Propeller and machine gun timing

[Matt] was looking for a challenge. Inspired by the machine gun setups on World War I planes he wanted to make a gun that can shoot between the blades of a spinning propeller. The original guns used an interrupter gear that synchronized machine gun firing with the engine mechanically. [Matt] set out to do this using a microcontroller.

To make this work there are two important pieces of information; how fast is the propeller spinning right now, and how long does it take for the pellet to pass the blade? [Matt] used an oscilloscope and some infrared sensors to establish the firing delay at about 20-22ms. Another sensor shows the propeller is spinning at 500 RPMs, with some simple calculations showing that there is indeed a big enough window of time to fire between the blades. After testing with a visible LED and then building out the rest of the circuitry he accomplished his goal. He even added a test function that purposely hits the blades just to see how accurate the system was. We hope this shows up in a Red Baron RC replica, or other flying arsenal.

[via Hacked Gadgets]


  1. biozz says:

    i thought some guy 60 years ago make a simple gar for this? XD

    but yeah i love the concept XD

  2. Mythgarr says:

    For those non-micro purists, you could probably implement this using a rotary encoder and an AND gate. Simplest digital solution I could think of would be to have the gun fire only at position 11..1’b (for an n-bit encoder) and tie those inputs into an n+1 bit AND gate with the active-high trigger.
    This setup mimics the mechanical operation of the interrupter gear pretty closely, where the physical bump on the rotor and the trigger must both be active in order for the machine gun to fire.

  3. biozz says:


  4. awz says:

    This would be awesome in a airsoft war with a rc plane, monitor and a semi auto machine gun, sit in a fort while gunning down your enemies. I can also see this working quite well on my neighbors cat…

  5. ChalkBored says:

    This is a project that would benefit from the usage of high speed photography in the documentation of it.

    @biozz, try nearly 100 years ago.

  6. wouter says:

    omg, this would be so awesome. I really really hope someone puts this in his RC plane. But I’m wondering how much work (and weight) it should be..

  7. bob says:

    RPM not RPMs*

    an edge detector for an ir sensor pointed at the blade would not require the encoder.


  8. Concerned Citizen says:

    This is dangerous!

    What if his workbench takes off and starts massacring people?

  9. saimhe says:


    …and the detector must be moveable so that one can set the timing like in a gas engine.

  10. Mattman says:

    Some of the casio Exilim series of digital cameras can shoot 1000fps, at low resolution. Since the bullet is moving at about 100 feet per second, we could get 10 frames per foot of travel of the bullet. It would be interesting to try…

  11. Andrew says:

    My amusement would be complete if he had only used a parallax propeller mcu.

  12. Spayum says:

    Or, you know….. he could’ve just built an interrupter gear. Which in the greater scheme of things would’ve gotten him a thousand times more respect form me. I’m tired of people stooping to using computers to solve problems that are easily solvable using analog solutions.

  13. alan turing's dog says:

    “I’m tired of people stooping to using computers to solve problems that are easily solvable using analog solutions.”

    That’s great. Let me guess, you routinely solve route-optimization problems using bits of string, and your carb-equipped flivver always starts on the first kick of the hand-cranked starter.

    You have a large collection of vintage tube operated radio and stereo gear, and you can design an accurate eighth-order notch filter using only a slide rule and a cup of tea. Your checkbook is accurately balanced on a small portable difference engine. The sign at your favorite tavern says “We don’t serve square waves here” and your motorcycle of choice is an aging gear-valved ducati which smokes a little.

    If this is the case, your definition of easy might be different from most folks. That’s OK. But having seen dismantled interrupter gear mechanisms designed for cannon (BF109) and machine guns, I’d have to say it might be time to cut a new cam for your inference mechanism.

    I’ll take 25 lines of assembly and a photo-interrupter any day over 25 lbs of dead weight and twisted shafts. General Electric apparently thought so too, which is why almost all aircraft machine guns are of the electrically fired Gat variety.

    PS – as a small semi-fictitious talking dog, I’m somewhat offended by your user name.

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