1. Schmitt trigger

2. Introduction
A Schmitt trigger is an electronic circuit that is used to detect whether a voltage has crossed over a given reference level.
A Schmitt trigger circuit is a fast-operating voltage-level detector.
When the input voltage arrives at the upper or lower trigger levels, the output changes rapidly.

4. Schmitt trigger

5. Symbols of Schmitt trigger

6. Positive and Negative Feedback
The input voltage vin is applied to the inverting input terminal and the feedback voltage goes to the non-inverting terminal.
This means the circuit uses positive voltage feedback instead of negative feedback, that is, in this circuit feedback voltage aids the input voltage rather than opposing it.

7. Feed back

8. Voltage Divider Feedback
The voltage divider feedback a negative voltage to the non-inverting input, which results in a larger negative voltage.
This feedback more nega­tive voltage until the circuit is driven into negative saturation. 

9. Schmitt Trigger transfer
Low state
High state Vo
Vt-
Vt+ Vi

10. Calculation of Resistor
To design a Schmitt trigger, potential divider current I2 is once again selected to be very much larger than the op-amp input bias current.
Then the resistor R2 is calculated from equation
R2 = UTP/I2
and R1 is determined from
R1 = (VOUT – UTP) / I2

11. Threshold level
Whenever the input voltage goes over the High Threshold Level, the output of the comparator is switched HIGH or LOW.
The output will remain in this state, as long as the input voltage is above the second threshold level, the Low Threshold Level.
When the input voltage goes below this level, the output of the Schmitt Trigger will switch.

12. Threshold level

13. Transistor Schmitt trigger
A transistor implementation of a Schmitt Trigger is rather important due to the single voltage supply that requires to operate the basic Schmitt Trigger circuit with two NPN transistors.

14. Transistor Schmitt trigger

15. Hysteresis
Hysteresis is desirable in a Schmitt trigger because it prevents noise form causing false triggering.
The difference between the two threshold voltages is known as hysteresis

16. Hysteresis
Hysteresis
Hold and measure
VThreshold
Vinput
Voutput

17. The End
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