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Timing Circuit

Discussion in 'Electronic Design' started by Jimbo, Jan 29, 2006.

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  1. Jimbo

    Jimbo Guest

    I need to design a circuit that I can tell it to start (digitally), and
    then it will tell me when 10 minutes have passed. I just need to be
    pointed in the right direction. What's the difference bewteen a
    timer/clock/counter. I have a good understanding of circuit theory but
    does anyone know of a good IC that can be surface mounted (and is
    cheap) that I shoulod use. Any help or tips are greatly appreciated
     
  2. Fred Bloggs

    Fred Bloggs Guest

    Go to http://www.ti.com and enter 74HC4060 in search box.
     
  3. Jimbo

    Jimbo Guest

    THANKS! This chip looks perfect. Good ol TI always seems to come
    through for me.
     
  4. Jimbo wrote...
    The 74hc4060 is an attractive chip with its on-board oscillator,
    but you should be aware cmos inverter-oscillators aren't very
    predictable (from IC to IC), nor very stable after the timing
    components are adjusted for your time delay. If you're only
    looking for a rough 10 minutes, fine, but if you want a semi-
    accurate 10 minutes you may prefer to use a second oscillator
    IC, such as a cmos 555, etc. Also, the 'hc4060 isn't the most
    attractive choice, because its 2^(14-1) = 8192-cycle timing
    requires a pretty low oscillator frequency for 10 minute timeout
    (13.65 Hz). Other parts have a higher divider ratio, such as
    Motorola's (now ON Semiconductor) mc14536, with 24 bits.

    The mc14536 also has an onboard oscillator, in case you still
    want to give that a try. I first started using the '4536 in
    the early 70s, and shipped thousands of instruments that relied
    on its cool 4-bit divider-select feature. We also featured the
    '4536 in our "hour of power" circuit in AoE (page 972), although
    I'm not happy with circuit we show there. $0.76 at DigiKey,
    very nice. http://www.onsemi.com/pub/Collateral/MC14536B-D.PDF

    BTW, TI also makes the '4536, as their cd4536B, if you prefer
    them. ST calls it the hcf4536B. The TI and ST datasheets,
    which were modeled after the Intersil/Harris datasheet, do a
    better job of showing how to use the powerful timer functions,
    http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=cd4536b
    http://www.st.com/stonline/products/literature/ds/4901/hcf4536.pdf

    Alternately, if you want to stick with a 16-bit IC, ON Semi's
    mc14541 is more attractive than the '4060, because it features
    an output flip flop with a polarity selection feature (and it
    also has an onboard oscillator), only $0.50 to 0.58 at DigiKey.
    http://www.onsemi.com/pub/Collateral/MC14541B-D.PDF This IC is
    also heavily manufactured and used. Here're Fairchild and TI's
    datasheets, http://www.fairchildsemi.com/ds/CD/CD4541BC.pdf
    http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=cd4541b

    BTW, you can get free samples of both ICs from ON Semi. And TI.
     
  5. Jimbo

    Jimbo Guest

    Thanks for the info. As far as accuracy, I'm looking for something
    that will work with a +/- 30 seconds or so. I'm assuming that any
    change in performance based upon manufacturer and/or temperature
    fluctations will not be a worry. Also, I don't really care as to the
    number of bits the chip has for an output. I'm thinking I'll just run
    the desired outputs through an AND that sets a S-R flip/flop. I'm sure
    there is a better way of doing this and once I start with the design
    maybe I'll come across some features that can help me cut down the need
    for extra circuirty. Perhaps that's what the mc14541 does. Doing this
    stuff in actual practice is quite a bit different than what I did for
    school. It seems that there are so many permutations of doing the same
    thing it's dizzing. It seems almost impossible to find the absolute
    cost effective and most efficent way of doing something. Too many
    options and not enough time.

    THANKS FOR YOUR HELP!
    Jimbo
     
  6. Jimbo

    Jimbo Guest

    Win,

    Is there a "system" for the naming of the IC out there? I keep reading
    things that tend to "hint" that there is a systematic way of naming
    these things. Isn't the formation usually "LETTERS" + "NUMBER" +
    "LETTERS". Have any idea as to the method these things are named?
     
  7. Fred Bloggs

    Fred Bloggs Guest

    That is only 5%.
    You don't need to do that unless you want the timeout to be
    programmable, in which case none of the suggested parts are that good.
    Once you get close, the differences in implementation become negligible.
     
