Fingerprint door lock

FINGERPRINT DOOR LOCK

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03 OCTOBER 2025

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+ +

This project features a fingerprint door lock powered by an ATmega328P +microcontroller.

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Overview

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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.

+ + + + + + + + + +

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+ +

+ +

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, gerber.zip

+ +

by W. D. Sadeep Madurange

This project features a fingerprint door lock powered by an ATmega328P -microcontroller.

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.

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.