Detects 1.8 to 230 Volts DC or AC, Minimum parts counting
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. Reversing polarities the Green 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.
P1 = Red Probe
P2 = Black Probe
D1 = 5 or 3mm. Red LED
D2 = 5 or 3mm. Green LED
LP = 1220V 6W Filament Lamp Bulb
A two colors LED (Red and Green) can be used in place of D1 & D2.
Source: Red Free Circuit Design