I suppose many of us are familiar with the concept of "Nerd Sniping"?
https://xkcd.com/356/
"Nerd sniping is a slang term that describes a particularly interesting
problem that is presented to a nerd, often a physicist, tech geek or
mathematician. The nerd stops all activity to devote attention to solving
the problem, often at his or her own peril."

So, I was totally in the middle of something else, stopped, broke out a
breadboard and wrote code to answer this question....
At about 15mS, I cannot tell they are out of sync.
By about 20mS, I can see that it's not "right" but it's hard to say which
is first.
At about 35mS, I can clearly see that they are out of sync and which one is
first.

My results are likely skewed by having one red and the other green, though
I did flip them and try again with about the same results.


Code attached below.  Please note that I'm not claiming it's good code....
:-)



void setup() {
  pinMode(D0, OUTPUT);
  pinMode(D1, OUTPUT);
  Serial.begin(115200);
}

  int T1 ;
  int T2;
  int j;


void loop() {
  j=3DanalogRead(A0);
  T2 =3D j/10;
  T1 =3D 200 ;
  Serial.println(j);

  digitalWrite(D0, HIGH);
  delay(T2);
  digitalWrite(D1, HIGH);
  delay(T1);
  digitalWrite(D0, LOW);
  delay(T2);
  digitalWrite(D1, LOW);
  delay(T1);
}



On Fri, Oct 12, 2018 at 2:06 PM Gordon Williams <gwilliams@ncf.ca> wrote:

> Hi,
>
> Hoping someone on the list may have some insight.
>
> I have 3 LED clocks that need to be synchronized for a nordic ski race.
> They show the time to the second.
>
> The way they are designed is they have a push button on the end that
> starts the timing.  The plan is to have a count down (3,2,1,go) with a
> person on each clock to push the button on "go".
>
> They will be synchronized close enough for timing purposes, but I want
> them visually synchronized as well so I don't get any complaints.
>
> Question: If the clocks are all in the centre of your field of view, how
> many milliseconds between the LED seconds digits changing can be
> detected? In other words, if you had 2 LEDs, what would be the maximum
> time difference between turning them on so you could say that LED A
> turned on before LED B?
>
> Thanks,
>
> Gordon Williams
>
>
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