I'm currently designing some small-scale robots capable of both melee and laser-tag combat. For the melee component, I'd like to create small, ejectable panels that hide the laser-tag receivers. When hit, these panels would pop off exposing the receivers.

However, I'm having trouble finding a suitable mechanism to fit my constraints. Ideally the mechanism would be: small/shallow, so as not to stick out too far from the robot and either 3D-printable or low-cost. My first thought was the mechanisms in pens which don't seem accessible without disassembling a ton of pens, or push-to-open cabinet latches which are too large to use. I've also ruled out something like an electromagnet and push-button as that would add unwanted circuit complexity. Is there another mechanism that would serve this purpose?

  • $\begingroup$ So, make the cabinet latches but smaller - mechanism still works... $\endgroup$
    – Solar Mike
    Commented Apr 29, 2021 at 20:37
  • $\begingroup$ All drawings I've seen of that mechanism tend to be very long. Scaling it to a useful length would probably make it too small to be printed on a standard FDM machine. But I could be wrong! $\endgroup$
    – kwelliott
    Commented Apr 29, 2021 at 21:51
  • $\begingroup$ It might be worth looking at the mechanism in SD card slots. $\endgroup$
    – Ethan48
    Commented Apr 29, 2021 at 23:23
  • $\begingroup$ @Ethan48 OP may have the issue of making the sd card mechanism large enough... $\endgroup$
    – Solar Mike
    Commented Apr 30, 2021 at 4:27

1 Answer 1


This design should be FFF printable (FDM is ™) down to a reasonably small scale:

latch spring mechanism

The panel gets a cylinder with a groove. The groove could be around the entire circumference; it was easier to create a simple slot for the drawing.

The robot gets a yellow cylinder with a matching groove, not around the entire cylinder. The release is a rod that can either be tilted out of the groove, or pulled axially. The ejection is effected by having a spring inside the yellow cylinder restrained from tossing itself with the panel by a string, staple, etc attached to the fixed end of the cylinder.

The latching rod could be a panel rather than a cylinder. A servo arm would easily serve the latching purpose. The shape of the mechanism does not have to be cylindrical. There's great flexibility in the concept, allowing for adjustment regarding 3D printing and materials on hand.

Additionally, consider to place a slot 90° in place of the panel slot, axial to the cylinder. The same applies with the slot in the yellow cylinder. The servo arm engages through the slot in the yellow cylinder, holding the panel in place. With sufficient travel, the servo moves from right to left in this drawing, pushing the panel away, eventually releasing it. This thought appeared after I created the post. If required, I will create a second drawing.

There may be commercially available ejectors, but I did not perform a search for such mechanism. Image created by me in Tinkercad.

  • $\begingroup$ what app did you use to draw the 3d? $\endgroup$
    – kamran
    Commented May 30, 2021 at 5:26
  • $\begingroup$ Note the last sentence in my answer? $\endgroup$
    – fred_dot_u
    Commented May 30, 2021 at 10:21

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