For no particular good reason, I think it would be interesting to build a Lego button that behaves somewhat like the button on a click pen: as you press it, it alternates between a "down" state and an "up" state.

I've researched how pen buttons work and done a fair amount of Googling. I've experimented on my own. I haven't yet been able to figure out how to build such a thing. It's surely possible. Any advice?

Edit: OR I'd accept, as a solution, the official (Googlable) name for such a mechanism. If I can find a number of examples online I assume I can figure out the Lego version myself.

  • The Lego trains have something like this. It's not a solution for your problem yet though, as the actual "clicking on/off functionality is in the battery holder brick. – S.L. Barth - Reinstate Monica Jan 2 '15 at 16:36
  • And I think the cycling happens electronically as opposed to mechanically since, at least on my train motor, the button always returns to the "undepressed" state. – Jason Swett Jan 2 '15 at 18:17
  • The technical name for what you're looking for is a latched push button. You might try latching instead of latched, and you might try switch instead of push button, but switch will also give you a lot of toggle switch results, which isn't what you want. – Nick2253 Jan 8 '15 at 16:35
  • If you are referring more specifically to the pen mechanism, you are looking for a bistable cam system. You can, of course, also search for pen mechanism, which gave me lots of pretty pictures from Google. – Nick2253 Jan 8 '15 at 16:38

Ok, I have managed to build a working cam mechanism using LEGO Technic. Only thing missing is a big enough spring or elastic band to pull the entire assembly upwards - Still thinking through that part but sharing in the hopes that others can take my idea further (My current best thought is to have rubber bands on the outside of the mechanism pulling the "clicker" upwards.). Here is a demonstration of the mechanism at work (was difficult to demonstrate with just one hand as I haven't lined the walls of the vertical holder enough and sometimes the up/down motion goes under an angle): video on youtube.


The entire mechanism: The entire mechanism

Clicker in the "up" position (i.e. retracted pen): Clicker up

Clicker in the "down" position (i.e. pen out) - my hand is in the picture as there is no spring to provide pressure upwards: Clicker down

The insert with the loop (I believe this is what is called a "cam"). There is a resting point on the top (the "down" position) as well as a resting point on the bottom (the "up" position): Cam inside

The "ball" - in this case a simple Technic axle with 2 1/2-width bushings on it - is supported in this mechanism, which allows the black part to slide freely from left to right. This "ball" moves around the loop defined by the "cam" from the previous picture: The "ball"

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  • This is impressive. Thanks. I was hoping for something smaller and with fewer pieces, but this is definitely an example of a working mechanism. – Jason Swett Jan 13 '15 at 13:48

What you can do is use an old-style spring loaded missile, the rubber tip is soft enough to let you trigger the launcher without putting the missile back in, and you don't need to trigger the launcher to lock the missile.

enter image description here

enter image description here

I you press the flat bar at the top it works about 90% of the time.

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  • Thanks for the response, but it seems like a solution should be possible without special parts (I don't own this missile part) and it seems like a solution should be possible that works 100% of the time. – Jason Swett Jan 2 '15 at 15:29
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    Fair enough. I'll have another think. – Windfire Jan 2 '15 at 18:04

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