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This is related to my earlier question on driving dual coil latching relays from a single logic signal.

This question is on how whether the TI CD14538BM96 monostable multivibrator can be used to convert an input signal with very long pulse durations into edge-triggered output pulses of a shorter, well-defined duration:

enter image description here

In case it is relevant to the answer, let's assume that I use the leading- & trailing-edge re-triggerable configuration.

How will this device behave in response to input pulses that are longer than the configured output pulse duration?

  1. Will the output pulse length always be of the configured length beginning with time of the last matching edge?
  2. Or will the output pulse length be dependent on the much longer input pulse length?

Initially I thought 2 would apply, but after some reflection I more or less convinced that 1 applies. Can somebody confirm this either way?

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2 Answers 2

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Like mentioned in the other answer, it is an edge triggered monostable. So, however long your input pulse is, the output pulse length will always be of the configured length beginning with time of the last matching edge provided you do not have multiple closely spaced input pulses within the duration when the monostable has triggered

If there is a possibility of getting multiple closely spaced input pulses within the duration when the monostable has triggered, you have to be careful and select re-triggerable or non-retriggerable mode whichever makes sense to your application.

Please take a look at the block diagram enter image description here

For your use case, you can draw a timing diagram and confirm for yourself these conclusions.

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Page 2 of the datasheet tells you that the device is triggered by the edge (not by the level), and that you want to use the non-retriggerable mode:

CD14538B retriggerable

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  • \$\begingroup\$ The non-retriggerable mode would be needed only if you wanted to time off the initial edge and reject any additional edges during the initial time "T", (as in the lower diagram). In either mode a single pulse that is longer than the calculated time "T" will not retrigger the device. \$\endgroup\$
    – Nedd
    Mar 26, 2023 at 17:12

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