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Frequency Measurement?

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bob9999
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Post Number: 14
Registered: 08-2007

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Posted on Sunday, 30 December, 2007 - 11:37 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I know that to someone this question will have an obvious solution. How is frequency measured with a multimeter? I bought an analogue multimeter in Japan many years ago which had the ability to measure frequency, (handy for Mains etc). I thought it may be a wheatstone bridge design with one arm as tuned circuit?? Despite much head-scratching I could not resolve the circuit.
While at sea I was facinated by seeing the vibrating metal strips used for frequency measurement on our generators ... odd person!
I've tried often to find a usable circuit but so far have failed. Any help would be appreciated.
Bob
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john_becker
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Posted on Sunday, 30 December, 2007 - 11:53 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Bob, there are frequency-to-voltage chips around -input the frequency and measure the voltage output on your DVM. In my RS cat there are over a dozen listed, eg AD537, LM331. Cant recall what I've used in the past.

J
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bob9999
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Posted on Sunday, 30 December, 2007 - 11:58 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks John. As usual you are a veritable mine of information. Would you know of a discrete circuit, (such as inginuity Unlimited used to promulgate)?
Have a nice New Year.
Bob
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chippie
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Posted on Sunday, 30 December, 2007 - 12:15 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The LM2917 is a freq-voltage convertor chip that would do the job......
eccentric millionaire financed by 'er indoors
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john_becker
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Posted on Sunday, 30 December, 2007 - 04:57 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Bob, thanks for that. I dont recall any IU type citcuits for this. Chippie, that could be the IC I used

J
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gordon
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Posted on Sunday, 30 December, 2007 - 05:43 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

EPE have published a few stand alone digital frequency meters, but I don't remember any recent frequency to voltage meter adaptors.

This circuit may be of interest, based on a 74121 monostable multivibrator.

http://www.circuit-projects.com/frequency-meter-for-10-hz-to-100-khz-input-by-74121.html
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bob9999
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Posted on Monday, 31 December, 2007 - 11:01 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think my first question must have been a little obscure. What I realy wanted was a circuit to measure frequency using discrete components. I know of the IC's which do the job, my real problem is that of curiosity!
I thought a wheatstone Bridge would do the job, one arm being a capacitor / inductor tuned circuit, the opposite a variable resistor.The component values are a mystery though. The measurement could be read off using a multimeter.
Sorry to be a pest and hoping this clarifies the situation,
Bob
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john_becker
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Posted on Monday, 31 December, 2007 - 11:42 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Bob, if you are after a discrete way then AC-couple your freq signal across a capacitor, follow it by a forward facing diode, feed that into another cap connected to 0V on one side, place a resistor in parallel with that cap.
Rate of charge and discharge of that last cap can be read as a voltage peak on your DVM.

Play around with the C & R values to find the best relationship for a given freq range. It's the Time, C & R relationships that rule the output. I covered those in the Basic software for my Teach In 2000 series.

Bit tricky but can be done. A dedicated IC is easier!

J
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bob9999
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Posted on Monday, 31 December, 2007 - 12:19 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

John, many thanks again, at long last I have the solution! I have lost count of the number of people I have asked. Must agree the IC method is easier but curiosity ruled this query. I will try the circuit, refering to my book of calculations again ... somewhere I have covered the study of time constants.
I take it the wheatstone bridge solution does not apply to this problem.
Regards,
Bob
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gordon
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Posted on Monday, 31 December, 2007 - 12:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

This site may be of interest, it covers the various types of Wheatstone bridge. The last circuit is able to measure frequency. I guess you would make R2 and R4 a variable potentiometer, and the frequency could be read off from a calibrated scale, when the meter reads a minimum.

http://www.tina.com/course/27bridge/bridge.htm
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bob9999
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Posted on Monday, 31 December, 2007 - 01:02 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Gordon,
Thanks for the info. I'll try this circuit, it appears as I had envisioned. I'll try the variable pot route too. I seem to remember having to zero my Japanese Analogue meter in a similar manner.
There are so many helpful sites out there, but it is knowing where to look!
Regards,
Bob
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dean_huster
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Posted on Monday, 31 December, 2007 - 01:37 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

"While at sea I was facinated by seeing the vibrating metal strips used for frequency measurement on our generators ... odd person!"

Nothing odd about that, Bob! I, too, have always been fascinated with resonant reed frequency meters -- great for generator applications. Older electronics are often more fascinating: sweep generators that sweep the frequency by vibrating the "rotor" of a variable capacitor with a solenoid or speaker coil; a Nixie tube display; "magic eye" tube null detectors. Maybe that's why I'm always captivated with the mechanics of the late 1800s and early 1900s, especially with clockwork and steam engines.

