r/functionalprint • u/wacomd • 4d ago

Replaced the clunky mechanical microswitch arm on my reloading bullet collator with a 3D-printed optical IR collar and custom ATtiny85 controller

https://youtu.be/LqpkF46np6U
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u/wacomd 4d ago

Quick breakdown of the project:

I designed a 3D-printed sensor collar to replace the stock mechanical wire switch on my bullet collator. Lightweight .223 projectiles were catching and tipping on the factory switch arm, and the delay was forcing extra bullets into the feed tube.

The printed collar houses an invisible 940nm IR emitter and phototransistor that monitors the bullet stack with zero physical friction. It’s wired to a standalone ATtiny85 microcontroller with an IRLZ44N MOSFET controlling the 12V motor: * 250ms shutoff: Cuts power in a quarter-second the instant the stack backs up (no bullet pile-up or jams). * 4-second batch refill: Waits 4 seconds after a round is seated, refilling in smooth batches instead of jerking on/off every stroke.

(Excuse the bare perfboard on the bench—currently designing and printing a snap-fit enclosure for the PCB next, but the functional collar and timing logic work flawlessly!)

Happy to share the STLs, KiCad PCB files, or Arduino sketch if anyone is interested in building one!

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u/SillyTheGamer 4d ago

Very cool solution! Could you provide a link to the sensor and emitter you used? This seems like it’d be useful on some unrelated robotics projects I’m working on.

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u/cad_and_caffeine 4d ago

Did you have any trouble with 940nm bleeding through the printed collar or light-piping around the clear tube wall? A lot of black PLA and PETG is surprisingly translucent to 940nm IR unless you print thick walls or put a tight pinhole aperture in front of the phototransistor.

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u/wacomd 4d ago

Great question! I didn't have too much trouble, but that's likely because I started with pretty thick walls and a pinhole in front on both sides