help determining unusual 555 astable component values

Discussion in 'The Projects Forum' started by tonkatoy, Jan 25, 2015.

  1. tonkatoy

    Thread Starter New Member

    Jan 25, 2015
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    Hello all--
    I need to power the electromagnetic clutch on my garden tractor intermittently. I would like to have approximately 5 to 10 seconds of "ON" time, preferably adjustable within that range, with an adjustable "OFF" time of from 5 to up to 10 minutes. It appears that a circuit like this: http://www.doctronics.co.uk/Images_swf/p555_exdca.swf
    is almost exactly what I need.
    Can both R1 and R2 be replaced by potentiometers? (I would assume so).
    I am having trouble selecting the proper values for R1, R2 and C1 to get the time ranges I'm looking for...this is a much lower frequency (I think that's the term to use) than what common examples of this circuit show. I am a machinist by trade, I know just enough about electronics to be dangerous. This can't be that hard of a circuit, I just need a little help to get me going in the right direction.
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,042
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    A 555 will give the short period but getting 5 to 10 minutes is problematic.
    A digital counter timer would work better for that such as this.
    You can use this long timer to trigger the 555 to give the short interval.
     
  3. tonkatoy

    Thread Starter New Member

    Jan 25, 2015
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    Crutschow--
    Thanks so much for the tip! Still a simple enough circuit that I think I can handle it...and elegant.
    Follow up question: can replacing R8 with a potentiometer vary the length of time the relay is energized, like with the 555 circuit? (I could also live with a fixed length of time in the 5-10 second range if I could determine the proper cap/resistor combination). More important would be the delay (un-energized) time...I need to have adjustability in the 5-10 minute range. So, if I use pin 6 (4-12 minute range) and substitute a potentiometer for R4 I should be able to meet my requirements without an additional 555 chip, right?
    None of the times need to be super precision. My application is to slow down the speed of a winch (doesn't need to move continuously) without a massive amount of mechanical gear reduction by cycling the electromagnetic clutch driving it. I thought of the concept as being analogous to a windshield wiper delay circuit...just operating a much lower cycle frequency.
     
  4. Dodgydave

    Distinguished Member

    Jun 22, 2012
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  5. tonkatoy

    Thread Starter New Member

    Jan 25, 2015
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    never mind my last post. Reading further, the circuit Crutschow linked to shows potentiometers doing exactly that. But the question about not needing the 555 is still relevant. I don't understand a need for that additional circuit. Unless the time the relay is energized can not be made that consistently short (5-10, maybe 15 seconds) reliably?
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Yes, you can add a pot to the 555 circuit to allow adjustment of the ON time.

    R4 in the CD4060 circuit is already a pot (connected as a rheostat) to allow adjustment of the OFF time.
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
    4,176
    397
    Either will work, 555 easily adjusted. 10k = 1k in series with 10k pot., 47k = 1M pot., 22 uF = 1000 uF. Check C leakage, try for 5 M or better. I've run 555 up to 3 hours. 4060 at 24 h good to +- 5 m.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You can't get both the short and long times from the 4060 circuit at the same time. You need a 555 for the short period that is triggered from the 4060 long period pulse.
     
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