Elenco 130-in-1 Electronics Playground - Manual (Page 101)

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Now you are going to use the operational amplifier
as a comparator and as a Schmitt trigger circuit. As
long as its input voltage exceeds a certain value, the
operational amplifier will produce a signal. View the
schematic: can you see how it works? The input
level that turns on the output is higher than the level
than turns it off. So once a Schmitt trigger circuit
turns on, it stays on unless the input drops
significantly. We call this type of operation a
“hysteresis loop.”
Build the circuit, but don’t press the key yet. The
operational amplifier serves as a comparator in this
state. When you alternate the control, LEDs 1 and 2
take turns lighting at some point. Note that this point
doesn’t alter whether you turn the control clockwise
or counterclockwise.
Now push the key and you have a Schmitt trigger
circuit, which makes a hysteresis loop. Turn the
control and see how the circuit operation is different
from before.
As the ratio of resistors RB/RA increases, the width
of hysteresis becomes narrower. Try using different
values for RA and RB, and notice how the width
changes.
Notes:
EXPERIMENT #82: INTRODUCING THE SCHMITT TRIGGER
Wiring Sequence:
70-36-26-121
27-83
63-28-130-131
34-33-67-90
68-134
84-69-138
89-137
119-124-135
122-132
31-129
Schematic