Can anyone help me make a simple adjustable timer?

Discussion in 'General Electronics Chat' started by sgermiller10, Mar 13, 2010.

  1. sgermiller10

    Thread Starter New Member

    Mar 7, 2010
    Can anyone help me to make a simple adjustable timer that can be used to switch a 9-12 Volt relay. I need the relay to turn on when a push button is engaged/pressed, and automatically turn off when the timer reaches the set value. I need the timer to turn off the relay after 0 minutes, to a max of 4 hours. There is no need for any sort of display, although a simple one wold be very helpful.

    :D:D:DThank You For Reading And Posting Any Posts!!!!!!!!!:D:D:D
  2. VoodooMojo

    Active Member

    Nov 28, 2009
    How accurate does the 4 hour thing have to be?
    The link here and the attached schematic shows a popular/typical method for what you are looking for.
    Increasing the C2 100uf cap with a larger one (4700uf+) will give you a pretty reliable, fairly accurate extended time timer circuit.
    delays of many hours is possible.

    Is is no harder than this:

    $12 US dollars
    can the parts be bought that cheap?

    If accuracy is needed, a different avenue may be pursued.

    EDIT: the words "fairly accurate" were added.
    EDIT: 4700uf+ was added
    Last edited: Mar 13, 2010
  3. KMoffett

    AAC Fanatic!

    Dec 19, 2007
  4. VoodooMojo

    Active Member

    Nov 28, 2009
    depending on your experience level, I would opt for the circuits Ken posted for better performance and accuracy.

    The circuit I posted works very well and accurate up to about 4 to 5 hours but after that it is a bit iffy. It could be off a couple minutes either way after 4 - 4.5 hours.
    That is with a 4700uf capacitor at C2 when looking for long time delays

    a suggestion would be to use switches at C2 to select different values of capacitance for C2 for more sensitive adjustments at lower delay time needs.

    like a 100uf for 0 to 15 minute range
    1000uf for 15 to 60 minute range
    2200uf for 1 to 2 hour range
    4700 for 2 to 4 hour range.

    readings for the graph were with 9 volts supply voltage
    Last edited: Mar 14, 2010