this tester I found in the junkyard. If it is connected to 240V, both indicators come on. If it is connected to 120V only the 120V indicator lights. This leads me to believe that the circuitry inside is simply a pair of large value resistors. The 240 lamp probably has a resistor of super large resistance to keep it from striking on 120v and the 120v lamp has a resistor of a lower value to make it light on 120v but also not blow up on 240.
|Lamp Type:||Neon (NE)|
|Filament/Radiator Type:||Low Pressure Neon Discharge|
|Download:||Download this File|
|File Size:||528 KB|
|Date added:||Jan 16, 2020|
|Dimensions:||1822 x 2464 pixels|
|Favorites:||Add to Favorites|
Pretty, please no more Chinese failure.