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Another fun battery one - most remote controls work by flashing an infrared LED we can't see with the naked eye, but every cellphone camera can see the flashes as purple light.

If you think a remote has dead batteries, open your camera app, point the remote at the camera, and push a button while looking at your camera app. You should see flashes. No flashes means the remote isn't working, probably dead batteries.

There's a lot of Bluetooth remotes out there these days that DONT do this so be aware of that too.