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Digital tachometer for my turntable
Digital tachometer for my turntable
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Old Yesterday, 03:36 PM   #1
JosM is offline JosM  Netherlands
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Default Digital tachometer for my turntable

Dear friends,

I would like to built a digital tachometer for my turntable. Not with a processor or stuff like that, but with "old fashioned IC's ( TTL or CMOS) and NIXIE tubes. Who Could Help me with a schema or article? Thank you.
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Old Yesterday, 04:53 PM   #2
Pyramid is offline Pyramid  United States
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Difficult to do with any accuracy without using math (uP).

For high frequency signals, a counter is used with a defined gate period. The slower the input frequency, the longer the gate period becomes especially as the accuracy (to the right of the decimal point) increases. To measure a frequency of 60Hz using a 1 second gate to count pulses would produce a result 60, with 1 Hz resolution. To get 0.1Hz resolution would require a gate time of 10 Sec which produces a count of 600 and the decimal is put between the two zeros.

A turntable platter is slow moving; counting single pulses per revolution would have a gate time of 60 Sec for a resolution of 1RPM. If you use a rotary encoder to produce many pulses per rev, you will need one with thousands of pulses per rev, or do some math: An encoder with 6K PPR will give a count of 3333 with a 1 second gate with resolution of 0.01 RPM at 33 RPM and a count of 4500 with a 1 sec gate at 45 RPM:

Count=RPM*PPR*Gate/60 where gate is in seconds.
or
Gate=Count*60/(RPM*PPR)

If you have an encoder with 2560 PPR, your gate time would be 2.34375 Sec for both 33 & 45 RPM and produce counts of 3333 and 4500.

To get resolution 3 places to the right of the decimal would require an encoder with 10's of thousands of PPR or a gate time of 10's of seconds.

You could also preload a counter with 78333 and count down at 25K pulses/sec using 1 revolution as a gate. This will give you accurate results for 45.000 and 33.333, but inaccurate results the farther away you get from either of those 2 numbers. The 74C192 counter IC is a BCD 10 up/down counter with preset inputs and could be used for this application. 7447 BCD to 7 Seg decoders could drive the digits.

Last edited by Pyramid; Yesterday at 05:05 PM.
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