The mechanical diode

A diode allows current to travel in only one direction. With that in mind, [Alex] built a mechanical diode that will only allow gear rotation in one direction to be transmitted through the system. But wait, by connecting two of these devices together he’s built something of a mechanical rectifier. An electrical rectifier converts alternating current to direct current and this mechanical version outputs clockwise rotation no matter what direction of rotation is coming into the device.

There’s video which we’ve embedded after the break as well as many pictures on his site but not much explanation. Here’s what we’ve deduced. The two large gears are inputs. Mounted on top of them is a smaller ratcheting gear that will only turn in one direction. This ratcheting gear selects whether the smallest gear on the left or right will rotate, which then feeds the output gear at the top of this image.


  1. googfan says:

    Useless, but cool.

    Now he needs to build a resistor.

  2. mungewell says:

    The 2 large gears are directly coupled so there are contra-rotating.

    The 2 smaller coupled via the center gear, so must turn in the same direction.

    The ratchets allow slippage in one direction (small gear slips anticlockwise), so therefore the center gear always turns the same way.

    Very cool,

  3. Admiral Michael says:

    Very cool but loud.

  4. s0crates82 says:

    this may be how energy is harvested from kites

  5. anon says:

    Isn’t this how a clock pendulum works?

  6. belg4mit says:

    It’s not useless. Now my hamster can charge batteries, whichever way he decides to run in his wheel; physical rectification is almost certainly more efficient than electrical…

  7. terahz1 says:

    A sprag does the same thing also. Allows motion one way (that is until you break it)

  8. mungewell says:


    Your hamster probably doesn’t change direction at 60Hz ;-)

  9. Mythgarr says:

    Wouldn’t it be more efficient and much quieter to use a differential transmission between the two? Each gear has a ratcheting system in opposite directions, the drill is the input to the differential, the outputs are connected to both. The gear with a ratchet preventing rotation will have a very high mechanical resistance compared to the gear moving in the direction of the ratchet, causing the energy to be almost entirely transmitted to the “correct” side.

    Then again, I didn’t think to build this and put it up on YouTube, so he certainly deserves credit.

  10. What a great job.
    This does have some uses. Any system in which one would like to have multiple methods of input with one desired output could benefit from a system such as this. For example, a windmill which would accept wind flow in either direction without having to rotate. In a system such as that the direction of the wind would not affect the preformance of the system as the resultant output would be standardized.
    Great work [Alex].

  11. misha says:

    @googfan – absolutely not useless ! This would be perfect to use for harnessing wave energy. A floating “wing” would give you a left/right tilt as a wave travels under it. This “rectifier” would allow you to convert that into a constant spinning direction for driving a generator or other shaft that harnesses the power in some other way.

  12. Mikey says:

    Can’t a ratchet already do this?

    I don’t quite understand what’s new here.

    Or why it’s so loud.

  13. reboots says:

    Freewheel bearings are common, and take many forms; most bicycles have one. See:

    These would make a cheaper, sturdier, and more efficient “rectifier” mechanism. The resulting project wouldn’t look as dramatic however.

  14. JGN says:

    Duh. Sprag clutches are already part of every automatic transmission made.

  15. Spork says:

    What reboots said.

    This was really obvious using two ratchets, but the cleaner more effective answer was the freewheel. I had to work with these when one went bad on a family members’ bike. They’re not fun to rebuild, but they last forever, the bike was probably 10 years old.

  16. Tim says:

    Ha, misha beat me to it – first thing I thought of when I saw it was harnessing waves to drive something mechanically.

    As for a mechanical resistor, that’s easy – it’s called friction. Also, a lot of electrical stuff can be reproduced using water or gas circuits. Years and years ago my brother built a water transistor.

  17. cynic says:

    It’s loud because it’s two ratchets, one is always slipping while the other is locking the large wheel it’s on to the smaller wheel it’s resting against.
    A video with a stationary camera and coloured spots on each gear would demonstrate this better.

    Mechanical hacks, Everything old is new again :)

  18. grovenstien says:


  19. James says:

    Its a Nice model. Maybe made for teaching?
    Either way it shows how a mechanism that looks complicated at first is pretty simple at heart :)

  20. backSLIDER says:
  21. googfan says:

    @ everyone who replied to me.

    I would say that electical rectification is much more efficiant using a full wave rectifier. There is alot of mechanical energy lost in those ratchets.

  22. Manuel says:

    This is very similar to the method used in the winding mechanism of many automatic watches. The weighted rotor turns two gears with opposite ratchets turning a single winding wheel. has a picture of the winding mechanism used in most Rolex automatic watches and many other types of automatic watch.

  23. reboots says:

    @googfan, remember, you’re also losing electrical energy in the forward voltage drop of the rectifier diodes.

