Circuit for LED test

Discussion in 'The Projects Forum' started by IDriveaford, May 21, 2010.

  1. IDriveaford

    Thread Starter New Member

    May 20, 2010
    Hey guys got another one for everyone. I need to make a circuit that is hooked up to the ignition of a vehicle or boat that will turn on 3 LED's for 2 to 3 seconds, then off, as a test of the LEDS.


    Low fuel level and temperature high LED. Turn ign to the run position; the LEDs turn on for 2 to 3 seconds then off again.

    Thanks alot! I have been searching for hours and can't find anything automatic. Everything is "push a push button to test them." I just want to be able to turn the ignition to run and have them do a self check (like in our cars and trucks).
  2. rickmartin

    New Member

    Sep 26, 2009
    You can feed the ignition signal through a resistor that is in series with the base of an NPN transistor, with an electrolytic capacitor between the transistor base and ground. Bridge a high value resistor across the cap to discharge it so the circuit will respond again if you restart the vehicle in a few seconds. With the correct values of R an C, the cap will charge and keep the transistor turned on for any number of seconds you choose. The transistor collector would connect to B+ and the emitter would connect to the anode of two diodes. The cathode of one diode will feed the low fuel LED and the other will feed the high temp LED, through resistors of course. This circuit will test your LEDs on ignition without interfering with the circuits that actually turn them on for fuel and temp warnings.