Detects 1.8 to 230 Volts DC or AC
This circuit is not a novelty, but it proved so useful, simple and cheap that it is worth building.
When the positive (Red) probe is connected to a DC positive voltage and the Black probe to the negative, the Red LED will illuminate.
Connecting the probes to an AC source both LEDs will go on.
The bulb limits the LEDs current to 40mA @ 220V AC and its filament starts illuminating from about 30V, shining more brightly as voltage increases.
Therefore, due to the bulb filament behavior, any voltage in the 1.8 to 230V range can be detected without changing component values.
Inside a huge PCB factory: https://www.youtube.com/watch?v=_XCznQFV-Mw
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