Keypad door lock
Opens a servo bolt for the right code; buzzes and locks out after wrong ones. A 4×4 keypad is 16 switches on 8 wires. The sketch drives one row LOW at a time and reads the four columns with pull-ups: a pressed key pulls its column LOW in its row’s turn. Type 2580 and # and the servo pulls the bolt open for 5 s; three wrong codes lock the keypad for 30 s. The code is a number in the sketch: on a real door, keep it in EEPROM and let the owner change it. On the real thing: an SG90 on the Uno’s 5 V pin is fine on USB, but if the board resets when the bolt moves, give the servo its own 5 V supply (grounds joined). The LCD backpack is at 0x27 or 0x3F: run an I²C scanner if the screen stays blank.
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Parts list
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- The right code opens it
- It locks itself again
- A wrong code is refused
How it works
What it does: Opens a servo bolt for the right code; buzzes and locks out after wrong ones.
Build it (guided or ready-made), then run its test procedure below. Every test has an expected value and a pass range, just like a factory test sheet.
Things to try
- Change CODE to your own four digits and upload.
- Press a key and watch the row pins on a scope: each row goes LOW in turn, thousands of times a second.
- A servo is not a lock: anyone can push it back. Real doors use a solenoid bolt or an electric strike, switched through a relay or a MOSFET.
Background
Push-button combination locks (Simplex, 1960s) became electronic keypads on alarms, safes and hotel doors in the 1980s.