerate of their well-being.

Now if you end up with a runaway case fan which acts out being a sawmill and cuts your computer or house or sister in two, we’d sure like to hear about that, but it’s still your bad. :)–Ed

What This Is

Last week, I came across this
at a friend’s site. They took a fan with transparent blades,
drilled 4 holes to the sides of the fan and inserted LEDs. It
looked like this:

Nice, eh? I thought the same.

I got the idea of improving the effect by making the LEDs blink very fast.


might have seen it in TV…sometimes when the wheels of a moving car are
shown in TV, it looks as if they stop, move forward or backward, although
the car is driving at almost constant speed. Wouldn´t it be nice to see the
same effects with the fan´s blades?

I thought about it some time and quickly
came to a solution. Yesterday, I posted it to the Hardforums
and I was told, that bit-tech had already done something
. Damn. I thought I was the first one.

However, the effect I get is
a bit different, because bit-tech just used short impulses (1/100 time
on; 99/100 time off), while my circuit works at a duty cycle of 50 % (1/2
time on; 1/2 time off). I build my circuit as planned and it works great.

What Happens

At certain frequencies, the blades seem to

  • stay still
  • rotate very slow (with changing direction)
  • have doubled or tripled (you see 20 blades
    or so)

    The effects often change a bit, because
    the rpm of the fan is never 100 % constant.

    My LEDs are too weak, to
    see the effects at daylight. But when it’s dark, everything is clearly visible.
    I´m sorry that I can´t show you any pictures on how it looks
    like – my web-cam just isn´t fast enough to capture these effects
    in the dark.

    How To Do This

    frequency of the blinking LEDs is adjustable from ~20 Hz to ~100 kHz.
    The frequency setting works with a very big potentiometer with the size of 1 Mega Ohm (1M Ohm), which gives
    you a wide range to find desirable stroboscopic patterns.

    Once you find an effect you like, the smaller
    10 kilo Ohm (10k Ohm) potentiometer allows you to fine-tune it.

    To save the LEDs from damage, I used a 40 ohm pre-resistor
    for them (I used 4 normal green LEDs in paralell). You can calculate the
    pre-resistor the normal way (input voltage is 5 Volt).

    The parts I used to generate the frequency are:

  • a Hex Schmitt Trigger 74HC14 (could be probably
    exchanged with a 74HC04)

  • one capacitor (0,047 µF; foil type; 63
    Volt rated)

  • two potentiometers (10k Ohm and 1M Ohm; no power rating

    The frequency may be generated in every other way, too (for example
    555 timer based). The transistor, a normal npn one, should be able to provide
    enough current for the LEDs.

    And yes: (I don´t know how often I answered
    the question already) potentiometers have 3 contacts; my circuit just uses
    2 of them; the middle contact and one of the side contacts (which one doesn´t


    The Circuit plan

    What You Need And What It Costs

    -1 piece 74HC14 (cost ~ 50 Cent)

    -1 piece BC547 (cost ~ 10 Cent)

    -1 piece 10 kohm potentiometer (cost ~
    5 Cent; type doesn´t matter)

    -1 piece 1 Mohm potentiometer (cost ~
    5 Cent; type doesn´t matter)

    -1 piece 40 ohm resistor (**depending
    on how much / strong leds are used) (cost ~ 2 Cent)

    -4 leds (my one are 1,5 V @ 20 mA) (cost
    ~ 50 Cent)

    -1 capacitor 0,047 µF (cost ~ 10

    -some PCB to solder the parts on




    If you have any questions or additions, visit
    the thread in Hardforums
    or send an email.

    FrankisGER aka Christoph Jadanowski

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