A question on count down alarm circuit

Discussion in 'The Projects Forum' started by harisudharsan, May 13, 2014.

  1. harisudharsan

    Thread Starter New Member

    May 6, 2014

    Just for my practice, I am trying to create simple gadgets using 74xx family chips.

    I am trying to make a simple count down timer that has the ability to trigger an alarm after x minutes. While setting the time, the 7 seg display shows only the minute and nothing else.

    For example, I can set the counter to 90 minutes and press start. It has to count down from 90 to 0 and trigger an alarm after 90 mins for 1 minute.

    As additional feature, Possibly I want to turn off the whole circuit after the alarm is over (for battery power saving). But that is my second step.

    I found the below circuit of counter to be good.. But it looks like counter
    cannot go backwards. Is it really so ??


    If so can some one know a counter circuit that can go backwards ?

    With Regards,
  2. wmodavis

    Well-Known Member

    Oct 23, 2010
    Google "UP-DOWN Counter". You'll find lots of possibilities.
    harisudharsan likes this.
  3. absf

    Senior Member

    Dec 29, 2010
    74HC190 decade up/down counter
    74HC191 binary up/down counter
    74HC192 4 bit synchronous up/down counter

    40192 4 bit decade up/down counter
    40193 4 bit binary up/down counter.

    harisudharsan likes this.
  4. crutschow


    Mar 14, 2008
    You can also use an UP counter and use gates to detect the count corresponding to the time delay you need. A CD4060, for example is a 14-bit counter with built-in oscillator the can use RC elements for non-critical timing or a crystal for more accuracy.
    harisudharsan likes this.
  5. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
    Decade counters come in two flavors, up only or up/down. So you will need

    1. two presettable up/down decade counters

    2. two BCD-to-7 segment display decoders

    3. a one-pulse-per-minute clock source

    4. a method to set the initial value into the counters

    5. misc. logic to control start, stop, alarm drive, etc.

    To start out, these can be thought of as five separate mini-projects:

    1. Select the chips for the decade counters. Work out the interconnections for a common clock signal, borrow, etc.

    2. Select the display decoder chips and displays, calculate the current limiting resistors, connect everything.

    3. Determine what defines the output state that enables the alarm, work out the logic, interconnects, and alarm device driver.

    4. Design the start/stop and load logic.

    5. Design a way to enter the initial value for the timer. You have two basic methods to choose from, either enter the number directly into the counters, or add control logic to have the counters rapidly count *up* to the desired value before you enable the count down timer function. This is the hard part.

    Last edited: May 14, 2014