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Counter/tachometer circuit?

Discussion in 'Electronic Design' started by Usual Suspect, Aug 6, 2007.

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  1. Printing equipment such as sheet-fed presses refer to speed in
    impressions-per-hour. I want to build a circuit that will display this number
    on 5 LED digits.

    The sensor will be a simple hall-effect or other sensor that is triggered up
    to 4 times a second (maybe less than one, probably not more than 5).

    It's a simple thing to start and stop a timer to calculate the period of the
    print cycle, but how to convert that to a meaningful IPH figure? Is a look-up
    table or maths done by microcontroller the only way? Or can some other means
    be used?

    I prefer the non-microcontroller solution, as I haven't yet broached the PIC
    [used as generic term] world...

  2. The fast response method is to take the reciprocal of the
    period between events and scale that to the desired units.

    There are industrial displays made to do just that. I think
    a common name for them is rate indicators. e.g.:
  3. I'd like to see what's possible re. building my own circuit for this one
    Is this approach possible without use of a custom uController? Or, for that
    matter, what are the possible solutions, excluding the need for programming?

  4. Lots of things are possible that are not practical. The
    simple way to compute rate is to count events during a
    precise period, and display the count. But to get 5 digit
    precision, you have to count 10^5 events, and that could
    take a while, unless you add something like a gear tooth
    pickup that gives you a couple digits of count per
    revolution. You scale the units of the count by setting the
    counting period appropriately. There are counter ICs
    available that also drive displays. If you can't find them
    with Google, I'll take a look and see what I can come up with.

    The reciprocal method is definitely easier with the math
    available through a microprocessor program. Analog or
    digital reciprocals are a pain, especially near zero rate
    (infinite period).
  5. Nobody

    Nobody Guest

    Store a table of reciprocals in ROMs.
    A *possible* solution is to multiply the impression frequency using a
    phased locked loop and a divider. I don't know whether this would be
    viable in practice; given the low base frequency, it may take a while
    to re-synchronise if the frequency changes. You would definitely want a
    proportional phase detector.
  6. linnix

    linnix Guest

  7. Rich Grise

    Rich Grise Guest

    I saw a guy design an RPM readout that used 1 slot on the disk. He counted
    milliseconds per rev. in one counter, used that number to preload another
    counter, then divided some clock by it to give a readout directly in RPM.

    Good Luck!
  8. Nobody

    Nobody Guest


    Connect the output from the interval timer to the address pins, connect
    the data pins to the BCD-to-7-segment decoders, hard-wire chip-select on.

    Once programmed, a ROM containing 2^M N-bit words is just a combinatorial
    logic chip with M inputs and N outputs.

    I'm talking about traditional ROM/PROM/EPROM chips with non-multiplexed,
    parallel address and data lines, not I2C ROMs or suchlike.
  9. Palindrome

    Palindrome Guest

    I like it.

    It would need one 128kx8 ROM for each of the 5 digits of the display,
    all with parallel inputs, to get the resolution required.

    Possibly what the poster has in mind is to reduce the ROM sizes by using
    supporting hardware to carve big chunks out of the address map.
  10. Arghh! It's horrible :)

    You could do the whole thing with an AVR and a MAX7219. Two chips vs a
    board full of counters, bussed EPROMS and seven-segment decoders.

    Best wire-wrap it, to complete the authentic 70's look :)
  11. Nobody

    Nobody Guest

    That assumes that you're including the 7-segment decoding in the ROM. If
    you use separate decoders, you only need 4 bits for each digit, so 5x4-bit
    or 3x8-bit. The size depends upon the resolution of the timer.
    I'm not sure that there would be much point. The only real advantage of
    ROMs is their simplicity; in most other regards, a microcontroller would
    be a better choice.

    Maybe he was thinking about scanning out the digits rather than generating
    them in parallel.
  12. Nobody

    Nobody Guest

    70's? Most of my ROM-based prototyping was in the mid-late 80's.

    Mostly because I had an EPROM programmer and a stack of EPROMs, but lacked
    a PAL programmer. You can't copy WordWise/View/etc ROMs with a PAL

    Flash wasn't around then, and the "development" (i.e. re-programmable)
    versions of single-chip microcontrollers cost a small fortune.
  13. I actually designed the very thing the OP is looking for then (an
    "items per hour" display. It used an 8748 (shudder). I had to build my
    own programmer, and write my own assembler in BBC Basic. As well as
    items per hour, it could display a total count and average production
    rate. It also had switches for a multiplier to deal with multiple
    items made per machine cycle.
  14. If you could live with 4 digits, we have a product that could be
    modified to do that. That's way more resolution that is useful anyway
    (you can't expect mechanical equipment to repeat within 10ppm).

    Or, have a look around for totalizers/counters/etc. that offer
    calculation functions. I bet there's something out there off-the-shelf
    for a reasonable price compared to making one from scratch, even if
    you need a fair number of them.

    Best regards,
    Spehro Pefhany
  15. MooseFET

    MooseFET Guest

    If you mux the display, you can fit it all in a single larger ROM.
    You already have the oscillator and counter in the period measuring
    part of the system.

    If you go with one of the variations of the 8051, the software is
    about as simple as the software for making the contents of the
  16. I have a Shimpo DT-5TG programmable panel mounted tachometer that will
    do exactly what you want. Its new/old/stock (never used) $150. Includes

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"©

    "Treason doth never prosper: what's the reason?
    For if it prosper, none dare call it treason."

    "Follow The Money" ;-P
  17. Rich Grise

    Rich Grise Guest

    This has been bugging me since I saw that guy's design, sometime in the
    early 1980's. The circuit was "simple", but "big" - i.e., he had something
    like three banks of counters.

    First, you'd have an ordinary start-stop counter to count some clock,
    I used ms in my example, which is 1 KHz, but it could be anything, as long
    as you know what it is.

    Then, that gives you, say, millisecs per revolution.

    So, I take that number and preload it into another counter, which
    is clocked at, say, y Hz. So, now, y divided by millisecs per revolution
    would give 1/y revolutions per millisecond, so if you want revolutions
    per minute, y would be, what, 60 KHz? to scale it to RPM?

    Hmmm... I guess it was just the two counter banks - the third row on
    his schem. must have been the decoder/drivers.

    Hmmmm... then somehow you've got to go from binary to BCD... Well, anyway,
    have fun designing this klooge, and let us know what you come up with
    either way, OK? :)

  18. Or, have a look around for totalizers/counters/etc. that offer
    Many of these totalizers/counters have a scaling feature. For application as
    a cycles-per-hour display, do I need a 3600x scale? ie, after each second of
    measurement (at 3 hz sensor input), the display should read "10800".

  19. You'll have to read the manual carefully to see if they can do exactly
    what you need.

    Best regards,
    Spehro Pefhany
  20. You'll have to read the manual carefully to see if they can do exactly what
    Yes, I frequently download user guides while researching a product.

    But my question was more about whether I correctly understand the definition
    of and need for "scaling" as applied to my case?

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