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SN74121

Rhode Roberts
Prodigy 70 points
Other Parts Discussed in Thread: SN74121, SN74LVC1G123

 I am trying to build a timing circuit, a one shot using a 74121. I am not seeing the expected pulse from pin 6 (Q), In fact I see a low level all the time. I have a single pulse (+5) from a momentary switch coming into pin 5. I am using a timing constant, with a variable 20K ohm resistor and a 33 uF capacitor.  I have tried 2 different SN74121's with the same results (thought one might be bad)  Here is a schematic of what I have built, but unlike the schematic, there is no pulse at pin 6.  Any ideas?

  • 0
    TI__Guru 58310 points
    Hi Rhode ,

    for a start , I would say the cap needs flipped to have the polarity right .
    The connection from Cext to Rext is required . The Rint can be left open .
    Alternatively , you could also consider the SN74LVC1G123 which also has an app note on the working . This device is done with BJT and is relatively older compared to the sn74LVC1G123 which is CMOS .
  • 0 in reply to ShreyasRao
    Prodigy 70 points
    Thank you Shreyas, Very helpful. Yes the cap is upside down, my bad drawing. But thank you for the great info. ~ Rhode
  • 0 in reply to ShreyasRao
    Prodigy 10 points
    "Flip the polarity"???
    From the very little information one can find in the datasheet, the positive side of the capacitor should be on Cext!
    I see 2 problems:
    1. Connect the node R/C to pin 11, not 9.
    2. The input will never see a low level logic input! You need to connect a resistor between the pin 5 and ground! Or connect your switch to ground. An open TTL input is like a high level (but bad solution).