Have been thinking about this for a couple years. I have old phones kicking around. Battery shot, hardware dated, but the camera(s) and mic and antennas still work. Would be cool if there were a way to set them up (powered) to stream audio/video or even take stills at intervals (or motion-activated) and then sync the content to the rest of the devices on my network.
I don’t know how complex the programming for something like this would be. But I suspect it’s trivial for those who do know.
The hard part is the hardware as you should really remove the battery from the equation to prevent catastrophic damage from constantly draining and charging an old worn-out battery. Unfortunately most older phones won’t run off wall power without a battery inside so one method is to solder some wires attached to ~4V to the battery contacts in the phone to trick it into thinking there is a battery present and allow you to run the device off USB power directly. This method might vary from phone to phone.