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Password Lock

Combine a 4x4 matrix keypad, status LED, and servo into a complete input-process-output system. You will scan rows and columns, store user input, compare a passcode, and control a physical lock mechanism.

Last updated

Build Time
90-120 minutes
Estimated Cost
$20-$35
Skill Level
Advanced
Raspberry Pi PicoRaspberry Pi PicoKeypadServoSecurity
  • Middle School
  • High School
  • Adult Beginners
  • Homeschool

Skills

What You'll Learn

  • Scan a matrix keypad without a dedicated interface module
  • Build and compare a multi-digit input string
  • Coordinate status feedback and servo movement
  • Separate logic errors from wiring and power faults

Preparation

Required Parts

Gather these components before starting the build.

  • Qty: 1Raspberry Pi PicoMicroPython installed
  • Qty: 14x4 matrix keypadEight-pin membrane type
  • Qty: 1SG90 micro servoLock actuator
  • Qty: 1Green LEDAccess feedback
  • Qty: 1220 ohm resistorLED current limiting
  • Qty: 1Regulated 5 V supplyServo power
  • Qty: 12Jumper wiresKeypad and outputs

Wiring

Circuit Diagram

Keypad rows use GP2-GP5 and columns use GP6-GP9. The servo signal uses GP16, and the status LED uses GP15 through 220 ohms. Servo and Pico grounds are common.

Build

Step-by-Step Instructions

Work through each stage in order and disconnect power before changing the wiring.

1. Test the keypad by itself

Wire the four row and four column pins. Run a small scan test or print each detected key before connecting the servo.

2. Add status feedback

Connect the green LED to GP15 through a 220 ohm resistor. Confirm it lights independently when GP15 is high.

3. Connect the servo safely

Power the servo from regulated 5 V, connect the grounds, and connect only the servo signal to GP16. Test open and closed angles before attaching a latch.

4. Run the lock program

Enter 2580 followed by #. The LED should light and the servo should unlock for three seconds. Press * to clear a partial entry.

5. Tune the mechanism

Mount the servo horn so neither locked nor unlocked position forces it against a mechanical stop. Treat this as a learning model, not a security device.

Programming

Project Code

Upload password-lock.py after completing the circuit.

password-lock.pypython
from machine import Pin, PWM
from time import sleep

keys = (("1","2","3","A"), ("4","5","6","B"),
        ("7","8","9","C"), ("*","0","#","D"))
rows = [Pin(p, Pin.OUT) for p in (2, 3, 4, 5)]
cols = [Pin(p, Pin.IN, Pin.PULL_DOWN) for p in (6, 7, 8, 9)]
servo = PWM(Pin(16)); servo.freq(50)
status = Pin(15, Pin.OUT)
entered = ""; password = "2580"

def angle(degrees):
    pulse = 500 + degrees / 180 * 2000
    servo.duty_u16(int(pulse * 65535 / 20000))

while True:
    for r, row in enumerate(rows):
        row.high()
        for c, col in enumerate(cols):
            if col.value():
                key = keys[r][c]
                if key == "#":
                    if entered == password:
                        status.high(); angle(90); sleep(3); angle(0)
                    status.low(); entered = ""
                elif key == "*": entered = ""
                else: entered = (entered + key)[-4:]
                while col.value(): sleep(0.02)
        row.low()

Problem solving

Troubleshooting

Keys produce the wrong characters

Check the keypad pin order and adjust row or column pin lists to match its datasheet.

The servo resets the Pico

Power the servo from a separate regulated 5 V source and connect the supply ground to Pico GND.

One key registers several times

Keep the release-wait loop and add a short debounce delay after each accepted press.

Common questions

FAQ

Is this suitable for real security?

No. It is an educational prototype; the passcode is visible in source code and the mechanism is not hardened.

How do I change the password?

Replace the password string and save the updated program to the Pico.

Can I add a display?

Yes. An I2C LCD can show masked input

Go deeper

Related Tutorials and Resources

TutorialGPIO ExplainedTutorialPWM ExplainedTutorialServo MotorsBuying guideBest Raspberry Pi Pico Starter KitEquipmentRecommended project kit

Project complete

Ready for LED Night Light?

Build an automatic LED night light that turns on when the room gets dark.

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Reuse your kit

More projects you can build with this kit

Keep using the SunFounder Raspberry Pi Pico Ultimate Starter Kit instead of starting with a new parts list.