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Experiment 3 - The Threshold of the Schmitt Trigger


Illustration functioning Schmitt trigger II.


Voltage profile.


Turning the potentiometer counterclockwise raises the input voltage Uin...

  • At a certain voltage U1 the transistor T1 turns on and
  • the voltage at the base of transistor T2 falls quickly and T2 turns off.

Thereby the voltage at the output switches to (Uout = 9 V). The LED lights up.

A characteristic of the Schmitt trigger circuit is the fact that the emitters of the transistors are coupled. This ensures that T2 remains a little bit longer turned on while T1 already turns on. Only when the input voltage reaches Uin = U1, the T2 turns off suddenly.

The voltage U1 is referred to as the ON threshold.

What happens, when you turn the potentiometer again in the other direction...?

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