Why do I get different readings from parallel-connected stopwatches?

[Buhl Dozier]

Hi all,

The system shown is to time an athlete running a sprint. At the starting end, the runner releases the start button to create a 0.5 sec pulse to the reed relays to start the digital stopwatches.

When the runner breaks the light beam at the finish line, the photo sensor outputs a 0.5 sec pulse to stop the timing.

3 stopwatches are used to verify accuracy, i.e. if all stopwatches agree, then I consider the system to be accurate.

PROBLEM: The stopwatches often don't agree -- The displays can be different by up to 0.04 seconds (40 ms).

=> I would like to know what could cause the different stopwatch indications?

Additional info:

- I can tell that the stopwatches do not start / stop at the same instant, based on mis-synchronized "beep" sound, so the problem is not with the stopwatches themselves. They all use the same basic chip and circuitry, which is accurate to about 3 sec per day which is much less than the time differences.

- The time differences occur randomly -- any of the stopwatches can be faster or slower than the other 2 for any timing instance.

- Response time of the reed relays is only 0.5 ms, also much less than the time differences.

Any help appreciated!

 

[davenn]

PROBLEM: The stopwatches often don't agree -- The displays can be different by up to 0.04 seconds (40 ms).

=> I would like to know what could cause the different stopwatch indications?

you have given no info on the stopwatches ??
and what is their stated accuracy ??

those differences may well be within their accuracy range

- I can tell that the stopwatches do not start / stop at the same instant, based on mis-synchronized "beep" sound, so the problem is not with the stopwatches themselves. They all use the same basic chip and circuitry, which is accurate to about 3 sec per day which is much less than the time differences.

OK so that part is outside the stopwatch control
but my prior comments still stand

my first gustimate would be the reed relays

 

[Kiwi]

Agree with Dave on reed relays possibly being the problem. Mechanical devices are probably not a good idea when high accuracy is required.

Also 5v reg has to start up and output a signal to the three reed relays. Could there be an occasional slow output voltage rise here causing the reed relays to operate at different times?

Assume that stopwatch inputs are 5v.
Could you tie all the stopwatch inputs together, and replace the reg and relays with a single pair of resistors as a voltage divider circuit to feed the stopwatches?

 

[davenn]

Also 5v reg has to start up and output a signal to the three reed relays. Could there be an occasional slow output voltage rise here causing the reed relays to operate at different times?

nice additional thought

 

[Bluejets]

Reeds are more than likely noisy as well especially without any debounce.

 

[Buhl Dozier]

Hi all,

Agree, sending 0.5 sec pulses to the 5V regulator is a bad idea, not knowing the output rise time (LM7805 spec doesn't say). But I took the regulator completely out of the circuit and still got random differences between stopwatches.

It turns out there's a slight delay in the start / stop action of the stopwatches themselves. I removed 2 of the stopwatches and tested them from the same switch, and the readings show the same random variation and .04 max difference.

Since the stopwatches are all manually-operated cheap "dept store" units not for any critical or official timing purpose, the delay is probably due to debouncing of the buttons. Unfortunately I can't bypass the debouncing or any other feature since the whole stopwatch circuit is in one potted chip.... but the .04 difference is actually only .02 for 1 stopwatch, which is good enough.

 

[(*steve*)]

Aagh. A reply I thought I had posted addressed that...

I considered that is the buttons are poled and debouncing is done in software, then a delay of 40ms is not unreasonable. These could easily be out of phase between multiple stopwatches.

 

[Buhl Dozier]

Aagh. A reply I thought I had posted addressed that...

I considered that is the buttons are poled and debouncing is done in software, then a delay of 40ms is not unreasonable. These could easily be out of phase between multiple stopwatches.

Yes, I think that's what is going on -- clock always running, and buttons polled in perhaps 10 ms frames (sometimes the difference between stopwatches is only .01). So even if the stopwatches all agree they can still be up to .02 in error, which is ok by me.

Also, the max difference between stopwatches is .04 regardless of elapsed time, i.e. no matter if duration is 15 seconds or 15 minutes, so the variation is probably all from debouncing, and unrelated to stopwatch timing / clock accuracy per se.