Delay off timer circuit needed

Discussion in 'General Electronics Chat' started by Dodgehemi61, Jun 18, 2011.

  1. Dodgehemi61

    Thread Starter New Member

    Jun 18, 2011
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    I would like to build a little circuit using a LM5555 timer that when 12 volt is applied to the input, it will activate the output and hold it for 45-60 seconds after the input goes away or 0 volt.
    What I want to do is add a small cooling fan on the heatsink fo my 60 watt mobile ham radio to help keep it cool when I am talking on it. With just the heatsink after a long trannsmit time ( two minuites) it gets so hot you can not put your hand on it. And I do not want a fan to run all of the time also. I have a nice little 40x10mm fan that only draws .06 ma.
    Thanks for your help.
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    3,795
    951
    Couldn't you do the same thing more reliably with an NTC thermistor?

    Put it on the heat sink and have it as a part of a voltage divider feeding an op amp comparator when the resistance decreases the op amp will go to the positive rail(if wired to do so) and activate a transistor switch with the fan. That way if you don't talk long enough to heat up the heatsink you don't get the fan running.

    Easier still is to just run the fan when you power on. Keeping the output as cool as possible prevents premature stress failure of the output transistor.
     
  3. Dodgehemi61

    Thread Starter New Member

    Jun 18, 2011
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    I would like to make a correction, the motor draws 60ma.
    Kermit2, that is a great idea using the thermister. I have a few LM355 I could work with. I thought about having the fan run all of the time, but the fan noise will get old after a short time. Al i need now is a circuit to work from. :) Thanks
     
  4. #12

    Expert

    Nov 30, 2010
    16,343
    6,826
    It's called a "one shot". Made with a 555 timer. Examples are on this site and the internet in general, now that you know what to look for. I'm feeling lazy right now. Could you do a few searchs for "one-shot timer" and see what you can find?

    ask again if you get nowhere.
     
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
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    If the fan runs on 12V, give it 9V and leave it on. It will be quieter that way. You could also use your 555 as a PWM and vary the fan speed as desired.
     
  6. Dodgehemi61

    Thread Starter New Member

    Jun 18, 2011
    3
    0
    Thanks everyone for the great information. I will do some looking around for the one shot circuit, and also slow the fan down with a 9 volt zener.
     
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