SN74LVC1G123: using as missing pulse detector

Part Number: SN74LVC1G123

Hi,

Is this IC able to use as a missing pulse detector? If yes, do you have an example circuit please? 

I have a pulse train of 2.5mS on and 2.5mS off. I need to detect when there is a pulse missing.

Thank you.

  • Hi John,

    You could use the SN74LVC1G123 as a missing pulse detector by setting the output pulse width slightly longer than 5 ms (the period of your signal) and then monitoring for when the output goes LOW to see any missing pulses.

    The most important design aspect of a circuit like this would be setting the output pulse length and ensuring that it's relatively constant.

    If the minimum pulse length is shorter than 5 ms, then your circuit will have a false output. Also, if the output pulse is longer than 10 ms, then the circuit will 'miss' a single missing pulse.

    The solution to this is to just set your pulse width somewhere in the middle and use good quality timing components (Rext and Cext) to avoid excessive changes with temperature.

    There is an example pulse generator circuit in the datasheet for the device, section 9.2:

    This circuit is set up to trigger on a button press using the negative edge triggered input (A\) as the trigger.

    You could use the same configuration and just replace S1 and R_PU with your pulse train input.

    You could also use B as the input for a rising edge trigger, and connect A\ to ground directly.

  • Hi Emrys,

    Thanks for getting back to me. My mistake for not mentioning in my initial post that the missing pulse (input to A\) could be either low or high for a continuous period of time.

    Following your instruction, the circuit can only do negative edge or poisitve edge trigger but not both right? To go around this problem, I need two sets of circuit? One to detect high and one to detect low. 

    Is that correct?

    Thank you.

  • Hi John,

    If a pulse is missing, then it would be missing both the low and the high edge, regardless of which state the signal is at (high or low). Just one edge should be sufficient to identify missing pulses.