I use an old iPad with brightness turned way down as an indoor IP security camera via the free Periscope HD app:

https://apps.apple.com/gb/app/periscope-hd-h-264-rtsp-cam/id...

I also use an old iPhone mounted inside the front door as a Touch ID enabled alarm control panel for a DIY HomeKit alarm system. This is using a Homebridge plugin I wrote:

https://github.com/vectronic/homebridge-alarm-panel