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From 5-24VDC ---> TTL.. HELP PLEASE!!!!

Discussion in 'Electronic Basics' started by Henry, Apr 11, 2004.

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  1. Henry

    Henry Guest

    Hi, I'm working on an interface board that have to get state inputs
    from sensors of many types with signals ranging from 5VDC to 24VDC
    (i.e. signals from 5 to 24V mean logic "1"), the problem is that this
    interface should be able to work with direct TTL outputs from sensors,
    too. The problem is that I was thinking about using optoisolators to
    isolate the target system but I know that a TTL signal won't be able
    to drive the LED from the opto, so, how do I do this???
    I can use a 5VDC and 12VDC power supply.
    Any help with ideas and/or schematics would be appreciated.
    Regards

    Henry
     
  2. John Fields

    John Fields Guest


    OUT goes high when TTL input is high:

    +5V +V
    | |
    E |
    TTL>---[R]---B PNP [R]
    C |
    | +---->OUT
    [R] |
    +--|------|--+
    | |A C |
    | [CR]-> B |
    | | E |
    +--|------|--+
    | OPTO |
    GND>----------+------+---->GND



    OUT goes high high when TTL input is low:

    +5 +V
    | |
    [R] [R]
    | |
    | +----->OUT
    +--|------|--+
    | |A C |
    | [CR]-> B |
    | | E |
    +--|------|--+
    | OPTO |
    | |
    ___ C |
    TTL>---[R]---B NPN |
    E |
    | |
    GND>----------+------+----->GND
     
  3. I think I would make a universal input with comparators, like the quad
    LM339. Connect the inputs through a high resistance voltage divider
    with overvoltage clamping to protect against spikes. If the LM339 was
    powered from 12 volts, it responds to inputs up to 10.5 volts.
    Connect one side of all comparators to a forward biased diode (.6
    volts) and divide all inputs by about 3 (say, a 200k to a 100k
    resistor to ground) (8 volts max. from a 24 volt input, .8 volts max.
    from a loaded TTL output of 2.4 volts. If you don't need microsecond
    response, load each of the resistive dividers with some capacitance to
    ground to improve the noise rejection.
     
  4. If you can guarantee that the low level input voltage is <= 500mV for
    all signal types you can use something like this (at least up to a few
    hundred kHz):

    1/6 74HC4050

    +5
    |
    in | \
    o--[10K]--x-------x------------| ----> 0/5V
    | | | /
    [100K] /---/ |
    | / \ 12V 0V
    | --- 500mW
    | | 1N5242
    0V 0V




    Best regards,
    Spehro Pefhany
     

  5. Greetings,

    Hmmm... IIRC TTL only guarantees an output high of 2.4V. The datasheet for
    the 74HC4050 suggests it has a minimum input high requirement of 3.15V when
    operated from 4.5V supply. After feeding through that resistive divider you
    are only guaranteed 2.18V highs. Assuming the sensors wouldn't mind this
    could be remedied by putting a pull up resistor on the input line to +5V.
    Of course if the sensor does output 24V, then some current would flow into
    the +5V rail and try to pull it up, so make sure it is properly loaded down
    to stay at 5V.

    Datasheet for the 74HC4050 here:

    http://www.fairchildsemi.com/ds/MM/MM74HC4050.pdf

    If the speed requirements were really slow and other requirements were
    reasonable I think I might consider just making an inverter using a bipolar
    junction transistor with pull up resistor and large base resistor. I might
    throw in an extra diode in series with the base to raise the turn on
    threshold somewhat. Alternatively one might consider just buffering the
    signal with some device like the 74HCT14 fed from a large value current
    limiting resistor. Of course this isn't fully orthodox practice since it
    makes use of the input protection diodes, but it will more likely than not
    work just fine (provided the input current limiting resistance is large
    enough).
     
