Experiment 4 - The Hysteresis of the Schmitt Trigger

If you reduce the input voltage with the potentiometer, the Schmitt trigger switches back to its original state. But there is a peculiarity...

Illustration functioning Schmitt trigger III.

Voltage profile.

Turning the potentiometer clockwise lowers the input voltage U_in...

• At a certain voltage U₀ the transistor T1 turns off and
• the voltage at the basis of the transistor T2 increases. T2 turns on.

Thereby the voltage at the output is U_out = 0 V. The LED turns dark and the Schmitt trigger is returned to its original state.

The coupling of the emitters of the transistors has the consequence that the threshold voltages U₀ and U₁ are not equal! This is known as hysteresis and is the hallmark of a Schmitt trigger circuit.

In practice, Schmitt trigger circuits using transistors, are rare since the calculation of the individual components is complex. You can build more easily Schmitt trigger circuits by using operational amplifiers or comparators In the following experiments you understand why...