Servo or stepper motor?

Discussion in 'Electronics & Electrics' started by Agg, May 5, 2017.

  1. Agg

    Agg Administrator

    Jun 16, 2001
    A Reported Post Near You
    So, wifey has put in a request. She wants to make a clockwork windup key thing which she can wear as part of a costume. So, imagine her done up as a doll, with this big key sticking out of her back, ticking away like a windup mechanism. She can make the dress and the key - I need to provide the mechanism.

    I haven't been able to simply find a mechanism on eBay or Aliexpress, so it looks like I'll have to make one. My thoughts go to Arduino. But do I want to use a stepper motor or a servo?

    It needs to::

    - rotate 360 degrees continuously (the servos I have are 180 degrees only)
    - tick along in visible increments, not just smoothly rotate
    - drive a fairly light but large key
    - be fairly low profile so it can be hidden in the back of a dress
    - be tolerant of some random giving it a wind

    Sooo.. from Googling I can't work out whether a servo or stepper motor or something else might be most suitable. Thoughts?
  2. oculi

    oculi Member

    Aug 18, 2004
    you can modify (most?) servos so they rotate continuously fairly easily, just need to remove the potentiometer and hard stops, and hook up directly to the motor then it just becomes a geared DC motor.

    unless you want speed control too in which case you need to mess with the potentiometer - then you can send "position" commands to it like a normal servo and it will respond by going fast or slow in whatever direction you tell it to go in

    here's a guide (which I haven't read) for the second case, if it doesn't make sense search for "convert servo to continuous rotation"

    stepper will get the job done but slightly harder to get to work, you can do it with a micro and some transistors or just with a stepper control module which are pretty cheap from memory.

    another easy way to do it is with a geared DC motor, such as or similar

    or you could use an actual windup mechanism (egg timer or similar but it might be a bit slow)
    Last edited: May 5, 2017
  3. AgB deano

    AgB deano Member

    Jan 31, 2009
    So to clarify, you want it to snap in a sudden movement versus a continuous movement? How much movement? Pronounced like 90 degrees? or something like 10 degrees?

    Given you want it to survive douchebags, the issue is gearbox, you'll have to be very careful what you use.

    If you do go the stepper route, although it will have a learning curve, you will be able to get it to look as good as it is going to get (it'll start when you want, go as far as you want and suddenly stop) and since you'll be able to directly drive your key (due to programming the speed and how much it turns directly), someone can just turn it and it probably won't care one way or another. You can also use the smallest servo you can find, it should still be able to drive a light load like that. EDIT: here is one that's just 15mm in height clicky

    Servo you'll want to drive with a pulse, so a 555 setup will be the easy way to go. Servo's aren't exactly heavy duty though and you must buy one that doesn't have a worm drive, as they do not like to be driven from the shaft unless they are built for it, so "hurrr durrrr" is a nogo. They also probably won't be happy in general if someone yanks on it. I'd suggest you build it to break easily where you want it to break.

    Another option could be a micromotor with inbuilt planetary gearbox (not worm). You could drive it using a 555 and a transistor/mosfet fairly easily. Set it for 0.25 seconds on, 1 secs off and pick the gearbox RPM to suit how much movement you want in that 0.25 seconds. Being a planetary gearbox, someone could turn it either direction without issue, you're certainly not going to bother the mosfet or motor anyway with those short pulses.

    EDIT: Using my terrible 3am-post-12-continuous-hours-working-on-essay-logic, a 360rpm motor/gearbox pulsed for 0.25 seconds will move 1.5 degrees, so if you wanted a 10 degree movement, it'll need to be 2400rpm? You'd just get a 2400rpm motor. The permanent magnets in it would stop it soon enough. Then the issue is it's a pretty big setup by now.. stepper or servo it is
    Last edited: May 6, 2017
  4. Ratzz

    Ratzz Member

    Mar 13, 2013
    Cheltenham East 3192
    Some kind of a spring setup, and you can just wind her up every now and then maybe? Does it really have to be electric?

