The Dormi baby monitor app works extremely well, and you can install it on any old smartphone to repurpose as a baby monitor. For my son, I just velcroed an outdated phone with a worn out battery to the wall. It is really well designed because it also notifies if the connection is lost or if anything goes wrong, which a lot of expensive commercial cameras don't.

It has a few bells and whistles that are worth having. We combined a Nanit camera and an Owlet sleep sock and that gave us god peace-of-mind at night. I only wish we’d topped it off with an analog walkie talkie as a backup in case the Wi-Fi went down.

We used a similar setup, but opted for two sets of analog walkie talkies (instead of one set), so that we could fall back to the secondary set if the batteries ran out in the first set (we replaced the batteries in the two sets of walkie talkies at one-month offsets so that they wouldn't run out at the same time). We also had the two sets running on different channels, to avoid concurrent outages caused by unexpected radio interference. We did the math afterwards; overall availability of the child monitoring system was four nines. Satisfactory.