This circuit is used to simulate the police car lights by alternating flashing strobes seen on British police cars. The IC1a forms a square wave oscillator having adjustable frequency with VR1 to give the best effect.
This square wave is buffered by IC1b and in turn drives the decade counter IC2. Outputs from the counter Q0/Q2 and Q7/Q9 are in turn diode ORed to give alternating double pulses.
C2 and C3 with R3 and R5 differentiate the pulse time in conjuction with the output drivers IC1c and IC1f. This produces a short pulse (30 ms) which enhances the flashing effect and adds to the illusion. The output drivers in turn drive a pair of hyperbright blue leds D5 and D6.
It is worth spending a little extra on using realy bright leds if the best effect is to be obtained. A 6 V camera battery or four AAA cells gives a long life in a small package. In use adjust VR1 to give the best effect.
Article sent by Ian Johnson (Sw) at email@example.com, thanks.