Experiment 3 - The Threshold of the Schmitt Trigger

If you increase the input voltage of the potentiometer, the Schmitt trigger changes state. What happens in the circuit...?

Illustration functioning Schmitt trigger II.

Voltage profile.

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

• 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 (U_out = 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 U_in = 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...?