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Looking for help designing a timer

Discussion in 'General Electronics Discussion' started by Robsta, Dec 30, 2017.

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  1. Robsta


    Dec 30, 2017
    Hi everyone,

    I'm new here, I came specifically looking for help with a timer I need to make. It's been about 20 years since I did any real electronics, and even then I could construct circuits and PCB's but was never any good at designing them- so any help would be gratefully appreciated!

    What I want to make is the following:

    A timer with LCD display that will measure to at least two if not three decimal places of a second. It doesn't really need to measure in minutes, and definitely not hours.

    I want to have count-up and count-down functions. When in count-down mode, I need to be able to set the starting time, obviously (say, 5 seconds, 10 seconds, etc), and also have an audible alarm/buzzer when the time reaches zero.
    It would be nice on the count-up function to have a delayed start, with a buzzer sounding at the actual start point (i.e, press start, have a few seconds delay, then an alarm sound, then the actual timing process starts).

    The above two features could be rolled into one with a count-down timer that allowed an over-run, and sounded an alarm at zero. That way, the delayed start could be set by the count-down with the over-run value actually being the count-up time- if that makes sense?

    I also need to start and stop the timer with a foot switch, but I guess that is by-the-by. It would be better if this was optional, so the timer also had it's own start/stop switch, with the option to switch externally as and when required.

    Could anyone point me in the right direction of a circuit that would do this? I already have a bench timer that supposedly does most of this, but it only measures to whole seconds, and when an external start/stop switch is connected, ALL of the devices buttons are disabled, so you have to disconnect the switch if you want to change the count-down time or even reset it. Stupid really. If I could somehow modify that timer to display in fractions of seconds, I'm sure I could re-wire the external switch connections to run off the device buttons, thereby bypassing the lock-out of all the other buttons- this would probably be the easiest option, rather than starting from scratch, but again I have no idea how I would go about altering the timer resolution.

    Many thanks in advance everyone.
  2. Bluejets


    Oct 5, 2014
    Stop watch has many of the functions.
    Maybe hack one of those.
    For $5.00 I'd say worth a try.
  3. kellys_eye


    Jun 25, 2010
    Do you need that accuracy? Seriously?

    Look to find a digital frequency meter with TIMING function (many have this) as they also have external trigger (start/stop) inputs too and can, by their nature, be as accurate as the clock-derived crystal frequency reference built-in to them.

    Here's a counter/timer on eBay current at $50
  4. Robsta


    Dec 30, 2017
    Thanks both.

    Modifying a stopwatch may be the simplest and cheapest way to go about it. I hadn't actually thought of that, so I'll look into it some more.

    Yes I definitely need two decimal places at least- it's going to be a speed-shooting timer primarily, and it's that critical (for example, I'm typically shooting the round under 6 seconds, but more than 5; registering a time of 5 seconds would be unfair as I'm closer to 6 seconds, but registering 6 seconds could lose me the top-spot).

    I was hoping to do this fairly inexpensively, but if the stop-watch idea doesn't pan out, there's a higher-resolution stop clock-only timer similar to the one I've already got which is only £10, so I might just use the pair in tandem; use the stop-clock operated with the footswitch (which will solve the problem of the buttons being disabled- there are no other buttons on this model), and use my existing timer for count-down only, manually operated. It's not as convenient as I would have liked, but it only means spending an extra tenner.
  5. NMNeil


    Oct 3, 2014
  6. Robsta


    Dec 30, 2017
    How does one determine the timing resolution?

    I was just thinking that maybe I'm over-thinking this; the timer I have has all the features I need, bar the resolution (and the issue with the buttons disabling, which I think is an easy fix)- all I need to do is increase the resolution (and/or the display), whether that be by altering the existing circuit, or copying it and replacing whichever component determines the resolution.

    Is it really this easy, or am I missing something?

    Thanks again all.
  7. AnalogKid


    Jun 10, 2015
    You're missing something - the changes you listed add up to a significant modification of an existing product. But you have given us literally zero information to work with.

    Schematic of "the timer I have"
    Photos inside and outside

    That one is easy - you tell us how many digits you need before and after the decimal point.

  8. Petkan


    Feb 9, 2011
    Timer's resolution is defined by the frequency counted. For instance a timer counting 1MHz has 1 us resolution.
    For instance 1 MHz may be derived from 16 MHz crystal oscillator. It this clock is say 0.01% so is the timer
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