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2.27 Schmitt-Trigger
Circuit 66: Schmitt trigger circuit with 4V hysteresis.
Setup information
Install the three transistors, the LED and the six resistors as shown in Fig. 2.66. Next are the 11 jumper
wires. Check the placement on the board again and only then connect the battery.
Experiment
With this circuit we accomplish a threshold value switch with hysteresis, namely the Schmitt trigger, is a
basic circuit widely used in analogue electronics. The Schmitt trigger responses at a certain voltage (in our
case that is 8 V) and switches through; the red LED turns on. If the voltage at the input drops, the Schmitt
trigger will switch off again at a defined voltage value that is below the switch-on voltage. The difference of
the switch-on voltage minus the switch-off voltage is called
hysteresis. The hysteresis of our circuit here (about 4V) is basically defined by the value of resistor R3.
Try connecting a low-power variable power supply to the input and run your experiment. You should be
able to set the voltage range between 1V and about 9V. This is a simple and vivid way to study the effect of
the switching hysteresis.
The circuit diagram for this experiment is shown in Chap. 3, Fig. 3.66.
Components required:
2 x 2N3904 transistors,
1 x 2N3906 transistor,
1 x red LED, 1 x 100 Ω
resistor, 3 x 1 kΩ resis-
tors, 2 x 2.2 kΩ resistors,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.66: Schmitt trigger with
4V hysteresis
91
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