One popular choice is Alfred, which lets you turn your old smartphone into a home security camera. It's easy to set up, and you can even view your camera feed remotely from your own phone.
Another option is Manything, which works in a similar way. You can use an old phone or tablet as a camera, and Manything will send you alerts if it detects any motion. Plus, you can view your camera feed from anywhere using the Manything app.
If you're looking for something a little more comprehensive, you might want to check out HomeAssistant. It's a bit more involved to set up, but it's a powerful platform that can integrate with a ton of different smart home devices.