Schmitt trigger

Discussion in 'The Projects Forum' started by ScorpFire, Oct 31, 2009.

  1. ScorpFire

    Thread Starter Member

    Jun 4, 2009
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    0
    Hi i learnt that schimitt triggers can compare an input voltage with an reference voltage and emit a High when the input > reference voltage.

    I have been searching around and dont know of any ICs that have this type of schmitt trigger, some are just hex inverters, NAND schmitt triggers and stuff


    Anyone knows which IC has the schmitt trigger im looking for?
     
  2. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    Schmitt trigger is a comparator in a specific configuration. So what you are looking for is a comparator.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    One way to identify schmitt trigger logic devices is by the symbol as shown in this link. The symbol visible inside each of the gates is intended to look like a simple hystersis curve.

    hgmjr
     
  4. yasir_66

    Active Member

    Jun 25, 2009
    71
    1
    try TTL 7414 & 7413.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    A 555 makes a good Schmitt Trigger.

    555 Schmitt Trigger

    A CD40106 is a close equivalent to a 7414, which is a hex schmitt trigger. Inverting schmitt triggers also make dandy RC oscillators.
     
  6. ScorpFire

    Thread Starter Member

    Jun 4, 2009
    13
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    hmm, i thought a comparator switches the higher logic level input onto its output?
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    I think what steinar96 was trying to say is that a comparator can be configured to provide hysteresis and that hysteresis is the mechanism that is used in the implementation of a schmitt trigger logic device.

    Schmitt triggers are often used to buffer signals with low levels of noise to prevent the noise from producing false logic transitions. Schmitt trigger devices are also used to cleanup incoming signals that have a slow rise-time or fall-time.

    hgmjr
     
  8. yasir_66

    Active Member

    Jun 25, 2009
    71
    1
    try 7413 & 7414.
     
  9. Papabravo

    Expert

    Feb 24, 2006
    10,163
    1,796
    The original circuit predates the integrated circuit. It basically uses feedback from the output to change the input threshold thus providing the hysteresis characteristic. It is not dependant on the use of a comparator.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You might find the data sheet on a comparator interesting reading. Look up an LM311, for instance.
     
  11. ScorpFire

    Thread Starter Member

    Jun 4, 2009
    13
    0
    Hmm yeap i see that with a pull up resistor i can get 0 or +5 output voltages, good thing...will try and get back


    Thanks for help
     
  12. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A CD4093 is a very useful Schmitt-trigger quad NAND. It'll operate on a fairly wide range of voltages (3v-16v), has outputs that reach nearly ground and nearly Vdd, can be used for oscillators, gated oscillators, squaring up waveforms, etc - and using NAND gates, you can implement any other logic gate function.

    http://en.wikipedia.org/wiki/NAND_logic

    They are low in power consumption, but don't have much output current capability.
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Our OP said
    and
    To me, this means that Schmitt input logic gates are not going to do what he wants. As others have said, a comparator with positive feedback, or a discrete Schmitt trigger, probably will.
    Of course, I could be wrong.:eek:
     
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