Fingerprint door lock (LP)

FINGERPRINT DOOR LOCK (LP)

18 AUGUST 2025

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Second iteration of the RF door lock. Old version worked -but drew too much quiescent current. Sensor and servo pulled 13.8mA and 4.6mA -idle. Linear regulators were a disaster. Battery didn’t last 24 hours.

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Redesigned the PCB completely. Tossed RF modules and the second MCU. Connected -R503 directly to the ATmega328P. Sensor now mounts on door exterior, servo -attaches to interior knob, MCU stays on the back to prevent tampering.

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PCB specs: 2-layer, 1oz copper, 0.3mm traces (1mm for power and servo). Ground -plane on bottom layer.

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Solved idle power draw with MOSFETs. 2N7000 and NDP6020P cut power to sensor -and servo before deep sleep. Through-hole MOSFETs that switch at 3.3V are -getting harder to find. NDP6020P already obsolete.

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Replaced linear regulators with MP1584EN DC-DC buck converters. No RFM–noise -isn’t a big concern. Buck’s pin breakout not great–wouldn’t fit commercial dev -boards.

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Squeezed more power savings by underclocking MCU to 8MHz/3.3V. Combined -with buck converters: 56% total power savings.

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Wake sequence: MCU activates FPM MOSFET, unlocks FPM over UART, scans and -matches fingerprint. Match triggers blue LED, servo MOSFET, PWM signal to -unlock. No match triggers red LED. MOSFETs off, back to sleep.

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Total power savings: 99.9% (30.6mA → 2.9μA). Verdict: Fixed.

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Commit: -7529094 -| Gerber: gerber.zip

by W. D. Sadeep Madurange