  8. Pooh Bear

    Pooh Bear Guest

    This has to be a classic job for a baby microcontroller. Typically a tiny
    PIC.

    That involves learning about microcontrollers and doing programming though.
    Doing it in hardware is 'easier' for a beginner but will use lots of parts
    in comparison. It *won't* be 'just an IC' although the microcontroller
    method can be.

    Graham
     
  9. Fred Bloggs wrote...
    Acckk!! You're right, I mentally added in an extra factor of
    60 and came up with a moderately-painful 1:1200 design. Hah!
     
  10. Jimbo wrote...
    ==========
    Oops, I made a mistaken post, with mental calculations off by 60x.
    Let's leave it here, below, but for sake of discussion let's assume
    that Jimbo's requirement was for a 0.5-second, 0.1% accuracy. :)
    In practice, with his 5% spec, he can use a '4541 IC with a 294k 1%
    resistor and a 27nF 5% film capacitor. Or better, 2% caps like the
    nice Panasonic parts offered by DigiKey. That way tolerances will
    be in the IC, and Jimbo may be able to dispense with adjustments.
    ===========

    Jimbo, your 30-second spec is one part in 1200, within the range of
    analog relaxation-oscillator stability with RC components, provided
    they are properly chosen. A 16-bit divider (15-bits in timer mode)
    like a '4541 will have its oscillator running at 32768/600 = 54.6Hz,
    which isn't bad. Using f = 1 / 2.3 RC, and R = 249k plus a 100k pot,
    we get C = 0.027uF, a moderate-value capacitor for which small stable
    versions may be practical. Still, I'm tempted to suggest instead an
    8-pin uP with a 32kHz resonator. Have you considered that possibility?
     
  11. Check out the 4536 as well. But a single 8-pin microcontroller can
    give you the required accuracy with no external parts, or 10x better
    with one external part. You would have to program it.


    Best regards,
    Spehro Pefhany
     
  12. Pooh Bear

    Pooh Bear Guest

    There is next to no sense at all in it.

    The first few numbers/letters may tell you something but not a lot.

    Graham
     
  13. PN2222A

    PN2222A Guest

    The first 1 or 2 digits is usually the heater voltage.

    Regards
    PN2222A
     
  14. PN2222A

    PN2222A Guest

    The first 1 or 2 digits is usually the heater voltage.

    Regards
    PN2222A
     
  15. Alan

    Alan Guest

    so you have a 300mA heater??
     
  16. John B

    John B Guest

    Actually the first letter is the heater voltage and the next one or two are the construction. For
    example ECC83 is a 6.3VAC heater dual triode and ECF86 is 6.3VAC heater triode/pentode.
     
  17. Winfield Hill wrote...
    Actually, I take that back, Jimbo no doubt will need a pot, even
    if he does use a 2% capacitor. The chip's timing factor of 2.3
    has a tolerance as well, although none of the manufacturers will
    give you a hint as to what it is. NSC's cd4541B datasheet does
    have a frequency-deviation graph from which one can see about 2%
    drop vs supply from 15 to 10V, and another -8% for 5V, along with
    a 1% increase for a 50-deg C increase in temperature. One useful
    thing we can take from this graph is that for operation near 5V
    one should use a constant of 2.5 instead of 2.3 in the equation.
     
  18. John Fields

    John Fields Guest

    ---
    The easy way out in order to essentially remove the power supply and
    temperature dependency of the chip, (without resorting to a crystal
    or ceramic resonator) is to use a 7555 an an astable to determine
    the clock frequency. With an initial worst-case accuracy of +/- 5%,
    and your suggested 2% cap and 1% resistor, that comes out to +/- 8%.

    However, using a low tempco rheostat and cap to set the center
    frequency takes out the uncertainty due to the chip's initial
    accuracy, leaving only the cap's tolerance and the pot's tempco to
    deal with (assuming the cap's tolerance includes its tempco), so you
    wind up with, essentially, a 2% machine.
     
  19. John Fields

    John Fields Guest

     
  20. Jimbo

    Jimbo Guest

    microcontroller? I've programmed them before but I don't know of any
    that are as cheap as these other chips. Also, I'm doing this to pay my
    way through grad school and I'm working with a very small upstart so
    any development boards/software is out of the question becuase of the
    price. This is going to be a "large volume"/"low margin" product so
    every extra dollar we can squeeze out of the design will have a huge
    impact on the bottom line. Are you thinking about something like the
    6811 or 6812?
     
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