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Help designing an accurate timer (1/100,000 sec) that displays time between every other pulse

I need to make a timer that will accurately measure (within

1/100,000th of a second) the elapsed time that passed between every other pulse (the pulses occur between 0.25 seconds and 3 seconds apart); in other words, it ignores the pulse in between and counts the time from the START of the first pulse, ignores the second, and displays the time it took to reach the START of the third pulse.

Then I need the cycle to repeat; so the START of the third pulse is not only the stop/display trigger, but it=92s also a new start trigger (pulse 1) for the next timing cycle to automatically begin measuring. So, if it received 10 pulses, it would display the elapsed time between pulse 1 & 3, 3 & 5, 5 & 7, 7 & 9. If too much time elapses (overflow) it will be obvious to me and so I don=92t need anything fancy to occur, it can simply display an incorrect time and move on, or wait for a new trigger to start the timing cycle over, either way is fine, whatever is easiest.

I=92ll need to use a photoconductive cell to generate the pulse triggers, but the width of the objects passing by can range from 6=94 wide down to 1/16=94 inch and so I need a way to accurately trigger off of something very thin moving across the beam.

I=92ve done some simple electronics projects, but not enough to design this on my own. Any help is appreciated. It doesn=92t have to be fancy, just accurate, and hopefully somewhat simple to make and calibrate.

Thank you in advance for any help. Mark

Reply to
Mark Main
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So that's 10us, is this the accuracy or resolution you require?

Record the time for all the pulses and then just extract the relevant ones.

Ok, what are the objects and what has led you to choosing a photoconductive cell when you don't seem certain that it is capable of detecting a wide range of object sizes?

--
DaveN
Reply to
DaveN

Rule #1: Don't build what you can buy.

Go to ebay and buy a counter with lots of digits.

One of the first things encountered after AND gates & OR gates.

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-2

How do you assure that the *objects* are positioned squarely on the conveyer? I don't believe you've thought this thru thoroughly.

=2E..and what font are you using that can't do normal single quotes and double quotes? I suggest you choose a plain-vanilla one.

Reply to
JeffM

Accuracy, I'm measuring and displaying time intervals; I just need them accurately measured.

y

s.

I thought it would be easier to ignore a pulse rather than adding two; but either way is fine; I'm just seeking the simplest.

ve

I've never used photoconductive cells and so I don't know how thin they can measure, especially at a fast speed. The sender and receiver will be around 4 inches apart from each other and so if a standard cell can be triggered from an object 1/16th inch thick passing rapidly in between them, then that's great... they are more sensitive than I expected.

Reply to
Mark Main

Squarely positioned isn't the problem, but I appreciate your mentioning it. I just need to know the time span accurately between the START of every other pulse; and the size of the objects can be quite thin. So I don't know what kind of sensor to detect a break in a beam spaced about a 4" apart.

Reply to
Mark Main

"Mark Main"

Squarely positioned isn't the problem,

** Yes it is - wanker.

You cannot get anything like 10uS timing resolution by optical means unless the objects have SHARP edges and are incredibly precisely aligned and pass extremely close to the sensor.

The problem is the inherent error in optical position sensing.

..... Phil

Reply to
Phil Allison

Flip flop ?

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

  SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
  THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"
Reply to
Jamie

It ill be very hard to get 10uSec resolution and accuracy with a photoconductive cell; a silicon photodetector diode is much faster.

Reply to
Robert Baer

y

quoted text -

I've been looking at adjustable focus laser diode modules on digikey. All I want to do is accurately measure the elapsed time between every other pulse regardless of how an object may be twisted slightly, or whatever... I just need to measure the time from every other pulse break. And I need it to be able to be as thin as 1/16" and I need the sending unit and receiver to be about 4" apart, I can get maybe as close as 3"... possibly slightly closer if I had to.

I also need to figure out how to do an accurate time count.

Reply to
Mark Main

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I'll design for 10uS and if I get less accuracy due to physical constraints then I'll have to live with that. I need to do this optically, and so what would you recommend for an optical solution that would work at 3" or 4" span between.

Reply to
Mark Main 

--
Your rule, not mine.
Reply to
John Fields

Use a laser diode and photodetector just like the ones that remotes use, but with a slit over the PHD/PHT. Your accuracy will depend on the speed of the items, the width of the slit, and the response of the PHD or PHT. A schmitt trigger could sharpen up the transitions, but you'd have to do some experimenting with thresholds and stuff, to calibrate it.

And frankly, ten microseconds is a fairly comfortable resolution, given the electronics they've got these days. ;-)

(PHD == PHotoDiode; PHT == PHotoTransistor.)

Good Luck! Rich

Reply to
Rich Grise

Go to ebay, and buy a counter that has a start/stop interval time function. Gate the pulses with a little logic, and start/stop the counter.

Oops - that won't work - I don't know if a boughten counter can stop the previous count and start the next count simultaneously.

For that, you might need to build - but building a counter isn't difficult, merely tedious. :-)

Good Luck! Rich

Reply to
Rich Grise

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Thanks. I see some that there are also some PHT's that can can be focused... I don't know how narrow the beam is when you do that. I'll play around with it.

Given the suggestions that I have received, I am going to try and figure out a way to also display the average of the last 10 cycles and update this every new cycle. This would require keeping the numbers stored and doing some math. I'd like to do a 10X and 100X if I can. That way I can use the 1X mode to tweak the speed, and then shift into

10X and 100X mode to fine tune things.

Now I'll be learning how to store numbers and in a revolving stack of

10 or 100 stored numbers.
Reply to
Mark Main

Many counters will count 10 cycles and display that. No math required.

Reply to
Robert Baer

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