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diff --git a/_site/projects/fpm-door-lock/index.html b/_site/projects/fpm-door-lock/index.html new file mode 100644 index 0000000..7067e46 --- /dev/null +++ b/_site/projects/fpm-door-lock/index.html @@ -0,0 +1,156 @@ +<!DOCTYPE html> +<html> + <head> + <meta charset="utf-8"> + <title>Fingerprint door lock</title> + <head> +<meta charset="utf-8"> + <title>Fingerprint door lock</title> + <link rel="stylesheet" href="/assets/css/main.css"> + <link rel="stylesheet" href="/assets/css/normalize.css"> + <link rel="stylesheet" href="/assets/css/skeleton.css"> +</head> + + + </head> + <body> + + <div class="container"> + <ul id="navlist" class="left"> + <li > + <a href="/">hme</a> + </li> + <li > + <a href="/projects/">tnk</a> + </li> + <li > + <a href="/about/">abt</a> + </li> + <li><a href="/feed.xml">rss</a></li> + </ul> +</div> + + + + <main> + <div class="container"> + <h2 class="brand center" id="title">FINGERPRINT DOOR LOCK</h2> + + <h6 class="center">03 OCTOBER 2025</h5> + + <br> + + <div class="threecol justify"><p>This project features a fingerprint door lock powered by an ATmega328P +microcontroller.</p> + +<video style="max-width:100%;" controls="" poster="pcb.jpg"> + <source src="video.mp4" type="video/mp4" /> +</video> + +<h2 id="overview">Overview</h2> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<h2 id="embedded-software">Embedded software</h2> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<h2 id="the-circuit-board">The circuit board</h2> + +<p>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.</p> + +<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> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<h2 id="epilogue">Epilogue</h2> + +<p>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.</p> + +<p>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.</p> + +<p>Files: <a href="source.tar.gz">source.tar.gz</a>, <a href="gerber.zip">gerber.zip</a></p> +</div> + + <p class="right italics">by W. D. Sadeep Madurange</p> + </div> + </main> + + </body> +</html> |