Blinking tower light

Discussion in 'The Projects Forum' started by trackrat, Apr 22, 2009.

  1. trackrat

    Thread Starter New Member

    Apr 22, 2009
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    I am building an N-Scale model railroad layout. I have a radio transmission tower on it and want to make the grain of wheat light on top blink. Not a simple "off-on" blink which I could easily do with my PLC, but a blink that gradually brightens and dims ove say a five or six second time period like they did in the old days. I was looking at RC time circuits, but cannot figure how to have a large resistance in series with the RC reistance and still get a filament to glow.

    Thanx,

    Trackrat
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    http://www.solorb.com/elect/pwm/pwm2/
    Maybe something like this. But instead of the pot on pin 6, use two more LM324 amps and make a long period (6 second) triangle wave generator to drive pin6.

    Ken
     
  3. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Howdy.

    Just an interesting point. Radio towers achieved the slow glow not by intent...but because the lamps had such huge thermal mass. Typically a top beacon lamp will have 2 X 600 watt bulbs up there. They are designed not to run efficiently, but to run a very long time, so they have very heavy filaments that run relatively cool :) Now, nearly all broadcast towers have gone to strobe lights...even during the night. You can tell they're strobed by shaking your head while looking at them...you'll see a series of "trails". (YOu can also do a Bronx cheer and get the same results. Just do this alone. :) )

    Eric
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    To answer your question, it will probably be best to drive the lamp through a transistor, and use an RC time constant on the base of the transistor to simulate the "cool down." For best realism, you might want to play around with the linearity....using a little bias adjustment. Brightness of a lamp is NOT directly proportional to voltage OR current, so you will have to fiddle with the time constant and "gamma" a bit.

    Eric
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Check out the "Fading Red Eyes" circuit about 1/2 way down this page:
    http://ourworld.compuserve.com/homepages/Bill_Bowden/page5.htm

    Yes, Bill Bowden used his circuit to drive LEDs, but there's no reason that you can't use higher-current transistors like 2N2222 or 2N2907's, and replace both the LEDs and the 100 Ohm resistors with your bulbs. However, your setup would probably last quite a bit longer if you used miniature LEDs for light.
     
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