Inside a 22mm digital panel indicator/voltmeter.

This is neat, and surprisingly cheap too. Instead of just being a panel indicator light it also displays the supply voltage. That's a very nice solution for quick diagnostics. The quality of construction seems OK. Very simple and well rated components for standard mains voltages, although the sense resistor may not be rated for the higher phase to phase voltages.
Power dissipation is low at about half a watt with predictably poor power factor (0.07) because of the simple capacitive dropper and low circuit voltage. There's no significant heat being dissipated on the PCB.

The music track referenced is an old fairground classic called Dominator:-
https://www.youtube.com/watch?v=-dohzrXT09w
Carl Cox mix:-
https://www.youtube.com/watch?v=iYvrXtmrLSY

Here's a universal ebay search link for the meters:-
https://www.ebay.com/sch/i.html?_from=R40&_sacat=0&LH_BIN=1&_nkw=22mm+voltmeter&_sop=15

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