  24. Fry-kun says:

    Big whoop. I have an old watch from Russia which uses this type of mechanism to wind itself up when the user’s hand moves around. The watch itself is at the very least 20 years old, the tech I’m sure is much older.

  25. andrew says:

    I think I saw one of these in the Saw movies.

  26. googfan says:


    yes, but the energy required to crank those ratchet springs > .7 volt drop.

  27. MkMan says:

    Very cool build!

    My first thought, though, is that a worm drive does this with two gears.

  28. physic.dude says:

    It is like the same thing inside a VHS tape player.

  29. horzza says:

    Did this years ago with a Lego differential and two ratchets back to one axle. Perhaps if instead of ratchets, the system operated like the system which locks a seatbelt in a crash or when you pull it too fast, in one direction, then it could eb more efficient.

  30. idbar says:

    Two diodes not only make a rectifier, they can also make an amplifier. This one appears to be a noise amplifier ;-) Good work. It would be nice to see a working application :)

  31. Jo says:


    Indeed, using roller clutches would have been another (much quieter and visually confusing) option.

    I remember a different type of full-wave rectifier was used in dot-matrix printers to always spool the ink tape in the same direction regardless of the direction the head was moving (thus allowing printing in both directions)
    IIRC, it used 4 gears in a T shape, the bottom one being the input, and the outside two the outputs.
    The middle gear was not fixed but swung on an arm around the input gear with some friction, enough to only mesh with one output at a time.
    If the input gear was going clockwise, it would swing the middle gear to the right, meshing with the right hand output, and clearing the left output.

    The arm might only have been a semicircular slot in the chassis for the middle gear’s axle.

  32. TNW says:

    About mechanical or electrical rectification, I think the first method may be more efficient, but it could also be less reliable due to tear and wear on mechanical components.

    Electrical rectification may be less efficient but will be more reliable because semiconductor diodes don’t have tear and wear. (yeah I know, except if driven outside specs :) ).

    Still a nice setup, it never hurts to build things like this and experiment, even if it has been done before.

    Btw, I know a good use of this experiment, hook it up to an Arduino with a RTC, instant alarm clock with a lot of noise and rattling ;)

  33. gus says:


  34. Deyjavont says:

    So, what is the dynamic resistance of this diode? ~26mV/Id ..err ~26mRevs/N ? And where is the current limiting resistor?

  35. @anon lock up clock escapement on wikipedia about how a pendulum works. This is way too complicated and trouble prone to use for electrical power generation from wave action. An alternator will create current no matter what direction it turns. Let the alternator spin back and forth and use solid state rectifiers to create DC. Using something like this to create 60 cycle AC directly, would create AC so dirty the power company would never let it connect to the grid.

  36. Tim says:

    Get back to me when he’s built a “capacitor”, or a “transistor”

  37. googfan says:

    @ tim




  38. John says:

    This has been done before, It’s called a one way clutch, or a sprag cluth, modern day automatic transsmissions use them

  39. See I told you this would happen.. Steampunk is alive.. Shun your silicon, steam is the way forward :)

  40. Joe says:

    Build a resistor? All he needs to do is to hook this to the gas pedal from a Toyota.

  41. smoker_dave says:

    Ratchet and pawl, nothing new. Probably invented pre-Victorian era.

  42. malikaii says:

    I think the guy filming the video was on cocaine or something. Dude, zoom out and stand still.

  43. M4RK says:

    Almost a century late dude!

    If you knew anything about horology, you’d know this is a basic technique that winds “automatic” watches for nearly a century.

    Currently an automatic watch is on my wrist right now, a Tissot Le Locle Chronograph with a Valjous 7550 movement.

    Here are some really cool graphic explanations of that mechanism:

  44. Ivan says:

    Nice hack putting it all together. Lamest article title on hackaday ever.

  45. Pouncer says:

    Cool a clock mechanism.

  46. M4CGYV3R says:

    My socket wrenches just want you to know monodirectional ratcheting gears are nothing new. Somehow I don’t even think using them in this particular way is a new concept.

  47. lologram for hackaday, delivered in a roflcopter made from chocolate. now thats a hack.

  48. fartface says:

    These have existed for at least 1000 years.

    next on Hack a day, round things roll! someone has invented the WHEEL by hacking off the corners of a square….

  49. XSdB says:

    I have been thinking of ways to create components (capacitor, diode, resistor, inductor and a transistor) that operate with water. Water instead of electrons. A capacitor, for instance, would be a rubber balloon diaphragm in the middle of a pipe. A diode a check valve, and resistor a restriction in the pipe. The transistor I have not figured out yet, nor the inductor. (the inductor may be a vertical reservoir or something… maybe)
    This guy has gone a step further with gears.

    I’d like to see a “water amplifier” .. a basic equivilant to an audio amplifier made with the above described parts.

  50. Skyler says:

    A more comprehensive hack would be building a mechanical 8088.

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