Problem using DM74LS123N to produce uS pulses

Discussion in 'The Projects Forum' started by Peter Pan, Apr 23, 2007.

  1. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Hello All,

    I have tried to assemble and run schematics using retriggerable monostable from book Marston, Digital Logic IC (page 193). Everything done correct according to the book, but the project does not work ... unfortunately. :( Output of the chip (pin 5) is just 1.8 V DC instead of
    series of pulses as expected.
    I have attached pix of the chip, schematics and voltage at pin 5 relative to GND. Only one half of the dual monostable is used (pins 5, 6, 7, 8, 9, 10, 11, 12) the other half is disabled by tying its CLR terminal high and grounding its A and B terminals.

    The pin outputs are:
    pin 5 1.8V DC
    pin 6 48 mV DC
    pin 7 1.34V DC
    pin 8 35 mV DC
    pin 9 - input square pulses
    pin10 5.3V DC
    pin11 5.3V DC
    pin12 4.2V DC
     
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  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Are you measuring pin 5 with an oscilloscope or with a voltmeter? A voltmeter can't track pulses and will provide an average or RMS (don't recall which) value of the pulse train.
     
  3. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    With PC oscilloscope Pico :) I also used it to make pin 9 input (seen on the pic attached to my 1st post in this thread)
    It can measure DC and AC voltage from a pulse train and for pin 5 it was DC 1.8V (flat plot of voltage vs time on PC scope's screen).
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Okay. I've had a closer look now.

    You've got 1.34Vdc on pin 7? Shouldn't that be 5.3Vdc? And, although its obvious I'll ask anyway, you have 5.3Vdc on pin 16?
     
  5. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    You don't indicate your values for R and C.

    Make sure that the RC time constant and desired output is less than the frequency you are feeding it, otherwise you're constantly triggering the device and it never works.

    In other words if you want an output pulse of 2ms, you cannot trigger it with a pulse that occurs every 1ms. The trigger pulse must be slower than the output time constant.

    Why do you have 35mV at GND, it should be 0V unless it is just meter error?
     
  6. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Yes, it is 5.3V DC at pin 16 as it has to be. About pin 7 - well, it seems you are right, it should be 5.3V DC instead of 1.34V DC - i'll check it :) .
    somehow i've missed it completely, many thanks!
     
  7. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
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    time constant is actually determined as t=0.5RC and R=220k, C=4700pF (both are indicated on pic attached). hence t is about a half millisecond and the triggering pulses (at pin 9) follow every few seconds - timing seems allright!:)
    35mV at GND is a scope' meter error as chip is well grounded and when measured by tester it gives 0V.
     
  8. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
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    Now I have checked it and have found out that voltage at pin 7 depends on value of R. When R=220k the voltage is 1.34V and when R=1k it becomes 3.6V DC at pin 7 (and about 1.6V DC on the resistor). it changes timing only, I believe.
     
  9. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Pin 5, the Q output, should never be at 1.8V, it should be either hi (~Vcc) or low (~GND). It indicates there is something wrong with Vcc, GND, or the way you are measuring.
     
  10. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Well, now I have found out that chip starts working well and produces pulses of about hundred millisecond duration each with repetition time 5 sec, triggered by input pulse train. But it does not produce very short pulses, of duration few microseconds (with the same repetition time) - what I really need. When I change timing capacitor from 220microF to 0.0066microF it stops to produce pulses. :( Maybe any ideas? Any advice is very much welcome!
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    What is the repetition time and pulse width of the incoming frequency?

    What is the desired pulse width at the output?

    Tell us more about your project.
     
  12. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    hi :)

    input is positive square pulse train with pulse width 2.5 sec and repetition time 5 sec.
    output has to be pulse train with pulse width few microseconds (2 microsec would be fine) and repetition time 5 sec.

    This project is a part of larger project which could be called design of home made NMR spectrometer where rf electronics has to produce train of very short (few microsecond) rf pulses with rf frequency 5MHz and repetition time 5 second.

    *I think it's better to make a new thread since it's working now (though not quite in a way I need it)!
     
  13. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Hello All :)

    I need to generate a train of pulses with microSec widths and repetition time of about 5 sec using one half of DM74LS123N. Repetion time is determined by input signal which is square pulse train (pulse width is 2.5 s for each pulse) and generated by external clock. Width of pulses produced by DM74LS123N is determined by C and R (shown on picture attached) and given by tp=0.37RC.
    For example, when C=10microF, R=1k then tp=3.7mS and my schematics generates these milliSec pulses quite well. However when I change C to 0.0066microF for example, it does not generate pulses at all. What could be a reason for this?!:rolleyes: Any advice is very much welcome!
     
  14. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    Here is pic of the chip pins
     
  15. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    ...and here is oscillogram of the input signal.
     
  16. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    567
    12
    Just in case you don't have one, see the attached datasheet:
     
  17. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    I believe that the equation is only valid for large pulse widths. In the Texas Instruments
    datasheet they have a graph for the smaller pulse widths.

    Check out --- http://focus.ti.com/docs/prod/folders/print/sn74ls123.html

    (* jcl *)
     
  18. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    Your setup allows a single microsecond pulse outputted each time the 0.2 Hz frequency transisitions from a high to a low.

    If your using a digital oscilloscope, your sampling could blow by that microsecond pulse like it wasn't even there. If your using an analog oscilloscope, the sweep certainly blew by it.

    If you want to see if you truely have an output, you'll have to trigger your oscilloscope internally, using the positive transistion, on the channel you have connected to the output pulse and set your sweep to 10 microseconds or 5 microseconds. If the scope triggers, you'll see the pulse, if it doesn't ... then there isn't an output.

    If your looking to output a group of pulses clocked by a 0.2 Hz signal, the manner you connected the 74LS123 won't work. I think the 74LS123 would be overkill if that's your requirement.
     
  19. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
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    the equation slightly changes for smaller pulse width but this effect is not substantial for the time since in my schematics short pulses are not produced at all.
     
  20. Peter Pan

    Thread Starter Senior Member

    Mar 24, 2005
    122
    0
    I use PC scope, which is digital and allows observation of very short pulses - i just switch it to uS range and watch whether voltage changes from high to low and back each 5 sec (given repetition time). it changes for mS pulses (thanks God) but does not for uS - here is my problem.
    ..i really think that there is something caused by capacitor - whether it's leakage or stray capacitance on pins is too large. i tried several different capacitars but it still does not make schematics working :confused:
     
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