Dual color (4 line x 3 column) led, 12 white and 12 blue are intermittently switched controlled by two NPN transistors BC337. The speed and duty/cycle timing of lighting duration is determined by NE555 oscilator. The number of repetition of sequential lighting of each group of led is determined by 5-stage divide-by-10 Johnson counter with 10 decoded outputs and carry out bit (CD4017).
The intermittent lights from two group of led imitate the police car strobe lights.
The speed of switching off and on of led are continuously regulated by variable trimmer resistor.
24 LED Police Flasher - [Link]