Hotel Sentry

Hotel Sentry turns a spare iPhone into a room guard. Leave the old phone plugged in on the desk of your hotel room and arm it. It listens for the pressure change a door makes when it opens, the sound of a knock or a handle, and any movement of the phone itself. When enough of those add up, your own phone and Apple Watch get an alert — with a photo of the room, taken at that moment. WHY A SPARE PHONE That drawer phone already has a barometer, a microphone, an accelerometer and a camera. It is a better sensor than most travel alarms you can buy, and it costs nothing to reuse. HOW IT DECIDES Hotel Sentry does not fire on a single noise. It scores signals over a rolling ten-second window: • Air-pressure blip — the signature of a door opening in a closed room • Recognised sound — a door, a knock, or speech • Sound spike above the room's own noise floor • Vibration of the phone • The phone…