Newsgroups: sci.electronics
From: dwalster@lincoln.gpsemi.com (Dave Walster)
Subject: Re: flasher
Date: Fri, 9 Jul 1993 11:48:50 GMT

Running a flashing LED straight from 12 Volts is not generally a good idea,
some of the LED's that I have come across will only stand about 7 Volts.
You cannot use a series resistor to drop the voltage as when the LED is
in the OFF state, there is no potential dropped across the resistor & hence
the LED 'sees' the full 12 Volts.
The circuit below was developed to provide the functions necessary whilst
also offering much improved reliability and safety.  I've installed it into
53 cars here since January, it's a popular and cheap circuit.

          1N4148
                                      6V2 400mW    FLASHING LED
            |\ |         BC328
            | \|                           | /|   |\ | /^
+12 V ------| /|-------\         /---------|/ |---| \|--+------ 0V GND
            |/ |        \       /          |\ |   | /|  |
                         E     C           L \|   |/ |  |
                         -------                        |
                            B                           |
             1KO            |             22K           |
                            |                           |
       O---/\/\/\/\/\-------+--------/\/\/\/\/\---------|

       ^ Connect to a switched live, i.e. one which goes to 12V when the
       ignition is turned on.  A good place I've found to put the wires is
       with the 0 Volt lead to the bulb side of the brake switch, the
       trigger wire onto the power side of the brake switch and the +12 Volt
       lead to either the cigarette lighter or the fuse box direct.

** Even though this circuit doesn't consume much power, it's a good idea to
   place a 500mA / 1A fuse in line for safety.

Hope this is helpful.
Dave Walster,
IC Engineering Dept., GEC Plessey Semiconductors, Lincoln, England.