Old Android phones pile up in drawers. Instead of letting one die there, you can point it at a doorway, a driveway, or a pet's favorite corner and let it record. Here is a straightforward, cloud-free way to do it, and the reasoning behind each choice.
Why an old phone is a good camera
A three-year-old phone still has a decent sensor, Wi-Fi, and a battery. The two things that usually get in the way are the screen (it drains power and announces that the phone is recording) and cloud services (a monthly fee plus your footage on someone else's server). Solve those two and an old handset becomes a capable local camera.
The core trick: recording with the screen off
The feature that makes this practical is screen-off recording. The camera keeps capturing while the display is fully dark, so the phone sips power instead of gulping it and does not glow in a dark room. On Android you get this either through a background-camera recorder app or, on some OEM camera apps, a "screen off" toggle buried in settings.
