- A magnetic reed switch to monitor the status of door https://www.adafruit.com/product/375
- A NFC chip that reads the key https://www.adafruit.com/product/789
- A key fob https://www.adafruit.com/product/363
- A speaker than plays sound https://www.adafruit.com/product/2210
- A distance sensor to detect whether the user is in the nearby https://www.adafruit.com/product/2210
- A 3D printed case
A typical user journey could be: when the user get into the door, he put the key on the key reader and it reads the key; if the key wasn’t there, a warning sound will be played to tell the user to put the key on it. The reader only starts to work when the reed switch detects a change of status (closed-open-closed) and the distance sensor detects the user (it is important to have double check because sometimes the door will be opened and closed just because the user want to make sure it is closed. When the user open the door again, the reed switch turns to open and simultaneously, the key reader will search for the key again, and if it is still detected, the warning sound will be played to tell the user that he should take the key.
And maybe the reed switch should be connected to another separated board (which can be connected to the main board with Bluetooth or WiFi) because the reader can be placed somewhere far from the reed switch this way.
And here is the exercise.