The Yamaha PSS-140 keyboard from the 1980s is good fun, running on either external power or batteries. If you do run on batteries, the omission of a power indicator can be a pain, making it too easy to leave the unit on when you've finished using it. Next time you come to it, the batteries will be flat.
Adding a power LED can help to avoid such disappointment. I show you how in this video.
Article - Simon Taylor, 12th March 2023