Event Detector Trigger board

Discussion in 'General Electronics Chat' started by Jonesy_135, Aug 7, 2013.

  1. Jonesy_135

    Thread Starter New Member

    Aug 7, 2013
    Hi All,

    I was hoping to get some help with a project im working on

    Im trying to build a trigger board to test an event detector

    The Event detector is used to test for solder joint failures
    so when a joint fails (thus becoming open circuit) the event detectors flags it up
    the detecttion threshold can be set from 200 to 5000Ω

    i need to build a trigger board that will simulate an open circuit event...
    so the track needs to go to GND then switch to a resistance (using a pot to allow me to adjust for the detection threshold)

    however the problem im having is is has to be 250-300ηs open circuit

    there for Relays and switches are out (as far as i can make out!)

    so far i have a 266ηs pulse from a 74LS123 which can be configured for either 0.2v or 3.4v output

    ive tried looking at transistor switches and diode switches

    i guess im just not experience enough to be able to put the pieces together

    So can anyone offer any suggestions to help?

    is there a relay out there that that will work?
    or use a transistor to switch from GND to the resistor.

    Any help would be much appreciated.

    i dont know how clear i have been with what i need, so ask all the questions you want :D

    Cheers guys!!
  2. wayneh


    Sep 9, 2010
    That pulse corresponds to 4kHz, so I think that is well within the capabilities of the 555 timer. You could put a diode on the output so that it can pull the output to ground, but it would float when high, allowing you to use whatever resistor you want to ground.
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    Does this mean microseconds (as you assumed), or nanoseconds? He used the wrong symbol, whichever it is.
  4. Jonesy_135

    Thread Starter New Member

    Aug 7, 2013
    I meant nanosecond, a 250-300 nanosecond pulse.

    Im a little confused at how the diode would provide a low resistance path to ground whilst the timer pulse is low, and then when the pulse goes high, increasing the resistance.... would it not do the opposite?

    Sorry for being dense, designing electronics has never been my strongest subject!
  5. wayneh


    Sep 9, 2010
    OK, well that short time rules out the 555.

    Maybe just turn the diode around in your head, so that the 555 can sink current but not source it.