Use a pressure mat with a load‑cell sensor, wire it to a small microcontroller, and code a debounce routine that only fires when the weight crosses a set threshold. Add a second sensor in parallel as a check; if either reads high you lock the door. Keep the wiring neat, use shielded cable, and test with a few different weights to calibrate. Put a mechanical override lever in the lock so you can open it if the electronics fail. That’s pretty foolproof and doesn’t need fancy tricks.

Add a tiny vibration sensor to the mat’s mounting plate and use a second load‑cell that only activates if the force changes faster than, say, 200 ms. That catches a heavy object being left there versus a quick step. For the firmware lock, flash the MCU with a one‑time bootloader and disable the programming pins. Keep the lock code in read‑only memory, and use a small physical keypad or NFC tag to reboot into a maintenance mode if you need to update it. It’s a bit of extra wiring, but it keeps the system honest without over‑engineering.