HELP! making adjustable timer for .25 - 2 seconds

Discussion in 'The Projects Forum' started by larry gardner, Sep 17, 2008.

  1. larry gardner

    Thread Starter Member

    Sep 17, 2008
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    Im new and really need help. Im wanting to build a timer for actavation of a solenoid from a min of about 1/4 second to around 2 seconds. I need it adjustable. I find a lot of schematics for timers but none for short durations. I am new to electronics and this will be my first project. I can change the actavation time if needed but I need it to be adjustabel in about 1/10 second increments.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    See the attached schematic.
    As it is, it will output a 0.2 second pulse when the input toggles low for a moment. If you add a 1 meg potentiometer in series with R1, the time will be adjustable from 0.2 seconds to about 2.6 seconds.

    You should use a diode connected across the relay coil to absorb the reverse-EMF pulse when the coil is de-energized.

    It would be a good idea to use a transistor like a 2N2222 to sink the current from the relay. Use a 1k resistor from the output of the 555 timer to the base of the transistor to limit base current.
     
  3. larry gardner

    Thread Starter Member

    Sep 17, 2008
    15
    0
    R1 is a resistor of 82.75k can I use a 82K or combine two resistors for 83k total. Where should I buy my supplies? I would like a one stop internet shop.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The numbers are approximate. An 82k resistor is a standard value. That will establish your "low end" for the time. Even 75k would be fine.

    Digikey is a first-class supplier, and they'll even ship your stuff via 1st class USPS mail if you ask them to. That'll save you money on shipping.

    Mouser is another first-rate shipper that'll get your stuff to you right away.

    Radio Shack is expensive, but there's likely one not too far from you. The reason they're expensive is because they have a storefront, and they stand behind what they sell. I recently exchanged a potentiometer that I'd bought at another Radio Shack 20 years ago, because it was out of tolerance, and it had never been used, I had the original packaging for it, and they still sold the exact same item.

    That's support.
     
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