555 timer input/output voltage

Discussion in 'General Electronics Chat' started by ddaytona1, Feb 7, 2011.

  1. ddaytona1

    Thread Starter Member

    Nov 3, 2008
    I'm building a 30 second timer to turn on a +12V seat belt light in a car when the car is 1st turned on.

    When the car key is turned on, 12V will be supplied to the 555, and remain there for the entire time the car is running. Is that a problem?

    I'm using the circuit in thread


    and also read http://forum.allaboutcircuits.com/showthread.php?t=14018

    I've calculated Rt and Ct to be 100uF adn 272Kohms to get my 30sec pulse output.

    Other Questions.

    Thread 14018 says that +12V to the 555 shouldnt be a problem. In that case, if I connect my +12 light to Pin 3 and gnd at Pin 1, should I expect the light to go on without problems OR do I really need the relay K1 set up shown in the diagram in thread 22021?

    In the diagram in thread 22021, is R1 10Kohm and C1 1uF ? and C1 is used in place of an on switch to quickly start the circuit when power is applies?

  2. mik3

    Senior Member

    Feb 4, 2008
    Why do you need the timer?

    How does the light turn off?
  3. ddaytona1

    Thread Starter Member

    Nov 3, 2008
    This is an old car that used to have a seat belt warning light that turned on for 30 seconds when the car was first turned on - then went out.

    I thought the light would turn off when the 3 second pulse supplied to pin 3 went low.