Connect with us

74HC123 and 74LS123

Discussion in 'Electronic Basics' started by Martin, Jan 5, 2005.

Scroll to continue with content
  1. Martin

    Martin Guest

    Im trying to build the project at

    Someone had told me that I should use 74HC123 TTL's instead of 74LS123's
    that are specified. Can anyone tell me if the pin outs are the same. Can I
    simply replace the LS for the HC in this case. I have used these (the HC's)
    and the project does not work. I don't know if this change is why it does
    not work or if it is somewhere else.
    Right now when I connect 5 volts (actually 4.90 using a regulator 7805) to
    the +5 to pin 16 of A. and negative to ground the relay clicks on turning
    the lamp on. (I dont have anything connected to the ring / tip. ) I'm sure
    this is not the way it is suppose to work.

  2. Andrew Holme

    Andrew Holme Guest

    A+B are two halves of the same chip. You only need one '123.

    Look at the chip datasheet: Google for "74HC123.pdf" or "74LS123.pdf"

    They are pin-compatible but the pulse width calculation is different:
    HC: PW = Rext * Cext
    LS: PW = 0.37 * Rext * Cext
  3. Andrew Holme

    Andrew Holme Guest

    The HC123 can be used instead of the LS123 but you will need to adjust
    the value of Rext and/or Cext to get the same delay.
  4. Martin

    Martin Guest

    Wow, I didnt realize that it was just one chip. That changes things quite a
    bit. I suppose I should have seen that due to the fact that the "two" chips
    didnt re use the same pins. I just thought that they both did two different
    things and needed two chips. I suppose then the designation IC3 A + B on two
    chips in the diagram means "Use the same chip".

    I did look at both the data sheets but Im afraid the information is beyond
    me. Do you believe that the one could be used with the other?

  5. Guest

    The timing cap pins (6&14) need to be grounded on the HC: the LS will
    work without this connection.
  6. Andrew Holme

    Andrew Holme Guest

    Yes, that's what he's saying: connect pin 6 and 14 to ground. It says so,
    in a note, on page 1 of the HC datasheet:
  7. Martin

    Martin Guest


    Im not sure exactly what you mean. Are you saying that I should put it
    together as described in the diagram but also have a jumper wire go from pin
    6 & from pin 14 (between the Negative side of the cap and pin 6 & 14) to
    ground. Could you clarify for me ?
    I doodled out what I think you mean but perhaps IM wrong ??

    Please cut and paste into notepad.

    |---- 14 CEXT Pin
    == C2 47u
    R4 R47k |
    -/\/\/\-------15 Pin

    |---- 6 Pin
    == C3 1000u
    R5 47k |
    -/\/\/\-------7 Pin

  8. Martin

    Martin Guest

  9. jhomppi

    jhomppi Guest

    Did you connect anything to pin 15 and pin 0 of the "B" chip.
    I have not looked at the datasheets but typically chips need
    power for their internal workings.
  10. Martin

    Martin Guest

    Well last night I re did things using one 74HC123 chip and grounded 14 and 6
    and it appeared to work.

    Im not sure what you mean by 15 and 0. on Chip B. Since on Chip A 15 is
    already used up So it cant be used again on B and I'm unfamiler with pin 0
    unless your talking SCSI language where 0 is the first device. But I dont
    think there is a pin 0, unless you mean pin 1? Which is again being used on
    Chip A.

    In either case I think it is working for me now. Thanks.
  11. jhomppi

    jhomppi Guest

    I am glad that you got it working.

    In case you do future projects here is some more explanation
    about pin 15 and pin 8 (not pin 0, sorry my eyes are bad).

    Pin 15 supplies power to the chip. Similar to electricity supplied to
    home appliances. Each applicance (ie chip) needs its own supply to
    operate correctly regardless of the other connections.

    Simple analogy for sci.electronics.basics:
    Suppose I buy a new TV and a new stereo amplifier.
    I connect the TV audio output to the stereo audio input.
    I plug the TV into the wall outlet and turn it on and hit
    the "on" switch of the stereo ..... no sound comes out of the stereo.
    I forgot to plug the stereo into a wall outlet. Both the TV and the
    stereo amplifier needs power to operate.

    The schematic with the single 74HC123 has both the A and B circuits
    powered from pin 15 of the single chip. If you use separate 74HC123
    chips then you must provide power to each of the chips.
    ie. Both 74HC123 chips must have their pin 15 attached to the +5V
    power supply. (ditto for the ground on pin 8).

    Be aware that sometimes (infrequently) schematics omit the power
    supply pins and ground pins for chips assuming that readers of the
    schematic know that chips need power.
  12. John Fields

    John Fields Guest

    Not always.

    For Philips' HC it's Tw = K RC, where K ~ 0.45 for Vcc = 5.0V and ~
    0.48 for Vcc = 2.0V.

    For TI LS, it varies from about 0.33 with Cext = 1µF to about 0.5 with
    Cext = 100nF
  13. John Fields

    John Fields Guest

    The 74HC123 is a _dual_ retriggerable monostable multivibrator. That
    means there are two one-shots (monostables) in the same package, with
    common power supply terminals. Pin 16 is used to supply +5V to both
    one shots, internally, and pin 8 is used to connect both one-shots to
    ground (0V), internally.

    Pin 15 is where the junction of the timing capacitor and timing
    resistor for one of the one-shots is connected, and pin 7 is where the
    junction of the timing capacitor and timing resistor for the other
    one-shot is connected.

    Pins 6 and 14 are where the other ends of the timing capacitors are
    supposed to be connected, and even though they're connected to ground
    inside the chip it's a good idea to connect them to ground externally
    as well. For some manufacturers, like National, it's evidently a
    requirement. For others, it's suggested in order to lower the noise

    In your application, even though it's not shown on the schematic, it
    would be a _very_ good idea to put diodes across the timing resistors.
    The cathode should be connected to the power supply side of the
    resistor, and you should use Schottky (or germanium) diodes rated for
    a breakdown voltage greater than the supply voltage.
  14. Martin

    Martin Guest

    Thanks jhomppi,

    I will squirrel the information away. I will re read the datasheet to see
    how they describe pin 15. I find the data sheets (for good reason)
    overwhelming and filled with a lot of shortforms that get confusing. Even
    when they explain it I get lost but I trudge on. Sometimes little details
    like you explain help me to suddenly get " the gestalt Ah"

  15. Don't thank him too much :)
    On most TTL-ish 16 pin parts (including the '123), power is pin 16,
    not 15, and ground is 8. On a 14 pin part, power is 14 and ground 7
    (still on the corners.)
  16. jhomppi

    jhomppi Guest

    Sorry for my bad eyesight.
    As other posters have noted I mis-read the pin assignments from
    the schematic.

    My apologies.

    However the discussion of providing power to chips is still
    valid if you omit my pin specific comments.

    I was trying to make a general comment about powwer to
    A newbie looking at a schematic of multi-function chips
    may be lead to believe that two separate chips are required
    since two rectangles are drawn on the diagram. If no power is
    supplied to the second chip then the circuit will not
    work. Generally speaking if the power supply pin of
    a chip is not connected then that chip will not
  17. Martin

    Martin Guest

    Thanks again. Yes I was a little confused by the 15 / 16 Input power I
    thought there was some confustion. Im glad its cleared up. I get confused
    wth fuzzy logic.

  18. Martin

    Martin Guest

    Again thanks for filling in more data.
    And thanks I will look at the diodes addition.
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
Electronics Point Logo
Continue to site
Quote of the day