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Press a button to start a timer project? Thanks.

Discussion in 'General Electronics Discussion' started by SimplicityDown, Jul 18, 2017.

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  1. SimplicityDown


    Jul 18, 2017
    I am trying to make a switch that when a button is physically activated it starts an l.e.d. timer display that counts time, until the button is released.
    How would I accomplish this, I have a small amount of experience with bread-boarding and soldering as a hobbyist, but would really appreciate not having to dig through a library of knowledge if a electronics sage out there could be so kind as to guide me in the right direction,
    Thanks you.
  2. kellys_eye


    Jun 25, 2010
    Circuits you need are:

    Clock signal generator (to whatever accuracy required)
    Gating device - simple (N)AND would do

    On a 'basic' level, a 555 can do the clock signal;
    a 74-series (N)AND gate (or NOR) will do the gating:
    and you can adapt a simple calculator to do the display (apply the clock signal from the gate device to the '=' key by driving a small FET wired across it and setting the calculator by keying '1+1==').

    No doubt someone will be along in moments to promote doing it using a PIC/Arduino/Supercomputer... seems to be the way these days <rolls eyes>.
    hevans1944 likes this.
  3. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    Wow! Calculators are less than five bux just about anywhere. This solution combines the counter, decoder, and display (usually the most expensive components of a project like this) all-in-one with just the addition of an FET to operate the "equals" key. Please let us know how that works out for you, @SimplicityDown!

    OTOH, an Arduino with a color TFT liquid crystal display shield can get the job done for about a hundred bux and you get the pleasure of learning how to program it. So, instead of an overnight project, you can stretch this out to several months by coming back here with Arduino questions.

    BTW, a PIC is too complicated for a rank beginner and Supercomputers are (probably) too expensive. Try @kellys_eye calculator suggestion first, come back here if you have more questions about how to implement it.
  4. AnalogKid


    Jun 10, 2015
    Any particular reason you want to re-invent the stopwatch? Also, there are many stopwatch and timer kits available.

    Depending on all of the other project parameters you didn't share with us, this might be a fairly simple project to do in all hardware (no PIC, Arduino, etc.)

    What is the maximum time period you want to count up to?
    Does the display have to include tenths or hundredths of seconds?
    Do you want the display to show seconds and minutes (more decoding), or just lots of seconds (easier).

  5. BobK


    Jan 5, 2010
    Yes, if I were doing it I would use a PIC and it would cost less, have fewer parts and be more amenable to changing it's behaviour if desired.

    What of those attributes do you take issue with?

    On the other hand, using a calculator with a transistor across the "=" button seems pretty ridiculous to me. If you are going to do that, why not just buy a stopwatch instead?

  6. kellys_eye


    Jun 25, 2010
    I take no issue with the attributes you list - except maybe the 'amenable to changing its behaviour' since the OP wanted a simple stopwatch function and doesn't indicate any need for 'change'....

    A PIC, the programmer, the time taken to learn the coding, the display (the 'complex' bit).... not much difference in time/expense between the discrete parts and the PIC version the way I see it :D - except the display may be more expensive since calculators are really cheap. It would also take, potentially, far longer if you take the code learning into account.

    Nothing against PICs/Arduinos etc at all really sometimes the direct approach is just as easy, just as cheap and just as good, if not a better, a learning exercise.

    Not many PIC/Arduino systems work using 'just' the PIC/Arduino - the peripherals, drivers, displays etc are all 'extra' to the controlling device and the student should really learn using the basics before moving onto the complexities offered by programmable devices.

    Dave over at EEVBlog used just such a calculator/switch arrangement to make a counter fairly recently!

  7. BobK


    Jan 5, 2010
    Agree that for someone who is not experienced with microcontrollers, the task is daunting. But for someone like me, who has all the necessary tools, knowledge, and experience, it is a no- brainer.

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