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Design a stopwatch

  1. Oct 10, 2015 #1
    << Mentor Note -- Thread moved from the technical Engineering forums so no HH Template is shown >>

    Project Description:

    Design a Digital Stop Watch using discrete electronic components
    Functions: Start, Stop, Reset
    Should be able to count up and count down
    The interval time should track the minutes, seconds and tenths of a second. Ex. 3:45.8 would be 3 minutes, 45 seconds and 8 tenths of a second
    The max interval (up or down) will be 9:59.0 after which watch should reset to 0:00.0
    The design may be done using available logic devices or using programmable logic devices (PLDs), but no microcontroller

    I would appreciate some guidance on where to start with this. Like what components do I need. I did some research and basically I need a 555 timer and decoders but I would like some more advice.

    Any, advice ?
     
    Last edited by a moderator: Oct 11, 2015
  2. jcsd
  3. Oct 10, 2015 #2

    Nidum

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  4. Oct 11, 2015 #3
    2nd for the Crystal O -- a very good understanding to have, and easier to make the SW accurate.
     
  5. Oct 11, 2015 #4

    donpacino

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    Well first... does this need to be a discrete hardware project with only hardware or a microprocessor driven project with software?

    If this has to be a digital hardware project, talking about software will not get you very far.
     
  6. Oct 12, 2015 #5
    In this case SW meant StopWatch .. I should not have abbreviated, obviously is need to be a discrete build
     
    Last edited by a moderator: Oct 13, 2015
  7. Oct 12, 2015 #6

    donpacino

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    sounds good. and upon inspection OP mentioned that there can't be a microprocessor, so I need to work on my reading comprehension.

    The simplest (and most 'modern') way to do it would be an FPGA or other pld, although discrete builds would work

    any way you do it, first you want to generate a clock signal at the smallest frequency you need (frequency dividers)
    then develop your counters
    0-9 from 1/10 of a second
    0-59 for seconds
    0-9 for minutes

    if you're a 'digital design novice' make it free running to start (only a reset) and get it working.
    then determine what you have to do to start and stop the counters
     
  8. Oct 12, 2015 #7
    I think I will use a 555 timer because i'm a bit more familiar with it. I tried a basic decade counter to start with using multisim. But when I start the simulation, the display shows this:
    2,3,8,9,0,1,2,3,4,5,6,7,8,9

    Why does it start from 2 and jump like that, then continue as normal ?, here's my circuit diagram:

    stopwatch.PNG
     
  9. Oct 12, 2015 #8

    donpacino

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    you need to add a reset line for it to work on the first go around
     
  10. Oct 13, 2015 #9
    How would i add a reset line ?
     
  11. Oct 13, 2015 #10

    donpacino

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    you have three inputs to your system...
    stop
    start,
    and reset


    look at your memory storing device (the counter) and determine what pin will cause the states to go to zero (or their initial states)?
     
  12. Oct 13, 2015 #11
    I looked at the pins and theirs no reset, I tried playing with the max/min pin but it doesn't reset
     
  13. Oct 13, 2015 #12

    donpacino

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    ok.
    a few things. you have outputs tied to ground, which can be a BIG no no.
    also you should not enable cten until you are ready to start, i recommend starting cten at logic hi, then moving it to logic low and seeing what happens. further looking at the chip, everything is reset to their initial values (which are a,b,c,d) when cten goes low.
     
  14. Oct 13, 2015 #13

    berkeman

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    When you look at the datasheet for the 74190 part, what does the LOAD input do?
     
  15. Oct 13, 2015 #14
    Okay, according to the responses I modified the circuit to this

    stop2.PNG

    @donpacino, When I start the simulation with the switch open and then close it the counter counts as normal which seems to fix the problem. Also I tied those outputs to ground because I am not using it, where must I tie them to if I am not using it ?

    @berkeman, The load input loads the value connected to A,B,C,D so to reset to zero, I used a push button
     
  16. Oct 13, 2015 #15

    berkeman

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    Unused outputs are left no-connected (or in a production design, they are connected only to test points for manufacturing test use).

    Unused inputs must be tied off, usually just to ground (or to ground through a resistor, to allow for manufacturing test via a test point on the input). :smile:
     
  17. Oct 14, 2015 #16

    donpacino

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    In addition to what berkemen said, what is your experience with electronics so far?

    If a output is driven high, and you tie it low, you are shorting the voltage supply to ground, with only a transistor or two in between. that could damage your chip or power supply
     
    Last edited by a moderator: Oct 14, 2015
  18. Oct 14, 2015 #17

    donpacino

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    ok great!!! now how do you implement the reset/start/stop buttons?

    you might be there already with some of them!
     
  19. Oct 14, 2015 #18
    I tested the reset button S2 connected from LOAD to 5V and it works. Also for switch S1 when I open the switch while it's counting it stops the count, I'm not sure whether it would work in practice. Also do I have to debounce the switches ?

    I am an amateur in electronics. This is my first project
     
  20. Oct 14, 2015 #19

    berkeman

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    Yes, and since this is a synchronous circuit, you should synchronize those human-speed inputs to the system clock, or you risk metastable states in your clocked logic.

    PDF presentation from Stanford.edu:
    http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=2&cad=rja&uact=8&ved=0CCsQFjABahUKEwjt6M_JzMLIAhXRLIgKHV5rBCg&url=http://web.stanford.edu/class/ee183/handouts_spr2003/synchronization_pres.pdf&usg=AFQjCNFW8d86AMDil5zoeQxtN5ocBejO7g

    :smile:
     
  21. Oct 14, 2015 #20

    donpacino

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    awesome, getting into electronics is great!!

    I would change the label for S1 and S2 to be your start/stop/reset function.

    Also a note about start and stop.... you have only one input CTEN that will start stop your circuit, but you need two buttons that control it. Think about what kind of logic you need to make that happen!
     
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