Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Combinational and Sequential Circuits

  1. Mar 23, 2012 #1
    Is there any way that I can detect a certain time in the day, and apply a signal HIGH for 30 minutes? The only way I can think of how to do it is via a frequency divider that sends a pulse for 30 minutes, but that requires >30 Flip Flops as we are working with 27MHz clock frequency.
  2. jcsd
  3. Mar 23, 2012 #2


    User Avatar

    Staff: Mentor

    You can use a Real-Time Clock IC, and read it from your uC:


  4. Mar 23, 2012 #3


    User Avatar
    Science Advisor

    If you had to do it with a 27 MHz clock and discrete components, you could use a few 74HC4020 chips.

    These are binary dividers and one chip will divide by 16384, so two chips would give you about a 10 second output square wave.

    A further chip of the same kind could divide by 8690 by some careful use of diodes and this would get you close to 24 hours.

    Since this may be homework, you should work out the details yourself.

    There are other chips that can do this, but a far better way would be to use a 24 hour timer set to turn on for 30 minutes at the same time each day.
    Sprinkler timers do this and so do the simple ones intended to turn a few lights on and off to make a house appear occupied while the owners are away.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook