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+---
+title: Prototype fingerprint door lock
+date: 2025-10-03T00:00:00+08:00
+author: W. D. Sadeep Madurange
+---
+
+This project features a fingerprint door lock powered by an ATmega328P
+microcontroller.
+
+<video style="max-width:100%;" controls="" poster="pcb.jpg">
+  <source src="video.mp4" type="video/mp4">
+</video>
+
+## Overview
+
+The lock comprises three subsystems: the ATmega328P, an R503 fingerprint
+sensor, and an FS5106B high-torque servo. The sensor mounted on the front
+surface of the door enables users to unlock it from the outside. The servo is
+attached to the interior door knob. The MCU must be installed at the back of
+the door to prevent unauthorized users from tampering with it.
+
+When no one is interacting with the lock, the MCU is in deep sleep. The sensor
+and the servo each draw 13.8 mA and 4.6 mA of quiescent currents. To prevent
+this idle current draw, the MCU employs MOSFETs to cut off power to them before
+entering deep sleep. Doing so is crucial for conserving the battery.
+
+Without power, the sensor remains in a low-power state, drawing approximately
+2.9 μA through a separate power rail. When a finger comes into contact with the
+sensor, the sensor triggers a pin change interrupt, waking up the MCU. The MCU
+activates a MOSFET, which in turn activates the sensor. Over UART, the MCU
+unlocks the sensor and issues commands to scan and match the fingerprint.
+
+If the fingerprint matches an enrolled fingerprint, the MCU activates the blue
+LED on the sensor, turns on the MOSFET connected to the servo, and sends a PWM
+signal to the servo to unlock the door. Otherwise, the MCU activates the red
+LED on the sensor. Finally, the MCU deactivates the MOSFETS and goes back to
+sleep.
+
+## Embedded software
+
+The embedded software, written in C with the help of the AVR toolchain,
+includes a driver for the sensor, servo control routines, and a battery
+monitoring system.
+
+In addition to controlling the sensor and the servo, the program strives to
+maintain precise control over the sleep mode, as well as when the peripherals
+are activated and for how long they remain active. I thoroughly enjoyed writing
+the embedded software. There's something magical about being able to alter the
+physical world around you by uttering a few lines of C code.
+
+The source code of the project, which includes a driver for the R503
+fingerprint sensor module, is enclosed in the tarball linked at the end of the
+page.
+
+## The circuit board
+
+For this project, I designed a custom PCB and had it fabricated by JLCPCB. Like
+the software, the circuit is chiefly concerned with optimizing power
+consumption and extending battery life.
+
+<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>
+
+To that end, the principal components of the circuit are the 2N7000 and
+NDP6020P field-effect transistors. They switch power electronically to the
+servo and the fingerprint sensor, the two most power-hungry parts of the
+circuit. The two MP1584EN buck converters play an axial role in efficiently
+regulating power to the MCU and the sensor.
+
+The ATmega328P typically operates at 5 V with a 16 MHz crystal oscillator. To
+further reduce power consumption, I modified the ATmega328P's fuses to run at
+3.3 V with an 8 MHz crystal oscillator.
+
+The bottom right area of the PCB isolates the power supply of the servo from
+the rest of the circuit. This shields components such as the MCU from the
+servo's high current draw, which can exceed 1 A. The IN4007 diode in slot U2
+serves as a flyback diode, protecting the MOSFET from reverse currents
+generated by the servo.
+
+Lastly, the 56 kΩ and 10 kΩ resistors in slots R10 and R11 form a voltage
+divider circuit. Its output is fed to the ADC of the MCU, which measures the
+supply voltage by comparing it to the internal bandgap reference voltage. 
+
+## Epilogue
+
+This project began nearly a year ago when I attempted to unlock our door
+wirelessly by writing to the UART ports of two MCUs connected to inexpensive
+433 MHz RF transceivers, as if there were an invisible wire between them.
+Although I failed, it led me down a rabbit hole of RF communications, MOSFETs,
+PCB design, and low-power circuits.
+
+During the project, I reinvented the wheel many times. I implemented a
+low-level network stack using only RF modules and an 8-bit microcontroller,
+designed my first PCB, and developed drivers from scratch. The project was far
+from a smooth sail. Bad electrical connections, soldering, desoldering, and the
+heartache of purchasing the wrong parts were routine. It was a long but
+rewarding journey from the messy breadboard to the shiny PCB. 
+
+Files: [source.tar.gz](source.tar.gz), [gerber.zip](gerber.zip)
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