I use my old iPhone as a camera to spy on my dogs during the day when I'm out. Using the free app ManyThing, you can access the camera, change it from front camera to back camera and get notifications on movement or sounds, which you can set the sensitivty and also the range. For my dogs, I block out the lower part of the camera so that the dogs walking around doesn't set off the motion alerts, and I set the sound down super low so if they bark or I leave the TV on, it doesn't constantly tell me about it.

The app is totally free for a month, and then you have to pay for the cooler features($5 a month), the main feature you lose as a non-paying user is the clips they automatically record on an alert. So if you get an alert you can go look immediately, but you can't review the footage that set off the alert.

It does not have night vision, but if you have an old phone laying around, it's basically free. Mine uses my home wifi, stays plugged in and I can check any time.