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Circuit 67: Schmitt trigger with 2V hysteresis
Setup information
Install the three transistors, the diode and the six resistors as shown in Fig. 2.67. 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 circuit,
which is a basic circuit widely used in analogue electronics. The Schmitt trigger responses at a certain volt-
age (in our case that is 5 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 2V) is basically defined by the value of resistor R3. Try run your experiment by connecting
a low-power variable power supply to the input. 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. 3.67.
92
Components required:
2 x 2N3904 transistors,
1 x 2N3906 transistor,
1 x red LED, 1 x 100 Ω resis-
tor, 1 x 220 Ω resistor,
2 x 1 kΩ resistors,
2 x 2.2 kΩ resistors,
1 x breadboard, 1 x 9V bat-
tery with clip connector
Fig. 2.67: Schmitt trigger with 2V
hysteresis
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