Guru
Joined: 24/12/2017
Location: AustraliaPosts: 1693
| Posted: 05:58am 31 Dec 2021 |
You still could use a USB power bank and leave the power bank supplied via a mains 'wall wart'.
With an opto coupler you could monitor the input to the power bank and use its output signal to indicate power failure and use that to initiate closing the file (or not).
Alternatively a supercap could hold the power up long enough for you to detect a power failure and close the file.
Bill