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 Post subject: Frequency display
PostPosted: Mar Fri 23, 2018 5:49 am 
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I am looking for a frequency display kit or schematic so I can display the tach frequency of a motor which is necessary for setting its speed.

The motor is a VCR video head motor which uses an external analog circuit to regulate the speed and uses a pot to adjudt the speed.

It is used in an RCA RP-190 45 player.

The frequency display needs to be accurate to the nearest Hz and also update fairly quickly.

Currently I am using the frequency function of a Radio Shack DMM and it updates fairly fast.

Any ideas?

Ideally I would want it powered by the power supply for the record player which is 12Vdc so that it turns on and off with the record player unless there's a way to have it auto turn on when a frequency is detected.


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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 5:57 am 
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Tube Radio wrote:
The frequency display needs to be accurate to the nearest Hz and also update fairly quickly.
Basic law of physics...

To get 1 Hz resolution you need a gate time of 1 second.
But that does not give you 1Hz accuracy.
To approach 1Hz accuracy you need a gate of 2 seconds or more.
To guarantee 1Hz accuracy you need a gate of 5 seconds or more.

The common solution for a periodic waveform is to measure the duration of one cycle or half cycle then convert to frequency by taking the reciprocal of the time.

- Leigh

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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 6:37 am 
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Oh ok.

Not sure how the Radio Shack DMM does it, but the display updates maybe every 1/2 second and it seems to be fairly accurate. Lab grade? No, but it is accurate enough for what I'm doing.

Now a LeCroy digital scope I have at work seems to update a little faster for 1Hz accuracy, which given it can tell you everything you wanted to know about a waveform it possibly takes some other measurement it can make quicker and converts it to frequency.


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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 2:18 pm 
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I assumed your motor control adjustment is set it once and forget it. Why would you need a permanent readout other than it's cool looking?


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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 2:25 pm 
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Leigh wrote:
Tube Radio wrote:
The frequency display needs to be accurate to the nearest Hz and also update fairly quickly.
Basic law of physics...
- Leigh

I would have said math....

You can get any accuracy you want by simply taking multiple measurements and averaging them. If you have a 1Hz gate and you want to measure 1Hz, you just need to be patient....;)

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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 2:46 pm 
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Unfortunately it isn't necessarily set once and forget it due to how simple the circuitry is.

Now to avoid discussing the circuit here I'll link the topic about the circuit.

viewtopic.php?f=12&t=333883

The display is basically so that I don't have to break out my DMM every time I need to tweak the speed to where the platter is exactly 45RPMs.

That said I did notice last night I could turn the record player on, let it run for about a minute then set the speed and turn it off and later on turn it back on and it will sometimes go to the correct speed, but not always.


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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 3:07 pm 
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What's the tach frequency? How much diy do you want to do? You can get very good resolution with a 32.768kHz watch crystal oscillator clocking a resetable counter, it can count crystal "ticks" then reset on every other tach pulse. Many ways to display the count such as in hex on a row of 8 or 16 LEDs, or BCD-to-decimal, or even with a "either side of center" type display. Can be done with a few ICs and LEDs, and/or on a Basic Stamp or PIC or Arduino. All depends on how much energy you want to put into it

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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 3:42 pm 
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richfair wrote:
What's the tach frequency? How much diy do you want to do? You can get very good resolution with a 32.768kHz watch crystal oscillator clocking a resetable counter, it can count crystal "ticks" then reset on every other tach pulse. Many ways to display the count such as in hex on a row of 8 or 16 LEDs, or BCD-to-decimal, or even with a "either side of center" type display. Can be done with a few ICs and LEDs, and/or on a Basic Stamp or PIC or Arduino. All depends on how much energy you want to put into it


The tach frequency is 748Hz.

If I knew about basic stamp, arduino ETC... I'd just build a digital speed control circuit which requires no adjustment at all and then wouldn't need the frequency display.

I think the main thing is the analog devices I am using and slight thermal drift.


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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 6:56 pm 
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Okay, well, would one of those low-cost frequency meters on eBay do the job, I'm thinking similar to one you're using with your rf gen? Maybe not fast enough to update?

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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 7:40 pm 
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Will look at those when I get home as I cannot see ebay at work.

So long as it has at least a 1/2 second update rate that will work.


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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 8:00 pm 
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Seller may be able to answer about refresh rates, the description only says there are several to choose from.

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 Post subject: Re: Frequency display
PostPosted: Mar Fri 23, 2018 10:54 pm 
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I know its not quite what are looking for but you could always do it the old school way with a strobe platter and neon light. I still have mine.


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 Post subject: Re: Frequency display
PostPosted: Mar Sat 24, 2018 3:11 am 
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I do have one of those I printed out which works, but I'd have to leave it on the record player all the time.


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 Post subject: Re: Frequency display
PostPosted: Mar Sat 24, 2018 3:35 am 
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Tube Radio wrote:
Not sure how the Radio Shack DMM does it, but the display updates maybe every 1/2 second and it seems to be fairly accurate. Lab grade? No, but it is accurate enough for what I'm doing.