    It would add to authenticity if she had to be wound up occasionally :) No idea if its a feasible idea.. maybe some shock cord that winds and unwinds like an old rubber band airplane only slower.. or even hidden cords in her dress that move the key with her leg movements or something? She walks, key turns.. she stops, key stops.

    No idea on details, just a random thought.
  5. RobRoySyd

    RobRoySyd Member

    Jan 24, 2008
    I'd go with a stepper but taking care of the requirement for people to be able to turn the key. So called "pancake" stepper motors can be bought, you don't have a lot of depth to work with so they would be a good solution:

    The key could attach directly to the shaft. Need to consider though that when a stepper is powered it takes a lot of torque to turn it so the usual circuitry used to drive these needs modification. The same would go for a DC servo as all motors can become generators. With a stepper this could be used to advantage e.g. provide a stepped feel to turning the key. Could use the "back EMF" from the stepper to light an LED or even charge the battery :)
  6. RussellK

    RussellK Member

    Jun 11, 2003
    I'd suggest modifying a standard servo as above for continuous rotation. Then use an Arduino (or two 555's) to send intermittent streams of PWM pulses to get the pausing rotation effect.

    As the key is light, just use a simple loose-fitting interface to the key that allows enough friction to rotate the key, but slip if the key is held or turned manually.
  7. Dice

    Dice Member

    Feb 18, 2002
    A pulley belt, no teeth.

    As far as motor - if you have lots of time, you could use a small geared motor like...

    (google for "ja12-n30-dc-micro-gearmotor-47-rpm-12-v" - and if you buy from china, be prepared for a month or so wait)

    I've used that motor in the ~100rpm variety for a uni project - I could not drive the shaft backwards by hand, so a belt or similar would be essential. The motor did usefully spin and provide surprising torque right down to around 0.5 volts
  8. merlin13

    merlin13 Member

    Apr 12, 2003
    back in BrizVegas
    Suggestion - make the shaft of the key big/wide enough diameter so the motor and battery pack/clockwork mechanism/hamster wheel/whatever mechanicals sits inside the end of the key shaft.

    Rather than all the mechanicals sitting on her back and the key being attached as extending out from those.

    Would also look "betterer" if it appeared the hardware that the key is supposed to be winding up is looks to be embedded inside her (oh God, I can see this generating aaaaall sorts of naughty connotations...).
  9. Technics

    Technics Member

    Apr 29, 2002
    Brisbane, AU
    I'd find the noisiest mechanical kitchen timer you can and disable the bell (or not). Then attach the key. Seems like involving electronics is just making it more complicated than it needs to be. Unless she wants to avoid being wound up once we hour.
  10. andrew_bg

    andrew_bg Member

    May 10, 2005
    Mount Victoria
    I would definitely go with a DC motor, cheaper, lighter and MUCH easier. Handle the stop/start with either an Arduino or a simple timer circuit.

    Im also in the camp to hide all the mechanicals in the key shaft and allow for a simple clutch so that if the key gets grabbed or snagged on something it can slip without damaging anything.

    You should be able to do it with some inexpensive small parts and simple timer to drive the motor for a pre-set amount of time, ie 0.2 seconds, wait 1 sec, drive it again loop.
  11. Clayton

    Clayton Member

    Oct 2, 2009
    What about a brushless outrunner direct drive with a custom drive driven from an arduino activating the correct winds. It will free wheel all you want. You can get some pretty sturdy and flat outrunners.
  12. Ma Baker

    Ma Baker Retired

    Apr 24, 2002
    This might be a quick solution.

    There's a few different ones here.
    Last edited: May 10, 2017
  13. Symon

    Symon (Plugging your Socket)

    Apr 17, 2002
    Santiago, Chile
    In case you aren't aware of it already, the lillypad may be a good way to handle the arduino side of things if you are putting the electronics in the dress.

    I do like merlin's idea though of putting it all in the key. You can then just have the motor shaft sticking out that then inserts into a mount on the back of the dress. You could even stick a rare earth magnet into the mount to help keep it in there, but still be able to remove the key easily if she needs to duck into the bathroom or something.

    Stepper motor would be the go.
  14. mesaoz

    mesaoz Member

    Jan 15, 2015
    Twitch / Brisbane

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