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If you think about TTL or CMOS schmidt gates in simple RC oscillators
it is quite simple. On power up capacitor is empty ("0") so output of the inverter is high. Capacitor charges through resistor till input reaches level for logical "1". In that instant, output is inverted to logical "0" so
capacitor discharges through same resistor. Once the capacitor is low enough, input is considerd "0" what forces output again to be "1" and whole cycle repeates.
If you add potentiometer in series with resistor, you can change frequency. If you replace resistor for two resistors in parallel and add diodes in series with resistors (oposite polarity for diodes), you can change independantly time to charge and discharge capacitor. This way
you can easily change duty cycle.
well here are more bits of info i can share with you;
a schmitt trigger is basically a bistable device with a very fast response and has hysteresis or dead band. its output changes at a specified value of the input lelvel and resets at a different value. now the difference between this values is what you call the "dead" band. so any changes in the voltage level within this band will not trigger this schmitt device. there are ics designed to react in this manner and you can also design this with the use of discrete devices as what "cruel" has referred you to.
as you see, in actual scenarios digital signals are really not that clean as what we expect, there are noisy signals as well as slow rising signals that sometimes it would not have enough strenght to trigger a digital device.
as far as i know for debouncing or signal conditioning this device offers a very good response.
further more, in reality many devices are edge triggered rather than level triggered, meaning the signal should have a very fast rise time so it can be recognized. likewise a slow rising signal with no sharp edge cant trigger a digital device. so the practical solution is to convert the sine wave to square wave by employing the schmitt trigger.
hope this could further your knowledge about schmitt trigger.