There are two ways to measure frequency. The common way that most people know about is counting the number of cycles in a fixed time period. This is the method used in most frequency counters that measure high frequencies.

For measuring low frequencies, the normal (and most accurate) method is to measure the time it takes to complete one cycle. That way, even with a 10 Hz input signal you can get 10 updates per second, and the resolution is only limited by the internal timer resolution. This is the method used on many DMM's.


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 Post subject: Re: Frequency display
PostPosted: Mar Sat 24, 2018 2:05 pm 
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BobWeaver wrote:
For measuring low frequencies, the normal (and most accurate) method is to measure the time it takes to complete one cycle. That way, even with a 10 Hz input signal you can get 10 updates per second, and the resolution is only limited by the internal timer resolution. This is the method used on many DMM's.


Oh ok so that would explain why the LeCroy scope seems to update for low frequencies quite fast as it already measures the time period for one cycle and can thusfore do a little math and derive the frequency.

So I would need a frequency display which measures the time it takes to complete one cycle.


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 Post subject: Re: Frequency display
PostPosted: Mar Sat 24, 2018 3:25 pm 
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Quote:
So I would need a frequency display which measures the time it takes to complete one cycle
Yes. Are you looking for a manufactured meter or a diy project? A power line meter might go up high enough in frequency but probably not, in which case you'll need to divide tach pulses by 10 or more to get in range of the meter. So, the meter will need superior accuracy and readout in tenths or better.

I suggested a good diy approach, which is the same as Bob's. Count how many "clock ticks" happen during one or more tach pulses. I would choose a 32.768kHz watch crystal oscillator. 32.768kHz crystals are easy to find and a dime a dozen. 32.768kHz is low frequency which gives a real advantage in ease of layout and low power. If you can devise a way to count how many "ticks" happen during a period of 10 tach pulses, you'll get an accuracy of 0.2% with about 75 updates per second. A count period of 20 or more gives even better accuracy and still with many updates per second.

Personally, I would do it with 5 building blocks
1. oscillator (optional)
2. resettable counter for watch "tick" counting (optional)
3. resettable counter for tach pulse counting
4. Basic stamp 2, which can provide 5v power for the other components and has all the memory and computer IO you need
5. Three or more leds for slow, good, fast indication

You can eliminate the oscillator and watch "tick" counter and instead rely on the Basic Stamp's internal routines to measure time periods. It won't be quite as accurate but probably will still be good enough. The Basic Stamp is relatively slow running and so won't be able to count tach pulses individually, that is why you want an external counter chip.

The Basic stamp programming is not hard to learn, especially if you've had prior exposure to any type of coding, even keyboard macros. Once learned, the skills will come in handy over and over.

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 Post subject: Re: Frequency display
PostPosted: Mar Sat 24, 2018 5:15 pm 
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richfair wrote:
4. Basic stamp 2, which can provide 5v power for the other components and has all the memory and computer IO you need
5. Three or more leds for slow, good, fast indication

You can eliminate the oscillator and watch "tick" counter and instead rely on the Basic Stamp's internal routines to measure time periods. It won't be quite as accurate but probably will still be good enough. The Basic Stamp is relatively slow running and so won't be able to count tach pulses individually, that is why you want an external counter chip.

The Basic stamp programming is not hard to learn, especially if you've had prior exposure to any type of coding, even keyboard macros. Once learned, the skills will come in handy over and over.


I used to do programs in basic when I was in my teens so perhaps I could learn it.

That said maybe I could make the basic stamp to where it receives the tach frequency and outputs a control voltage for the motor. Then no frequency display would be needed as I could set the frequency it needs to be at in the programming and wouldn't have to adjust anything at all.


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 Post subject: Re: Frequency display
PostPosted: Mar Sat 24, 2018 8:48 pm 
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That said maybe I could make the basic stamp to where it receives the tach frequency and outputs a control voltage for the motor. Then no frequency display would be needed as I could set the frequency it needs to be at in the programming and wouldn't have to adjust anything at all.
Hold yer horses podner, that's a much different project! Basic Stamp is good at doing one thing at a time and with a bit of cleverness can gather time and tach pulse counts at nearly the same moments, good enough to calculate frequency, then update the display during the next tach pulse, then start over. It's on-board IO ports can hold on/off values while it thinks about other things but if you are asking it to generate a variable speed control signal, that signal would have to disappear while time intervals are being counted. There are other classes of processors some of which have on-board, independently-running PWM outputs. By and large, those will require much more support circuitry.

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 Post subject: Re: Frequency display
PostPosted: Mar Sat 24, 2018 9:15 pm 
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Oh ok figured it wouldn't be that simple.

Just checked this afternoon and I had to readjust the speed control so it is indeed not a set and forget kind of thing.


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 Post subject: Re: Frequency display
PostPosted: Mar Sun 25, 2018 2:59 am 
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You need a phase locked loop to reference to a stable oscillator. Problem will be an oscillator frequency that gives an integer division to 748Hz for the phase detector.

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