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CD74HC221: MONOSTABLE MULTIVIBRATOR OUTPUT ISSUE

Sushant Patil25
Prodigy 205 points
Part Number: CD74HC221
Other Parts Discussed in Thread: SN74LS221

We are using CD74HC221 monostable multivibrator for our one of the project. We are using external capacitor (10uF) and resistors (100K) for 800mS pulse width calculated as per formula provided in datasheet. But i am not getting the outputs as expected (IC is not responding for input).

When i am using 10uF and 50K resistor, i am getting 400ms pulse width as per datasheet. SPDT switch output is given to CD74HC221. Let me know the suggestion to get output.

  • 0
    TI__Guru* 96591 points

    Hi Sushant,
    The capacitor C7 (and C6) is reverse polarity - not sure if that's the issue, but I wanted to point it out.

    Can you provide scope shots of pin 9, pin 7, and pin 5 (preferably together)?

  • 0 in reply to Emrys Maier
    Prodigy 205 points

    Polarity of capacitor i have changed and tested but there is no luck for 10uF and 100K combinations. Also tested for 10uF and 50K resistor with reverse polarity, it works fine (gives 400ms pulse width)

    I have tried with different combinations of resistor and capacitor, up to 400mS only i am getting results as per expectations. But RC combinations for above 400mS it is not working.

    Kindly go through below images has been taken as per your suggestions.

    1. Tested with R = 50K and C=10uF

    2.Tested with R=100K and C=10uF

  • 0 in reply to Sushant Patil25
    TI__Guru* 96591 points
    Thanks for the scope shots.

    You can see in the first shot that the capacitor is naturally at 5V and is discharged down to ~2V when the CD74HC221 is triggered by pin 9. As long as the capacitor is below a certain value, the output has a high pulse.

    In the second scope shot, the capacitor voltage is holding at about 2.5V and the output is holding high (the expected behavior of the CD74HC221 when the capacitor is held below threshold like this).

    The rise time for a 100kohm + 10uF RC circuit is ~4*R*C = 4s -- since the inputs are triggered every 2 seconds, I would expect to see some rise on the capacitor. Is there a chance the 100k resistor got mixed up with a larger value (or got damaged)? If it were a larger value (say 1Mohm) I could see why the capacitor would appear to have no charge after 2 seconds. (~4*R*C = 40s).
  • 0 in reply to Emrys Maier
    Prodigy 205 points
    Above circuit we have tested on bread board alone also, Means there is no possibility of mixing resistor with large value.
    100K resistor also we replaced and tested no outputs. same circuit with part SN74LS221, works fine. But with CD74HC221 we are getting this issue.
    Let me know the suggestion to come out of issue.
  • 0 in reply to Sushant Patil25
    TI__Guru* 96591 points
    I will order some samples for this device and test in our lab. I should be able to get you some details by the end of the week.
  • 0 in reply to Emrys Maier
    Prodigy 205 points
    That's great, thanks for support.

    Because of this issue only our project is on hold. Waiting for solution and feedback.
  • 0 in reply to Sushant Patil25
    TI__Guru* 96591 points
    Hi Sushant,
    I only got a few minutes in the lab today, but I tested this device with a variety of large RC values without any problems (even 220uF capacitors discharged fully and recharged correctly). Are you certain that there's not a bad connection somewhere? It looks like the device is triggering once and discharging the cap, but then it isn't being recharged, which indicates a disconnect on the resistor side.
  • 0 in reply to Emrys Maier
    Prodigy 205 points

    When i am keeping circuit in run condition for more than 2 hours after 2 hours it stops giving pulse. On our board there is no heating issue. Same behavior observed on bread board also.

    Waiting for valuable suggestions.

  • 0 in reply to Sushant Patil25
    TI__Guru* 96591 points
    Ok, so this issue only shows up after several hours of operation? Or is it from the start?
    After you reset the system, does the issue stay with the part, or does it go away until 2 hours of runtime again?

    Just to be sure, are there any other details about your test that you haven't included?