Counter Displays

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Robert Reed
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Counter Displays

Post by Robert Reed » Thu Apr 17, 2008 7:42 pm

So, by now we have all seen enough panel meters from enogh manufacturers to fill a stadium. They range from volt meters to current meters to temperature and just about anything you can name. But they all share one thing in common - a digital display. The one meter I cannot seem to find is a basic panel mtd. frequency counter at comparable low prices. Up to now I have always built my panel mounted counters 'stick style' and oh what a pain. Most of these are 6 or 7 digit displays with an ultimate counter rate of a couple of MHz. The meter I need at present will have 5 digits with an ultimate counting rate of 100 KHz. Is anyone aware of a source for these types of counters at comparable prices to the asian imports. All I need is the basic counter as I don't mind supplying the time base, prescaler, latching and clearing functions myself. Such a unit would sure save me a ton of work.

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MrAl
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Post by MrAl » Thu Apr 17, 2008 8:12 pm

Hi Robert,

Do you need the display too or just the counter?
LEDs vs Bulbs, LEDs are winning.

Robert Reed
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Post by Robert Reed » Thu Apr 17, 2008 8:19 pm

MrAl
Yes, most definately the display. As mentioned, the counter circuitry can be pretty basic. I just hate wiring up the display section, even when its multiplexed.

rshayes
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Post by rshayes » Fri Apr 18, 2008 8:56 am

Most of the cheap digital panel meters use a dual-slope A/D converter. The signal is integrated for a fixed period (1000 counts in the case of the Intersil 7106). The integrator is reversed and the time required to reach zero with the reference voltage is counted. In effect, an internal counter is preset to 1000 and counted down to zero.

It might be possible to take an external counter and count the external signal for the desired gate period, resulting in a count below 1000. When the dual-slope converter starts the negative integration period, continue counting using the A/D clock until the counter reaches 1000, and then apply a signal to the comparator that will indicate the end of what would normally be the end of the negative integration period.

Since the internal counter is BCD, this will give a decimal readout even if the external counter is straight binary.

The external logic could be contained in a CPLD such as the Xylinx 9500 series. A few parts to detect the timing of the A/D converter and to drive the A/D comparator might be required.

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Re: Counter Displays

Post by Bigglez » Fri Apr 18, 2008 10:11 am

Greetings Robert,
Robert Reed wrote:The one meter I cannot seem to find is a basic panel mtd. frequency counter at comparable low prices. Up to now I have always built my panel mounted counters 'stick style' and oh what a pain.
.....
Such a unit would sure save me a ton of work.
If labour effort and building each article from scratch are
your hurdles, here's some suggestions:

(1) Use a PCB (literally "wireless", identical copies)
(2) Use a uC (to reduce component count, firmware flexible)
(3) MUX the display (to reduce component count)
(4) Use a modular oscillator (reduce component count)

Comments Welcome!

Robert Reed
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Post by Robert Reed » Fri Apr 18, 2008 11:23 am

RS
Interesting, however I am not sure of the accuracy. I generally work from 1 ppm to 10 ppm for the time base. Also need 5 to 7 digits depending. This one will require 5 digits with a time base accuracy of 10 ppm. Have to count 100 MHz to the nearest KHz so there is no point in extending accuracy to greater than 10 ppm.
Big
The problem with PCBs are twofold in my case -1) number of digits will vary in my case. 2) Boards for each unit may have to be custom sized in order to 'shoehorn' them in.

I always MUX as much as possible but four digits is the most I can do on my available stock of chips. Adding 2 or 3 digits still requires a lot of parrellel segment wiring.

I generally use mod osc or the cylinder 32 KHz crystals tied to the divider chips osc. section for time base.

Not really into PICs at this point. Would be nice thogh if I could get them preprogrammed.

As a note of interest, Dallas and Maxim used to produce an 8 digit counter on one 18 pin chip (less display of course). This was one great chip that sold for about $12 but is now obsolete. Have searched hi and low for these on the web and only located one Canadian outlet for them. Unfortunately, they will not sell across the border. This chip does it all with very few additional components and will handle a 10 MHz count rate simplifying prescaling somewhat.

