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    In the diagram, the input voltage increases from zero, along the bottom horizontal line. The output voltage remains at zero on the vertical line.

Schmitt Trigger

    However, when the input voltage reaches 1.7 volts, the output shoots up from zero to 5 volts. Reducing the input voltage, as shown in the top horizontal line does not cause the output to drop to zero immediately. This only happens when the input voltage is reduced to 0.9 volts. The input level at which the output increases to maximum, and the level at which it drops to zero are different.

    This is called HYSTERESIS.


In the lower diagram Schmitt Trigger action is demonstrated in another manner.

The black graph represents a noisy logic signal received from the moon.

This is the input to the Schmitt Trigger.

The green graph is the output signal.

The output remains at zero until the input exceeds 1.7 volts.

The output then shoots up to 5 volts and remains at 5 volts until the input drops to 0.9 volts.

The output then drops to zero.

    An almost perfect output is recovered from a very noisy input


With no signal in,Tr1 has no forward bias and is cut off.

The collector voltage of Tr1 is high, turning Tr2 on.

The emitter current of Tr2 flowing through R2 produces 1 volt across R2.

Since the base of Tr1 is at zero volts, the base/emitter junction of Tr1 is reverse biased by 1 volt.

The input signal has to exceed this voltage plus 0.6 volts (1+0.6 = 1.6 volts) to forward bias Tr1.


The input signal increases from zero.

Once the input voltage exceeds 1.6 volts,Tr1 begins to conduct.

Its collector voltage starts to fall and the base voltage of Tr2 falls.

The emitter current of Tr2 through falls, reducing the voltage across it.

This further increases the conduction of Tr1, producing a cumulative effect.

Tr1 comes on very rapidly and Tr2 goes off.

Tr2 collector voltage goes high.


When the input voltage falls, it has to go below 0.6 volts before Tr1 collector current starts to fall.

Again there is a cumulative action which rapidly turns Tr1 off and Tr2 on.

Tr2 collector voltage falls.

The difference in the values of Tr1 base TURN ON and TURN OFF voltages is known as HYSTERESIS.

    The Schmitt Trigger can be used to clean up noisy signals or to speed up slow rise and fall times of pulses.

    The Schmitt trigger is a comparator application which switches the output negative when the input passes upward through a positive reference voltage. It then uses negative feedback to prevent switching back to the other state until the input passes through a lower threshold voltage, thus stabilizing the switching against rapid triggering by noise as it passes the trigger point.

Schmitt Trigger Circuit

Schmitt Trigger Action

    The Schmitt trigger action uses a comparator to produce stable level-crossing switches in contrast to the action of a straight reference comparison

Schmitt-Trigger Action

    action is a double threshold comparator process. The current equation at A gives:

Schmitt Trigger Formula


Schmitt-Trigger Formula

    This dependence upon the output voltage gives the dual threshold. The two output states give the thresholds shown at right.

Schmitt Trigger Dual Threshold