Covert REX trigger

Request-to-exit trigger disguised as an emergency door release

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An idea to disguise a mechanical "Useless Machine" in a ubiquitous wall box to release a Request-to-Exit button on demand, for physical penetration testing.

This project is very much in the "is this bonkers?!" stage of iteration.

Here's a bit of "proving the concept" video:

The project consists of a servo, battery, charge circuit, microcontroller, and RF relay. When the RF relay closes, it signals to the microcontroller to start a 3 second timer. If the 3 seconds elapses without the relay opening again (to allow for power tests) the servo does a smooth 180 degree sweep, lifting the lid up and out, with the intention of pressing a green mushroom REX button.

This the current motion and hardware for Version 1 (I'm planning at least 2 more):

An earlier WIP video, showing triggering from a Flipper Zero:

And another video showing the measuring of the force when the lid is extended:

And last but not least here's as "how it would work" video with V1 hardware:


(NOTE: This is WIP and will change in the near future) This is the STL file for the basic v2 mechanism, as shown in

Standard Tesselated Geometry - 2.30 MB - 05/19/2024 at 10:23


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  • Base updates

    Robert Wallhead4 days ago 0 comments

    Here's a couple of pictures of the updated mechanism base, with places for all the components, routing for the wiring and some protections for the battery:

  • Progress on v2, demo of v1

    Robert Wallhead05/19/2024 at 03:41 0 comments

    I've just put together the following demo of v1 in action:

    And v2's mechanism works pretty well on the first print!

  • Finishing V1​

    Robert Wallhead04/27/2024 at 23:21 0 comments

    Version 1 is complete! Even though I identified a number of shortfalls with this first iteration, I wanted to complete the build of this one as a standalone unit. Everything fits, which is great, though I'd like version 2 to have places for each component and something to assist with wire routing. Here's the finished interior:

    And here's a video of the motion:

  • WIP on v2 3D-printable linkage

    Robert Wallhead04/14/2024 at 05:48 0 comments

    I've uploaded a pic to the gallery showing the WIP design for the v2 3D-printable linkage

  • Work to date

    Robert Wallhead04/14/2024 at 01:09 0 comments

    As this has progressed enough to at least be functional, I've decided to publish it. The videos and pictures here are of my "V1" of the project. I only need to install the battery and charging circuit to consider this version complete.

    Testing has shown this can generate 150-170g of pressure at full reach, which is reasonable as the cheap 9g servo I'm using is supposed to generate 1.8kg/cm of torque, and the distance between the servo's pivot and where it was pressing on the scales is about 10cm. The problem is that REX buttons are specified as taking anywhere between 200g and 1.5kg (expressed as approximately 2-15N) of pressure to trigger, though the one's I've observed have all been at the lower end.

    V2 then really has to target 300g/3N to be considered worthwhile, which will require a servo that's more than twice as powerful. I'm considering this one as a candidate: with an estimated 3.2kg/cm at the 5V I'll be working at.

    Additionally, V2 will be built using a 3D-printed 4-bar linkage so that this project is repeatable, and all the files and code will be published at that point.

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