Project for a 12 hour timer to relay

Discussion in 'The Projects Forum' started by Flakman, Jan 6, 2010.

  1. Flakman

    Thread Starter New Member

    Apr 7, 2008
    6
    0
    Happy New Year all:

    I've searched through the forum without luck at this time. I am in the planning stages of building a project requiring a 12 hour timer. This timer will activate a relay for about 2 seconds every 12 hours. I was thinking about using a 555 timer, but not sure if the timer and caps can handle the 12 hour 0V time with any decent degree of accuracy. I don't need to-the-second accuracy, but would like the timer to be within 5-10 minutes. However, at that rate, the activation time may end up being waaaaay off after a week or so.

    The other alternative would be to use a mechanical timer. As the mechanical timer would have an on cycle of no less than 10-15 minutes, I would need something that would only pulse the relay for no more than 2 seconds and then go back to 0V after the timer supplies power.

    Thanks,

    John
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    I would recommend a microcontroller solution. If you want I will be happy to elaborate.
     
  3. Flakman

    Thread Starter New Member

    Apr 7, 2008
    6
    0
    I am open any possible solutions.

    Thanks.
     
  4. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    230
    OK, here's a relative simple circuit that will do what you want with a wall plug timer. It forms a monostable pulse generator that pulls the relay on for about 2 seconds when power is applied to the wall wart. Set the time to go ON at two 12 hour intervals for the minimum time...15 minutes for a motor driven one or 1 minute for a digital timer.


    Ken
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I liked that last one, but if it is too simple (and you really want to roll your own) you could use a 4060 with a crystal time base.
     
  6. Flakman

    Thread Starter New Member

    Apr 7, 2008
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    0
    KMoffett: I like that one, too! Brilliance in its simplicity. Thank you. I think that will work perfectly! You guys rock!

    John
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    Yes it is but you may want to consider using a programmable plugin digital timer available at Home Depot ~ $35.00. Ken's clever circuit would remain unchanged.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Nice solution, Ken!

    My .02: select a DC plug supply aka "wall wart" and relay pair that have similar ratings.
    If a relay has a coil with 250 Ohm windings, try to find a wall wart rated for 12v @ 50mA.
    12v / 250 Ohms = 0.048A=48mA

    You might also find that a wall wart rated for 10v @ 400mA will work just fine.

    Wall warts are generally not regulated. They will output within 10% of their rated voltage at their rated load current.

    Use a capacitor rated for at least twice the expected voltage in the circuit. Higher is OK.
     
  9. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    The wall wart I bench tested the circuit with was rated at 12VDC/1A...~16VDC open circuit. Smaller would be more convenient. I noticed Radio Shack had mini 12V relays in with 200Ω, 300Ω, and 400Ω coils.The 400Ω relay would allow you to reduce the capacitor size to 470uF/25V...a lot cheaper than a 1000uF/25V.

    Ken
     
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