Clock Mechanism Goes Crazy For Arduino

You’ve doubtless seen those ubiquitous clock modules, especially when setting clocks for daylight savings time. You know the ones: a single AA battery, a wheel to set the time, and two or three hands to show the time. They are cheap and work well enough. But [Playful Technology] wanted to control the hands with an Arduino directly and, in the process, he shows us how these modules work.

If you’ve never studied the inside of these clock modules, you may be surprised about how they actually work. A crystal oscillator pulses a relatively large electromagnet. A small plastic gear has a magnetic ring and sits near the electromagnet.

Each time the polarity of the electromagnet flips, the ring turns 180 degrees to face the opposite magnetic pole to the electromagnet. This turns the attached gear which is meshed with other gears to divide the rotation rate down to once per 24 hours, once per hour, and once per minute. Pretty clever.

That makes it easy to control the hands. You simply detach the electromagnet from the rest of the circuit and control it yourself. The module he used had a mechanical limitation that prevents the hands from moving well at more than about 100 times normal speed.

We wondered how he made the hands reverse and, apparently, there is a way to get the drive gear to move in reverse, but it isn’t always reliable. Of course, you could also replace the drive mechanism with something like an RC servo or other motor and it sounds like he has done this and plans to show it off in another video.

We’ve seen the opposite trick before, too. If you really want an easy-to-control analog clock, try this one

28 thoughts on “Clock Mechanism Goes Crazy For Arduino

  1. I used the alarm clock variant, and by setting the alarm hand to noon I could detect when the hands were in this position. Therefore I could advance the clock to a specific time.

  2. The explanation given on the video is wrong.
    The coil is being driven by a chip that has a quarz controlled oszillator and frequency devider inside.
    Usually those clock chips produce two pulses per second .
    They make use of a H-bridge that drives the current trough the coil for say 13ms each second but at a distance of half of a second between each other.
    Sorry Im not english.

    1. Many years ago I made one with a pic to divide by 730 to give one rotation of the hour hand per year. The I had some nice artwork for the zodiac.Friends thought it depressing to see how far the year had progressed since last they last glanced at it.

  3. Please DON’T publish “hacks” that have only shitty video for clickbite.Serious projects have github or proprietary web pages .You are too serious to deal with stupud peoples make shitty videos for money .

  4. Regarding unreliable reversal: in the old synchronous motor clocks, the motor itself was perfectly happy initially starting in either direction, and a stop mechanism was used to catch reverse motion and push the motor back into forward operation. Removing that stop you did a clock that was perfectly happy running either forward or backward, but what you had to unplug and replug repeatedly until it was running in the desired direction. On some clocks, the stop could be reversed, yielding a clock that reliably ran backward: this was a fairly common hack, often accompanied by replacing the clock face markings with a mirrored version. Commercial versions were also available, based on the same one-part repositioning.

    1. This is true. Years ago when I was an engineer at a radio station I built such a clock for our writers in the production department. They loved it and had it there for years. You should have seen the look on peoples’ faces when they first saw it. BTW it kept perfect time……

    2. These clock motors use shaped pole pieces to put a bias on the rest detent position. It really does have a preferred direction to turn when it gets the magnetic impulse from the coil, akin to the shaded pole motor. There is no mechanical stop mechanism.

      If you get your pulse energy just right you can induce an oscillation about that detent point, and with the right pulse length and phase you can persuade the rotor to reverse.

      Similarly, capacitor-start motors use a separate starter winding to induce that preferred direction. Some of these are available with a separate starter winding, not connected to the main winding through a common connection. This allows reversal of that winding, and allows the motor to start in the opposite direction, just like swapping two phases on a 3-phase motor.

  5. All my wall clocks and watches turn counter-clockwise.
    I just fell in love with the concept when my Math teacher had such a watch when I was in high-school in the late 80’s.
    For my home-made reverse clocks I buy regular clock motors and simply flip-over the metal blade forming the magnetic poles, to make them turn counter-clockwise.

      1. And that’s indeed the answer. There’s not a single specific thing that’s “Arduino”.

        Fun fact: atmega chips contained in many of Arduino models have an operating mode specifically for clock-type devices. In that mode the CPU core runs on the internal oscillator (up to 8MHz if I recall right) and you can connect an external 32.768 kHz crystal (i.e. a standard watch crystal) to provide clock ticks.

    1. 10ppm is about 30 seconds per month. That’s table stakes for a clock.

      Where it starts getting interesting is that for a typical crystal the 10ppm spec just means that a typical production run will have most of them land somewhere in there. You can trim them down to zero (in software or hardware), but that doesn’t mean they’ll stay there without pretty strict temperature control.

    2. I did a project on this a few months ago. I found all the arduinos in my spares box tended to run a little fast, by a few seconds an hour. The actual amount varied from device to device.

  6. Just FYI for people searching, often these are referred to as clock ‘movements’ rather than ‘modules’.
    And if you’re after the sort of clock where the second hand moves smoothly, rather than in ticks, that’s a ‘continuous movement’.

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