Small motor timer circuit.

Discussion in 'General Electronics Chat' started by regpye, Feb 24, 2010.

  1. regpye

    Thread Starter New Member

    Nov 19, 2008
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    I have searched the threads and not been able to find what I am trying to do.

    I am constructing a small machine and I need to turn on a small motor (windscreen washer motor 12v)
    The motor will run from a 12v DC supply and should come on every 10 minutes or so (not critical for the time) and run for approx 5 seconds and then off again for another 10 minutes, and so on.
    I was thinking of using a 555 to trigger a mosfet, but my knowledge of the timing circuit is inadequate to work out the values.
    I would be most appreciative if someone can steer me in the right direction.
     
  2. Wendy

    Moderator

    Mar 24, 2008
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  3. regpye

    Thread Starter New Member

    Nov 19, 2008
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    Thanks Bill,
    I have looked at the second circuit that you mentioned, it has a P channel mosfet. I have tons of N channel mosfets left over from another job, they are a bit over rated for this application being 82 amp handling, but they are cheap and I have them in stock. (IRF2807)
    Not sure of the calculations, but I think R1, 15K, R2 150K, C1 470 would be close. Could you check my calculations please.

    Thanks
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Remember 10 min = 600 sec, so R2 would be around 1.3 meg. ohms.??
     
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, for a 470uF cap, about 18k for R1 and 1.8 MEG for R2 should get you in the ballpark.

    The capacitor will need to have pretty low leakage rate, or R2 won't source enough current to charge it up.
     
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  6. regpye

    Thread Starter New Member

    Nov 19, 2008
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    If I changed the cap to be 220uF I would have to reduce the R1 and R2? I am not that good at this stuff, but I am learning.
    Also to drive an N channel mosfet, could it be driven directly from pin 3 of 555? or would I need to have a transistor between them? The mosfets that I have available are IRF2807, much bigger than I need for the small pump.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    If you reduced the cap to 220uF, you would have to increase both the resistors to compensate. Use the formulas that are in Bill_Marsden's schematic. R1=33k, R2=3.9MEG

    Yes, you can drive the MOSFET gate directly from pin 3 of the 555 timer; I suggest using a 20 Ohm resistor connected directly between the gate and pin 3 of the timer.

    Keep in mind that large electrolytic caps may vary from their rated capacitance by 20% or even more. Your mileage with the values I've given are only approximate.
     
    Last edited: Feb 26, 2010
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  8. regpye

    Thread Starter New Member

    Nov 19, 2008
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    OK, Thanks guys,
    I have built the circuit and after a bit of tweaking it is working fine.
    Had to increase R1 to 47K and on another circuit board (same design) it needed 56K. Maybe a difference in the capacitor values, even though both marked 470uF.
    I added a couple of LEDS to show the on and off functions.
    Now I can build the rest of the machine.
     
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