  6. That will work, if you can find the right kind of junction FET you
    can save 2 resistors.
    Did this once with a mpfxxx , in such a case the current is limited.
    Not very reproducable, so different for different FETS.
    JP
     
  7. Look at data sheets for 2N7000 fetlingtons and the older 2N2222. These were
    designed for TTL level translation circuits and are available in quad DIP
    packages. There are also TTL input or output bus driver type IC's, for
    multiple lines. Also look at line drivers.

    http://home.btconnect.com/woodgreen/tech_tips/techtip/art8.htm 2N7000
    examples

    http://www1.jaycar.com.au/images_uploaded/2N7000.PDF 2N7000 diff.
    packages, including quad DIP


    You could also use a quad open collector output comparator IC to drive a
    Quad optocoupler IC.

    http://www.national.com/ds/LM/LM139.pdf
    http://www.nteinc.com/specs/3200to3299/NTE3220.html


    Or one of these http://www.linear.com/pdf/lt1161.pdf LT1161, for more
    demanding needs / POWER.
     
  8. budgie

    budgie Guest

    Use any 74HCT buffer or inverter, with input via a suitable resistor and with a
    diode clamp on the HCT input.
     
  9. j.b. miller

    j.b. miller Guest

    Easiest way would be to use an IDC5 optocoupler...as they were designed for
    this application...
    jay
     
  10. Roger Gt

    Roger Gt Guest

    X-No-Archive: yes
    : Hi, I'm working on an interface board that have to get state
    inputs
    : from sensors of many types with signals ranging from 5VDC to
    24VDC
    : (i.e. signals from 5 to 24V mean logic "1"), the problem is that
    this
    : interface should be able to work with direct TTL outputs from
    sensors,
    : too. The problem is that I was thinking about using
    optoisolators to
    : isolate the target system but I know that a TTL signal won't be
    able
    : to drive the LED from the opto, so, how do I do this???
    : I can use a 5VDC and 12VDC power supply.
    : Any help with ideas and/or schematics would be appreciated.
    : Regards
    : Henry

    For a universal logic input a 74C914 is useful, providing the
    grounds are common.

    For an output a 74LS07 with a pull up will allow voltages to 40
    VDC. So 24VDC is really not a problem.

    Do you also need to generate the voltage (24VDC) to provide the
    output?

    There are many converters for this purpose, 12V in 24V out.
     
  11. Henry

    Henry Guest

    Hi Jay, do you know where I can get them? (I tried mouser, jameco and
    newarkinone and I couldnt find them) Are they easy to implement?
    Thanks for your help.
    Regards

    Henry
     
  12. These are industrial opto-isolated input modules, cost about
    $10-$20/each. Digikey and many other distributors should have an
    equivalent if that's what you are looking for- okay, try this number:
    Grayhill 70M-IDC5

    Best regards,
    Spehro Pefhany
     
  13. Henry

    Henry Guest

    Hi, the idea is that I have to design the circuitry because a
    constraint on the hight of the board because it is supposed to be
    packed on a low profile case (no more than 15mm). I looked at the
    module and it looks perfect for my case but it is 1" tall... so there
    is no way I can fit it on the case.
    So I have to use discrete components to create the interface.
    Thanks anyway for your suggestion... I wish I could use one of those..
    I saw they even work with a mixed VAC/VDC input... how can they do
    that?
    Thanks to everybody who's helping me out in my quest.
    Regards
    Henry
     
  14. Do you need optical isolation? I kind of suspect you do as most likely
    interfacing with a uPC and mixed 5 and 24V signals in. If that's the case,
    best to isolate and not even tie the commons together, which rules out some
    of the level shifting digital IC's. Some Opto isolators have parallel
    opposite LED's on the input and are used to do the AC signals. I have two
    circuit designs that will work for your 5 - 24 Vdc input signals, but will
    take a couple hours of my time to draw up, if you have not found a solution
    yet. Need more info on your needs too, as to PCB space and # of I/O points.

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    Hi, the idea is that I have to design the circuitry because a
    constraint on the hight of the board because it is supposed to be
    packed on a low profile case (no more than 15mm). I looked at the
    module and it looks perfect for my case but it is 1" tall... so there
    is no way I can fit it on the case.
    So I have to use discrete components to create the interface.
    Thanks anyway for your suggestion... I wish I could use one of those..
    I saw they even work with a mixed VAC/VDC input... how can they do
    that?
    Thanks to everybody who's helping me out in my quest.
    Regards
    Henry[/QUOTE]
     
  15. Ken Smith

    Ken Smith Guest

    +5V
    !
    V LED of low current opto
     
  16. Diagram needs work?
     
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