The Story

After getting locked out of my dorm room a few too many times, I devised a better way to access my room: with my fingerprint. The small device I developed over two iterations would pull the inside door handle just enough to unlock it. The product used cheap, off-the-shelf components that cost under $80 and were reliable enough that we never needed our keys.

https://www.youtube.com/watch?v=6_gyTXoZGik

Design and Analysis

Mechanical Design

• Double linkage mechanism allowed the handle to reset automatically without reversing the motor direction.
• Arm geometry and linkage ensured the resulting motor force remained nearly constant despite linearly increasing handle torque.
• 3D-printed gears, linkages, shells, and alignment jigs reduced costs.

Electronics

• Raspberry Pi Pico microcontroller.
• Custom printed circuit board simplified design and improved reliability.
• Hall effect sensor and magnet on the arm provided inexpensive positioning feedback.

Software

• Detection during motor stall.
• Improved security with fingerprint scanner device tracking after power-on.
• Easily add and remove fingerprints without flashing the code.

Testing

• Ran continuously for 100 cycles without issue.
• Tracked time between opening cycles to monitor for degraded performance.