LED flasher circuit

Discussion in 'General Electronics Chat' started by ledharper, May 10, 2012.

  1. ledharper

    Thread Starter New Member

    May 9, 2012
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    I am into model railroading, specifically Lionel, which is powered by variable AC through a transformer. Typical volatage is 0 to 18V but the transformer is capable of about 24V. Normal running voltage is 6 - 14V.

    I want to build a simple driving circuit to alternately flash two red LEDs in a crossbuck to simulate a warning at a railroad/road crossing. The system will be activated by a micro switch through the pressure of a train wheel passing over it. I see several problems that will need to be addressed.

    !.) Placement of the micro switch to start flashers before the train actually reaches the crossing.
    2.) As the switch will be repeatedly activated by each wheel, how will that affect the flasher? (There could be as many as 60 wheels.)
    3.) Cutting off the flasher after a decent delay, after the train has passsed.
    4.)Setting/controling an approiate flash rate.

    I know that voltage dropping resistors (750 Ω?), and a diode will be need to control current flow for each LED but what about 555 timers to control flashing speed and delay in for stopping the flashing after the train has passed? I have some LEDs, 750Ω resistors and 1N4001 diodes ordered but I don't know what else I will need.

    I have a little experience in elctronics, wiring, soldering, etc., but not much experience in theory.

    Any help you can give me would be deeply appreiated.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    So what u need is an Astable.

    Read
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    That is my working copy, where I will accept comments and feedback on the article.

    The released blog version is here...

    LEDs, 555s, Flashers, and Light Chasers

    Side comment, there are photosensor modules that will work much like microswitches, except no moving parts. You can build them or buy them. They sense the reflection of a IR beam and trigger off that.

    You probably need a retriggerable monostable to keep the lights lite. This is pretty straightforward. I would recommend you break this up into several projects, and address them one at a time.
     
  4. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    It will probably take two 555 circuits to do what you want. The first one is a monostable (also called a one-shot) which will turn on for a predetermined amount of time (long enough for the train to pass.) The second one is an astable, which will switch the LEDs on/off in alternating fashion for as long as the astable gets power from the monostable. There actually is an IC called a 556 (NE556 or similar) that contains two 555s in a single package.

    Of course, a microcontroller circuit is much simpler to build to accomplish what you want, but you have to be able to program it.

    Either way, what you want is so common, I bet that you can do a little Googling, and find a completed project (or several) on-line.
     
  5. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    An attempt to put all suggestions down in black & white: U1 keeps U3 from timing out untill all cars have passed; U2 triggers U3- repeated triggers have no effect on U3. When U3, pin 3 is high, U4 is allowed to oscillate, alternating the two LEDs at about 1/2 sec. Power can be derived from track AC V by rectifing, filtering & regulating. Brightness of LEDs regulated by 150Ω resistors.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That looks like it should work. You can save one part (whoopee!) by replacing U1 with a diode.
    EDIT: This assumes that the switch is closed long enough for the cap to discharge. I doubt that this is a problem.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Looks like there was a little cut/paste done there, 1960's style.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Thanks Ron - who needs U2- gone with U1. No paste used, just " Scotch " tape fron back in 1940s.
     
    Ron H likes this.
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The editing I did was with Microsoft Paint.:)
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Even better!:D
     
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