Schmitt Trigger and Gates

[wuchy143]

Hello,

I was wondering how a schmitt trigger is used in conjunction with a gate. For example the 74HC132 Nand/Schmitt. https://www.fairchildsemi.com/ds/MM/MM74HC132.pdf

I understand that the schmitt trigger is using hysteresis with a comparator but don't see the implications of using it with the 74HC132.

Any thoughts?

-wuchy143

 

[kpatz]

The Schmitts are on the inputs of the gates. They're handy if you need to feed an analog or unconditioned signal into the gate.

 

[wuchy143]

Hi kpatz,

I went to college at UNH Durham. Cool.

Anyway, so I guess from that you are saying that there are schmitt triggers basically at the pins which are able to apply histeresis so you know that you have, for sure, an input of "0" or an input of "1" to the gates. So it's a pre-configured schmitt trigger which only job in life is to determine if whats at the pins are a "0" or "1".

Does that all make sense?

Thanks for the reply!!

-wuchy143

 

[kpatz]

That's basically it. On gates without Schmitt triggers you can get unpredictable or undesirable results if the input voltage is in that no-no zone between "low" and "high". The Schmitt ensures the gate gets an actual 1 or 0 (high or low) even if the input voltage is out of normal TTL or CMOS spec, say due to noise. They can also be handy for making oscillators and astable/monostable multivibrators, or to measure an analog voltage by timing how long it takes for the voltage to charge a capacitor through a resistor up to the Schmitt threshold (this is sometimes done to achieve a simple/crude A/D conversion with a microcontroller or a timer/counter).

I live in Manchester, about an hour from Durham.

 

[wuchy143]

Makes perfect sense. Thanks!!