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+---
+title: Fingerprint door lock (LP)
+date: 2025-08-18
+layout: post
+project: true
+thumbnail: thumb_sm.jpg
+---
+
+Second iteration of the [RF door lock](../fpm-door-lock-rf). 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.
+
+<video style="max-width:100%;" controls="" poster="pcb.jpg">
+  <source src="video.mp4" type="video/mp4">
+</video>
+
+Redesigned the PCB completely. Tossed the RF modules and the second MCU.
+Connected R503 sensor directly to the ATmega328P. Sensor now mounts on door
+exterior, servo attaches to interior knob, MCU stays on the back to prevent
+tampering.
+
+<table style="border: none; width: 100%">
+  <tr style="border: none;">
+    <td style="border: none; width: 49.9%; background-color: transparent; text-align: center;">
+      <img src="breadboard.jpg" alt="PCB" style="width: 100%">
+    </td>
+    <td style="border: none; background-color: transparent; text-align: center;">
+      <img src="pcb1.jpg" alt="Design" style="width: 100%">
+    </td>
+  </tr>
+  <tr style="border: none;">
+    <td colspan="2" style="border: none; background-color: transparent; text-align: center;">
+      <img src="footprint.png" alt="PCB footprint" style="width: 100%">
+    </td>
+  </tr>
+</table>
+
+PCB specs: 2-layer, 1oz copper, 0.3mm traces (1mm for power and servo). Ground
+plane on bottom layer.
+
+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. 
+
+Replaced linear regulators with MP1584EN DC-DC buck converters. No RF, noise
+isn't a big concern. Buck's pin breakout not great--wouldn't fit most
+commercial dev boards.
+
+Squeezed more power savings from the MCU by running it at 3.3V/8MHz. Combined
+with buck converters: 56% total power savings.
+
+Wake sequence: MCU activates sensor MOSFET, unlocks sensor 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.
+
+Total power savings: 99.9% (30.6mA → 2.9μA). Verdict: Fixed. 
+
+Commit:
+[7529094](https://git.asciimx.com/fpm-door-lock/commit/?id=75290945b2fd84b3bc108fd46419ee478eaac3ca)
+| Gerber: [gerber.zip](gerber.zip)