Who invented the Yahrtzeit tablet (panel)?
At first glance, this could not have been common before 150 years ago, as there is no room to place candles where the electric lights go.
Who invented it, and is it mentioned anywhere in seforim?
Old shuls in Europe have these plaques as well, but without the lights. Is your question when (and why) lights were added, or when these plaques were first used? I recall from Medieval Jewish History classes that the concept/importance of Yahrtzeit was popularized in the aftermath of the Crusades.