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ScorpFire
Joined: 31 Oct 2009
Posts: 3
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 31 Oct 2009 12:35 Schmitt trigger.... |
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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?
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betwixt
Joined: 04 Jul 2009
Posts: 389
Helped: 64
Location: Wales, UK
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31 Oct 2009 12:55 Re: Schmitt trigger.... |
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What you describe is not a Schmitt trigger, it is a comparator.
The difference between a normal input and a Schmitt input is that there is a difference between the high threshold and the low threshold. A normal logic input has a specific voltage that is regarded as high, anything below it is regarded as a low. A Schmitt has two thresholds, the high threshold is at a higher voltage than the low one so there is a 'no mans land' region between them. To be high, the input must be above the high threshold and to go low it must drop below the low threshold.
They are normally used where there is noise on a logic signal so a larger swing is needed to prevent the state changing by the noise.
There are many types of Schmitt logic gates and most micro-controllers have Schmitt input pins as well. Look at data sheets for the 74LS04 and 74LS14 to see their characteristics.
Brian.
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ScorpFire
Joined: 31 Oct 2009
Posts: 3
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31 Oct 2009 16:08 Re: Schmitt trigger.... |
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| can we set the threshold levels? or is it predetermined
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betwixt
Joined: 04 Jul 2009
Posts: 389
Helped: 64
Location: Wales, UK
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31 Oct 2009 20:15 Re: Schmitt trigger.... |
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Fixed.
Either a fixed voltage or a proportion of the supply, depending on the manufacturing technology. I don't know of any that have programmable differences.
Brian.
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hage-bai
Joined: 01 Oct 2007
Posts: 5
Helped: 2
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01 Nov 2009 6:27 Re: Schmitt trigger.... |
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Are you dealing with an analog signal here or is your required solution being driven by CMOS or TTL logic?
Is your input signal noisy? If you want to ignore temporary unwanted transitions and need hysteresis then the Schmitt trigger is needed.
If the input is analog, consider an op amp schmitt trigger circuit which uses resistors to set threshold voltages.
If you are dealing with defined logic levels such as TTL, then use one of those 74LS1xx parts.
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Kral
Joined: 28 Mar 2005
Posts: 1182
Helped: 187
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02 Nov 2009 17:39 Re: Schmitt trigger.... |
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ScorpFire,
The various Schmitt trigger buffers and inverters that are available have fixed hysteresis. The Schmitt trigger is a comparator that has positive feedback to provide its hysteresis (difference between the high and low reference levels). You can build a Schmitt trigger from a compator (or op amp) and provide the positive feedback through external resistors. In this manner, you can provide well defined high and low trigger levels.
,
Historical note; Originally, the term "Schmitt trigger" refered to a specific circuit using vacuum tubes (valves). Later the circuit was adapted to a specific circuit using transistors. Still later, the term "Schmitt trigger" was applied to any comparator circuit employing positive feedback for hysteresis.
Regards,
Kral
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