Minute and Seconds Down Timer

Discussion in 'The Projects Forum' started by Kyle Ruiz, Mar 28, 2016.

  1. Kyle Ruiz

    Thread Starter New Member

    Oct 20, 2015
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    I need help with a school project we're doing. The output is to have a timer that counts down starting from 1:59 to 0:switches and then stop until the circuit is reset.
    -The minute hand is shown by an LED (lit = 1 minutes , not lit = 0 minutes) and the seconds are shown in a 7-segment display
    -The 0:switches part means that a set of switches will indicate where the timer will stop ex. 0:15 - 0001 0101 , 0:30 - 0011 0000

    I'm currently using a 74191, a 7447 for the seven segment display, and a 2Hz 5V astable multivibrator for the clock pulse.

    Question: How do I make the timer stop using switches? Is there anyway for the switches to be considered invalid ( 00 ) if it is higher than 5 / 9? A schematic would be appreciated.

    Additional Note: We're only allowed to use basic ICs like the basic gates, flip flops, synchronous up down counters, decoders etc.
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi K,
    Have you considered using diodes between your switches and the 74191 output pins, to form AND gates.?

    E
     
  3. Kyle Ruiz

    Thread Starter New Member

    Oct 20, 2015
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    I can use AND gates.
    Can you explain how it affects the circuit to make it stop and which connections need to be made? (since there are a total of 8 switches and 9 outputs taking into consideration the LED).
    appreciated
     
  4. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    You arrange the Gates to only give a signal when the state of the 74191 outputs matches the setting of the Stop switches.
    eg: for '9' that would be 1001 in binary.
    You have not said how the 2 Sec pulse generator is configured.? The output from the Gate/s would stop the 2 sec pulse gen.
    Do you have a diagram of the 2 sec gen that you could post.?

    E
     
  5. Kyle Ruiz

    Thread Starter New Member

    Oct 20, 2015
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    I'm using a simple 555 timer which is then fixed onto our trainer kit. The generator itself is independent of the project and we're only using it for the pulses. So I don't think we can completely shut down the pulser. Sorry for the lack of info.
    upload_2016-3-28_20-59-5.png
    Its a varying pulse generator, the pot is varied so the output frequency is 2Hz (for a readable output)
     
  6. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi,
    Look at this pdf for ideas on how to configure your gates.

    If you pull the 555 RST pin low on a count match, it will stop the 555.
    Note: disconnect it first from the Vsupply!
    E
     
  7. Kyle Ruiz

    Thread Starter New Member

    Oct 20, 2015
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    I'm sorry if I wasn't a bit clear, we can't change how the pulser is configured since its an independent circuit. The output of the pulser is hanging and we just jump the clk input of the 74191 *edit to the hanging pin of the pulser.

    *We're using a breadboard for the timer but the pulser, seven segment display, and LEDs are all soldered onto a PCB
     
  8. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi K,
    OK, the pdf I posted was just an example of combinational logic.
    If you cannot modify the 2sec pulse generator you will have to Gate the pulse driving your counter.
    eg: say your switches and logic output a Low logic level at the set time period, this Low could be one input of an AND gate and the other input would be the 555 output pulse.
    So when the logic Low at the end of the required time period occurs, it will inhibit any further input pulses to your counter.

    Can you post your circuit showing your proposed switch/logic solution.?

    E
     
  9. Kyle Ruiz

    Thread Starter New Member

    Oct 20, 2015
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    0
    Thanks mate, I managed to get an idea on how to make it stop.
    I changed the 74191 to 74190 and used XNOR gates so when the logic of the switches matches the logic of the outputs of the 74190 it creates a logic 1 output which goes into AND gates and into the CTEN pin of the IC which when logic 1 stores the current binary output and stops the pulses.

    Truth be told the project was already a month due and I started the day before the deadline. Everything worked out anyway.
    Again thanks for the help.

    *edit here's the schematic we used (we made some minor changes in the end but its more or less this, never mind the output the circuit is currently running in the multisim), for anyone who might view this thread in the future.
    [​IMG]
     
    Last edited: Mar 29, 2016
  10. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
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    hi K,
    Changing to a 74190, a Decade counter is much better choice than the 74191 Binary counter.

    Pleased to hear you have managed to sort it , good luck with your future studies.;)

    Eric
     
  11. Picbuster

    Member

    Dec 2, 2013
    374
    50
    Please move this to homework help this should not be in this forum.
    picbuster
     
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