One Flash LED 555 Timer question

Discussion in 'The Projects Forum' started by ryfitzger227, Dec 4, 2011.

  1. ryfitzger227

    Thread Starter New Member

    Dec 4, 2011
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    I have a pretty simple question. It just confused me.. I want to hook a LED through a resistor to terminal 3 (output), on a 555 timer, and have in flash once. So basically as soon as you connect power to the 555 I want the LED to come on, but after the cap has drained I don't want the LED to come on again until power is disconnected and connected again. Could a 555 timer do this? Or will I need something else.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Sure.

    Look at the attached schematic and simulation.

    R1 and C1 set the time that the LED is on. If you want the time to be 2 seconds, increase R1 to 180k, 3 seconds = 270k, etc. up to about 10 seconds. Then you need to start increasing C1 instead.

    With R1 at 100k and C1 at 22uF, the time will be about 2.2 seconds.

    D1 is there to discharge C1 quickly when the power is turned off.

    You will need to adjust R2 for the LED you're using.
     
    Last edited: Dec 5, 2011
  3. wayneh

    Expert

    Sep 9, 2010
    12,124
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    You don't need a 555 to accomplish this, although it will work. I believe the circuit you want is called a monostable

    How long do you want the LED to flash after the power is on? The attached will provide a brief flash when power is applied (to the open righthand side), and will drain fairly quickly once power goes off. You can vary the result by varying the cap and the resistors, but in general it works best for a short flash.
    [​IMG]

    Gaaa! Beaten by Sarge's quick fingers again. And I need to correct a mistake; the cap won't drain unless the input line is pulled to ground, not just "off". I used this on the output of a 4017, which flips between +V and ground, not just on/off.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I designed a circuit with an 555 to "power on everything" for about ten minutes with the touch of a button then shut off completely. The button turns on a FET running the 555 wired as a one shot. The output of the one shot holds the FET on until it times out, then everything goes off.
     
    Last edited: Dec 5, 2011
  5. Wendy

    Moderator

    Mar 24, 2008
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    Wookie, you have a major error on your schematic. Pin 2 is an input to a comparitor, it should be tied to pin 6. Without tying pin 2 down somewhere it is an indeterminate state.

    [​IMG]
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Pin 2 can simply be connected to pin 5.
     
  7. T.Jackson

    New Member

    Nov 22, 2011
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    Yeah pin 2 can go to pin 5. Many textbooks depict this. 2 to 5 and you'll be out in a year as Peter May would say.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Pin 6 is better for this application, but your schematic has it floating.
     
  9. T.Jackson

    New Member

    Nov 22, 2011
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    Are there 26 or 27 transistors in the 555 timer?

    I have forgotten. I used to remember stuff like this; hell I had teachers telling me to get a life! That's coming from a teacher!
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    23 transistors, 15 resistors, and two diodes.

    Bill, pin 6 was never floating in the schematic I posted.

    It really doesn't matter whether pin 2 is connected to pin 5, pin 6 or Vcc for this application. If pin 2 is tied to ground, then the circuit won't work.

    T.Jackson,
    Check out these pages:
    http://semiconductormuseum.com/Transistors/LectureHall/Camenzind/Camenzind_Index.htm
    Hans R. Camenzind was the gentleman who invented the 555 timer while working at Signetics.
    He also designed the 565 and 566 ICs, the latter of which became hugely popular with synthesizer builders.
     
  11. Wendy

    Moderator

    Mar 24, 2008
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    I never said pin 6 was floating, I said pin 2 was, which you have fixed. Such drama! :D

    My personal preference is almost always to connect pin 2 to pin6, since it makes a simple Schmitt Trigger.

    Come to think of it, the same scheme could be used with any inverting Schmitt Trigger chip. Handy if you need other gates.
     
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