[Big Clive] had some 22mm digital AC voltmeters, made to put in a panel. There was a time when this would have been a significant pain, since it required you to make a large square hole. Of course, in a world of CNC and 3D printers that isn’t as big a deal as it used to be, but the ones [Clive] has are nice because having a round footprint you can drill a hole for them with a hole saw or a stepped bit. Of course, he wasn’t satisfied to just use these inexpensive meters. He had to tear one apart to look inside. You can see his review and teardown in the video below. The meters are available in a range of AC voltages, although [Clive] didn’t think the ones he had would safely handle their rated maximum.
Inside, the modules reminded us of cordwood construction in a way. Most of the electronics are on a small round board. But several components connect to the board and the bottom cap in a vertical orientation. The meters are available in several colors, but [Clive] likes the red ones as they appear brighter than the others. The voltage reading compared favorably to a Fluke meter.
The verdict is that for the price (under $2 from China; a little more in the US) they are handy, but you might not want to consider running significant voltage through them. You could use these instead of pilot lights and get the voltage reading for free. We were surprised that someone doesn’t make a DC version in the same form factor, at least if they do, we couldn’t find it. If you don’t mind drilling the square hole and letting a bezel cover your sins, you could use a universal meter with an Arduino instead for many applications.