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circuit needed

Discussion in 'Electronic Basics' started by Jamie, Aug 9, 2004.

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  1. Jamie

    Jamie Guest

    Could someone suggest a simple circuit that would light an led for about 2-4
    seconds when power is applied and then turn off and stay off until power is
    cut and applied again?

    Thanks in advance JJ
  2. Tom Biasi

    Tom Biasi Guest

    Look at some 555 circuits.
  3. Namely, look for "one shot" or monostable 555 circuits.

  4. John Schuch

    John Schuch Guest

    Try this:

  5. This should be OK:
  6. Looks wonderful, Terry! And right about 3 seconds, or so.

    (Personally, I prefer to _NOT_ bus the power around in a schematic. The extra
    lines detract from the thrust of the idea. I suppose they are excellent for
    those worrying about soldering up connections, though.)

  7. R.Spinks

    R.Spinks Guest

    Should not the resistor (R1) and capacitor (C2) be in opposite spots to
    provide a negative going pulse on powerup?
  8. No, that wouldn't work.

    As shown the RC time constant is long relative to the typical ramp up
    of the power supply. So pin 2 remains below required 1/3 threshold for
    a short time, triggering the mono. Then it stays +ve so that it's
    still high when mono ends, avoiding re-triggering.
  9. John Fields

    John Fields Guest

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