Homemade microsecond timer

[steve_locher25]

I'm trying to build a 9 digit LED 7 seg display timer that can measure uS (microseconds).
I would love to have it measure nano seconds, but looking at the data books, it seems my idea will not work for nano because of rise / fall times, max freq of 555 timer, propagation delay etc etc.
But, there are still some projects I'm working on where it would be nice to be able to measure just how long a pulse width is from a one shot for example, and so I've decided to try to build it to measure at least 1 microsecond.
Basically, since I still don't own an oscilloscope, I'm building a somewhat precision timer.

On hand I have a NE555P, but, not sure of it's max freq (100KHz?? and if so, would not be fast enough?? for microseconds I need 1 MHz, clock speed, right?)

I've already hooked up a 3 digit LED 7 seg display, and after tuning it with a 1Meg pot, I had it measuring .01 seconds like a charm. (was running MC14553BCP into a MST6911C at 5Vdc)

Found a TLC555CP that is rated at 2.1 MHz maximum, thinking about firing that up after I add the 2nd and 3rd set of displays. (3 sets of 3 digits when finished)

For tuning, I'm using an LCD display kitchen timer, and then adjusting the pot, after so many minutes go by, a little faster here, little slower here, until the seconds on both are incrementing at the same time. I think when I get into the millisecond range, I'll have to add another smaller pot to fine tune it, etc.

So, my questions are:

1) am I wasting my time because even after I spend all that time tweaking it, it might not be the same the next time I use it (drift? temperature changes? etc.)

2) should I try a crytal instead of a 555 timer?

your thoughts?

Steve

 

[(*steve*)]

The more digits you want to show, the higher precision your clock has to be.

If your clock is accurate to 1% then more than three digits is overkill (you'll be +/- 5 counts in the 3rd digit at full scale).

Yes, you are wasting your time for exactly the reasons you state -- use a crystal oscillator Use one of the ones in a metal can with 4 pins. They're cheap and you can (if you're clever) change ranges by mounting them in a socket and replacing them with a faster or slower one as desired. You can also use a divider

for lots of digits, investigate the 74C926. You can get them on eBay for a reasonable price and a pair of them will give you an 8 digit counter with very few additional parts (8 transistors and some resistors -- and of course the displays)

 

[55pilot]

I second what (* steve *) has said. If you want a circuit that will be accurate, you need a crystal oscillator or at a minimum a ceramic resonator. For what you are doing, a metal can oscillator makes the most sense.

---55p

 

[steve_locher25]

Great, more homework

Never messed with crystals before, this project just quickly took to the back burner until I teach myself crystal oscillator circuits

Oh well

74C926 looks great though, I'll keep that in mind for later

time to post my next project in the forum.....

 

[55pilot]

Never messed with crystals before, this project just quickly took to the back burner until I teach myself crystal oscillator circuits

That is not what Steve or I recommended. We have both recommended that you buy a metal can oscillator. It is a crystal based oscillator, but all the crystal stuff is done for you. You plug it in, apply the voltage and out comes a square wave at the right frequency.

You can teach yourself crystal oscillator circuits. They are not that hard. But you do not HAVE to.

---55p