A very nice and simple AC/DC indicator circuit diagram is presented in this page . The AC/DC indicator circuit diagram is based on two transistors one PNP and one NPN transistor . The circuit will give a visual indication using two LEDs . Also this AC/DC indicator circuit will provide an information if the signal tested is positive or negative .
If the red LED will glow the signal is positive DC , if the yellow LED will glow the tested signal is negative .
When the tested signal is AC , both LEDs will glow ( LED1 and LED2 ) .
The tester require a dual 9 volts power supply circuit . Don’t use it for high voltage AC or DC signals .