Is there any solution on making a drop of a counted number of balls when one ball passes through the system?

For example: One ball falls into a hole on the top and ten balls (plus/minus (not important) the ball that fell in) come out.

I have thought about letting one ball drop into the system and activating a lever which pushes open a tunnel with a lot of balls in it and the lever then self-closing afterwards. To limit the number of balls dropped there should be a closing mechanism on the upper side of said tunnel that closes when the tunnel itself opens, so only the balls in that section would get dropped...

But how would you do that without jamming?

If someone cares about the ball-size and weight, that should not really matter because the basic construction can always be a little bit reconfigured if the ball size is between 12 and 15 mm. Same for the weight.

EDIT: There has been some misunderstanding apparently, I am sorry for that. It should be noted that this mechanism should be repeatable for as long as you have a supply of balls coming in from a second source (auto-refilling of the reservoir). While this itself is not an issue because you can simply make a big pressure-based ball reservoir that gets triggered everytime the small reservoir empties, it is an important aspect because it means the whole mechanism should not only be able to run one time.

2 Answers 2


I've seen this done in a GBC module that counts the balls that pass by. It worked with scales that eventually tipped over, each scale representing a decimal of the total number of balls that passed by.

Can't exactly find the GBC module but found this clock from jkbrickworks that uses the same principle.

  • This solution seems like the best because it's probably the most simple and reliable way of solving a problem.
    – Ambo100
    Commented Jan 10, 2018 at 16:39
  • This is not a bad solution, but it has one issue: Since it is designed for the reverse purpose it does not refill. After you dropped the ball in and got 11 balls out, the next ball that goes in stays in there instead of 11 balls going out again.
    – uncanny
    Commented Jan 10, 2018 at 22:14

I don't know if this is practical, and I can't test it as I don't have enough compatible balls to build GBC type mechanisms, but what comes to mind is:

  1. Have the balls run down a trough.
  2. The trough ends in a segment of trough which is 10 balls long and pivoted about the long axis.
  3. The triggering ball causes the trough to spin about that axis, dumping the balls sideways into a receptacle.
  4. The top end of the moving trough is designed so that it is flat and blocks further balls from sliding down (without jamming) until it returns. This is the critical area to prevent it from jamming.

For ease of operation, the trough's weight should be balanced about the axis. The weight of the balls should assist in the dumping movement — either not enough to start it, or the trigger mechanism prevents movement until triggered.

  • Very interesting idea. I will try it and report back
    – uncanny
    Commented Jan 23, 2018 at 19:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.