I remember as a kid some device had one of these, but I can't for the life of me remember what it was.
It is likely to have either been the recording level indicator on an old valve reel-to-reel tape recorder, or the tuning indicator on an old valve FM radio.
alt Hacker News
It is likely to have either been the recording level indicator on an old valve reel-to-reel tape recorder, or the tuning indicator on an old valve FM radio.