Button Actuation Mechanism

Mechanical Design Internship — Meemo · Summer 2025

Annotated section view through the button mechanism showing the cap, stem guided through the shell wall, retention collar and the tactile switch beneath

Section through the button stack. The whole mechanism lives between the inside face of the shell and the top of a switch.

Overview

As a mechanical design intern at Meemo I designed the three-button input mechanism and the status-indicator integration for the Wristbuddy, a parent–child locating device worn and used one-handed, often without looking.

  • Designed a capped-plunger button stack through the top shell
  • Arrayed three buttons on the same pitch as the switches on the board
  • Integrated an addressable RGB status indicator into the shell
  • Verified the packaging in assembly against the fully populated board model
  • Printed the mechanism and tested it by hand against the switches

Goals & Requirements

  • Three front-panel inputs findable by feel and pressed one-handed
  • Work entirely within the fixed enclosure and board — switch placement frozen
  • Retention from below, so the button cannot fall out through the front
  • Travel matched to the switch, with over-travel limited so a press can’t exceed the switch’s rated stroke
  • One status-light opening driving all the device’s status colours

Design

Each button is a capped plunger. The cap sits proud of the shell so it can be found and pressed without looking. The stem passes through a through-hole in the shell wall that guides it and keeps it square, so an off-centre press still actuates cleanly instead of binding. A retention collar below the shell face is larger than the through-hole, so the button cannot fall out through the front — and it also sets the rest position, which is what fixes the travel. The gap from plunger tip to switch dome sets how far the cap moves before actuation, and the collar bottoming out limits over-travel. Three of these sit in an array on the same pitch as the switches on the board.

The top shell exterior showing the three button caps in an array

The array as the user sees it.

The button pockets and retention features seen from inside the top shell

The same features from inside — pockets, guides and retention.

The status indicator is one addressable RGB package driving all the device’s status colours from a single opening, rather than a separate LED per state.

The shell with the recess for the addressable RGB indicator

The indicator recess in the shell.


Motion study of the two halves closing — the board carries the plungers down onto the base.

The housing assembled over the fully populated circuit board model

Closed, against the populated board.

Outcomes

  • Three-button mechanism travel-matched to the board’s tactile switches, verified in assembly against the fully populated board model
  • Single-opening addressable RGB status indicator integrated into the existing shell
  • Printed part actuated cleanly when pressed off-centre, retained by its collar, and felt like a button under hand testing

Scope note: this is mechanism design and packaging — I did no wiring and no firmware; the indicator is placed and integrated mechanically, not driven. Validation was functional hand-testing of one printed part, with no instrumented measurement and no tolerance stack, and no photos of that printed build survive.

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