Dean
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bob9999
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Posted on Monday, 31 December, 2007 - 02:28 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Dean, a kindred soul I feel. You are talking to the person who, in days gone by, linked a Trix motor to a Trix motor, one wired to the other as motor and generator. Kick start with a battery and, with my then limited knowlege, wondered why I hadn't obtained perpetual motion! Ah. The enquiring mind.

I still love to strip and repair clockwork, steam engines are also a hobby. I've a couple in the shed. What is the facination of watching machinary at work? Fred Dibnah knew!

I also have a steam engine in the bedroom, of German origin. That one though has never been run, just looks the part. There is somthing outstanding in German engineering, always seem to 'finish' the job! Mamod were rough by comparison. The Grandchildren get to play with the engines in the shed though, hopefully a new generation to carry on with 'hobbies'.

Kids seem to spend too much time with computers to realise there is a whole world of exciting things to do. Too much depends on 'cred' these days I think! ... My last grump for 2007.
Bob
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atferrari
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Posted on Monday, 31 December, 2007 - 05:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hola Bob9999

At sea as a seaman? Where? For how long?

PM if you like.

Feliz Año Nuevo.
Agustín Tomás - Buenos Aires - Argentina
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dingbat
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Post Number: 33
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Posted on Wednesday, 16 January, 2008 - 12:37 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Bob9999,
- have you had yr frequency - Q answered?
If not, perhaps you can say "why" you need to measure frequency and what is the range you are interested in....etc,
Historical Note:-
Vibrating-reed F-meters were particularly suited to alternator supplies, as they were very robust and easy to read. Moving-coli meters would be more delicate and subject to variation with voltage, although a general purpose multimeter might include a zener.

Is this to measure continuously, or just to check when you feel like it?
What is the supply voltage?
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bob9999
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Posted on Thursday, 17 January, 2008 - 12:43 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Dingbat, thanks for taking the time. John Becker pointed out the solution for me, (above). It was realy a question about an old multimeter I had, (see above). I take your point about vibrating reed meters, my use was for a general purpose F-meter, for mains voltage, occasional use.
Regards, Bob
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obiwan
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Posted on Thursday, 17 January, 2008 - 08:08 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Sometimes, it's amazing how they did things "back then", and looking at how we do it today, we wonder how they ever managed in the "old days". Sometimes I think we're regressing, if not how we do something, in our thinking, becoming limited somehow.

A lot of that stuff, if you could get your hands on it, or make it, would make a great science fair project for a kid in school. But, I don't guess a lot of people looking at it would know what it is anyway.
Do Not Hit The Fly That Lands On The Tigers Head.
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steerpike
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Posted on Thursday, 17 January, 2008 - 09:33 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think the system John described is what is called a Pulse-count-detector used in some FM receivers instead of a ratio or foster-seely detector. If you search on that term, you'll find a few practical circuits - some using valves.
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grab
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Posted on Friday, 18 January, 2008 - 10:58 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If it was analogue, then it was almost certainly using a capacitor- or inductor-based circuit to measure the frequency. I doubt it can have been very accurate though - anything other than a pure sine-wave would give you a wildly wrong result. Modern ones just use a comparator and count crossings instead, which is much more accurate.

Obiwan, I take your point about wondering how they did it "in the old days". A friend where I used to work was really into his control theory. One day he brought in a book about Victorian control engineering, basically to show how all the mechanical widgets of the time were doing negative feedback control using mechanical methods, instead of the electronic or software control we'd do these day.

Of course, they were doing it like that because they had to, not because they wanted to. Necessity is the mother of invention, and all that. So mechanical controllers gave way to electronic controllers with discrete hardware, because it's easier to change a box of electronics than an entire gearbox. And discrete electronics gave way to software, because it's easier to change software. Some things still use mechanical methods or discrete electronics though, because their inherent simplicity makes them more reliable. But this is explicitly making the tradeoff of reliability against ease of changing the control parameters - now we have easily-changeable controllers, we can decide whether to make that tradeoff or not. If the Victorians needed to tweak something, they had no choice but to get the spanners out and rebuild it.

Graham.
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hackinblack
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Posted on Sunday, 20 January, 2008 - 11:05 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

for interest this IC is the one used in Veglia instruments (rev counter)in FIAT,ALFA ROMEO etc. and probably many others.
\popattach{7561,datasheet}
a whole lot easier to get more accurate readings,than playing around with descrete components....unless you ENJOY doing that{wink}
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mark
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Posted on Friday, 25 January, 2008 - 11:15 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think some older analogue multimeters just had a diode limiting circuit and capacitor in series with the meter circuit. So higher frequencies gave a higher reading on the scale. Probably non-linear and also not very accurate - but it added a 'feature' to the spec for the price of a capacitor!

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