Bigglez
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Post by Bigglez » Fri Apr 18, 2008 6:45 pm

Greetings Robert,
Robert Reed wrote:I always MUX as much as possible but four digits is the most I can do on my available stock of chips. Adding 2 or 3 digits still requires a lot of parrellel segment wiring.
I better understand your limitations. Two more solutions
I should have added to my short list are to use intelligent
displays, either dot LEDs or LCDs.

As these LEDs typically have four "windows". Two of
these in cascade would give you eight digits, and if you
don't need eight some can be blanked. As the LED
displays have ASCII decoders internally, you can annote
your display with any letters or numbers, suffixes or units.
The LCD modules are simple, typically only requiring five
data wires plus power and ground. They also use ASCII data.
Here's two LED examples:
Image
Image

Here's the LED datasheet and the LCD datasheet. (Caution large file downloads).

Comments Welcome!

rshayes
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Post by rshayes » Sat Apr 19, 2008 3:51 am

On second thought, a CPLD may be all you need. These are available from Digikey for around five to ten dollars. For example, the XC9572 has 72 cells. Each cell has an array of gates and a flip-flop. A six digit counter will require 24 cells. A storage latch will require another 24 cells. Scanning the storage latch will require 4 cells. BCD to seven segment conversion will require seven cells. Digit selection would require 6 cells. This is a total of 65 cells. This might fit in a 72 cell array, but the next size up, 108 cells should be an easy fit. The rest of the cells can be used for the time base and gateing. This comes in a 44 pim plastic surface mount package, so board area should not be a problem. The LED arays can be the same types used in the voltage and current meters.

The programming software is downloadable from Xylinx. The programming interface can be as simple as two chips and about 20 parts.

L. Daniel Rosa
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Post by L. Daniel Rosa » Sun Apr 20, 2008 1:39 pm

Funny you should mention this. One of the (many) projects on my list is a frequency counter. I plan on making mine as a "backpack" to use a 2*16 LCD. It would be limited to 50MHz due to the nature of the microcontroller's input, but could use a prescaler.

The primary function would be simple enough. I may just get bogged down with featuritis again. Autoranging to limit resolution displayed for unstable inputs and provide a longer sampling window for steady ones (show to 0.001 Hz resolution if you wait long enough). Possibly burst capture to count pulses _and_ measure frequency. Serial output for automated data collection.

Anyways, now that I know I'm not the only one I'll let everyone here know when I've made it.

Dean Huster
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Post by Dean Huster » Mon Apr 21, 2008 4:41 am

L. Dan, for higher resolution on lower frequencies, do what Tektronix did with their DC510. Once the frequency is below a certain value, the counter switches to a period measurement and then uses the processor to convert period to frequency. MilliHertz resolution without the 1000s gate time.

Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

L. Daniel Rosa
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Post by L. Daniel Rosa » Mon Apr 21, 2008 3:34 pm

Dean, I'll probably do that on one of the later editions. The switchover frequency is the square root of the operating frequency, so below 1kHz (4MHz clock, 1MIPs) I'll get better resolution as an intervalometer.

Robert Reed
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Post by Robert Reed » Tue Apr 22, 2008 8:07 pm

Thanks for all the suggestions, but what I really was hoping to find was a basic panel meter counter. However I will 'mull' over all of these and see if I can lessen my burden.

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Karl Williams
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Post by Karl Williams » Fri Apr 25, 2008 6:11 am

Hi Robert,

If you ever do need a small amount of parts from a Canadian supplier that won't ship directly to the U.S. you can have them sent to my place and I'll forward them on to you.

Take care, Karl

Robert Reed
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Post by Robert Reed » Fri Apr 25, 2008 12:26 pm

Thank you Karl for your kind offer. I am presently trying to relocate that supplier, so I may PM you in the near future if succesful. If I recall, Kitchener is right across the lake (Erie) from me.

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haklesup
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Post by haklesup » Fri Apr 25, 2008 1:34 pm

Thanks for all the suggestions, but what I really was hoping to find was a basic panel meter counter. However I will 'mull' over all of these and see if I can lessen my burden.
You mean like this for less than $250, seems reasonable
http://www.laurels.com/counters-timers.htm

Or were you looking to build your own and was looking for a display with little overhard in terms of connections like a serial input display?

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