I'm building an automatic door lock, which is moved by a servo ( this one ). However, I want to still make it possible to open the lock manually, but it's not possible if the servo is attached to the lock mechanism.

Do you have ideas on how to go around this problem?

Here's a diagram of the lock mechanism

Lock mechanism diagram

  • Can you give some more detail about how the lock is intended to function? We may be able to come up with some hitherto un-thought of solution, but we need to know how the whole mechanism will function to be able to visualise it better. – Majenko Jan 8 '16 at 21:20
  • @Majenko I've edited the question with a diagram of the lock mechanism – jonathanGB Jan 8 '16 at 21:58
  • Excellent, that's exactly what we need to see. – Majenko Jan 8 '16 at 22:13
  • 1
    I'm voting to close this question as off-topic because This question belongs on another site in the Stack Exchange network -- eg Robotics Stackexchange – James Waldby - jwpat7 Jan 9 '16 at 0:38
  • Ok my bad, but will it still be visible? Other people could find this thread useful. – jonathanGB Jan 9 '16 at 0:40

So it's actually the worm gear that is causing your problem, since that can't be reverse driven by the cogs.

The simplest option, I think, would be to have the middle cog normally separated from the worm gear and only re-engage it when you need to drive the mechanism with the servo. You could maybe use a second servo to move it into the right place, or maybe a solenoid, or something along those lines. Have it slide along a short track or slot with a spring to ensure it disengages when the solenoid is released.

  • Do you think it would be simpler/better to change the servo with a DC motor? – jonathanGB Jan 8 '16 at 22:21
  • If you still have the worm gear in there you'll still be in the same boat. Most electronic locks work by releasing the other end of the tennon from the mortice instead of retracting it - it's simpler that way. – Majenko Jan 8 '16 at 22:26
  • but with a DC motor rather than a servo, you can rotate it manually without problem. The only thing to manage is the current generated by this free rotation. But, if we stay with the servo, which of the 2nd servo or a sollenoid do you think will be best (if you have an opinion) ? In terms of complexity to implement and also constancy. – jonathanGB Jan 8 '16 at 22:29
  • A solenoid is the normal preferred option. You sometimes see clutch gears that allow the outer teeth to rotate independently of the central axis, which would be in place of the lock spindle cog - but those are expensive and generally custom designed. You see them in laser printers quite often for the paper feed. – Majenko Jan 8 '16 at 22:32
  • You sometimes see ratchet clutch cogs too, where if you rotate the teeth in one direction the turn the central spindle, but in the other direction they don't engage with the spindle - and the spindle can rotate separately too. Again complex and expensive... – Majenko Jan 8 '16 at 22:34

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