Detects 1.8 to 220 Volts DC or AC
Minimum parts counting
D1________5 or 3mm. Red LED D2________5 or 3mm. Green or Yellow LED LP1_______220V 6W Filament Lamp P1________Red Probe P2________Black Probe
This circuit is not a novelty, but it proved so useful, simple and cheap that
it's worth building.
When the positive (Red) probe is connected to a DC positive voltage and the Black probe to the negative, the Red LED illuminates.
Reversing polarities the Green LED illuminates.
Connecting the probes to an AC source both LEDs go on.
The lamp limits the LEDs current to 40mA @ 220V AC and its filament starts illuminating from approx. 30V, shining more brightly as voltage increases.
Therefore, due to the lamp's filament behaviour, any voltage in the 1.8 to 220 Volts range can be detected without changing component values.
Note: A two colors LED (Red and Green) can be used in place of D1 & D2.