CD4047B: triggers on both flanks

Tim Kurz

Prodigy 10 points

Part Number: CD4047B
Other Parts Discussed in Thread: SN74LVC1G123, CD40106B

Hi everyone,

i am using a CD4047B as a monostable multivibrator.

It should detect negative flanks.

This is how i connected it:

Additional i have a 1kOhm pull up on Pin 6 and a Lowpass (1kOhm, 1µF).

The problem is: the IC triggers on both flanks. See picture:

Yellow is Q,

Green is -Trigger

Am i missing something here?

Thanks for your help. Tim

11 months ago

0 Clemens Ladisch 11 months ago

Guru 265480 points

I suspect that the trigger edges are too slow and lead to oscillations in the input buffers; see the input rise/fall time specifications on p. 4, and [FAQ] How does a slow or floating input affect a CMOS device?

The SN74LVC1G123 has Schmitt-trigger inputs without a rise/fall time limit.

0 Tim Kurz 11 months ago in reply to Clemens Ladisch

Prodigy 10 points

You were right about this. The RC Input Lowpass has to be just the right dimension. If it is too big, the edges are too slow and cause false triggers. If it is too small, the jitter causes false triggers. Thanks.

0 Clemens Ladisch 11 months ago in reply to Tim Kurz

Guru 265480 points

When you exceed the datasheet's rise/fall specifications, then the problem might still happen. You should use a device with integrated Schmitt-trigger inputs (like the SN74LVC1G123), or add Schmitt-trigger buffers (like the CD40106B) for the slow signals.

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CD4047B: triggers on both flanks