Elenco Electronics EP-130 - Introducing the Schmitt Trigger

-
101
-
Now you are
g
oin
g
to use the operational amplifie
r
as a comparator and as a Schmitt tri
gg
er circuit. A
s
l
ong as
i
ts
i
nput vo
l
tage excee
d
s a certa
i
n va
l
ue, t
he
operational amplifier will produce a signal. View th
e
schematic: can you see how it works? The input
level that turns on the output is higher than the leve
l
than turns it off. So once a Schmitt tri
gg
er circui
t
turns on,
i
t stays on un
l
ess t
h
e
i
nput
d
rop
s
signi
f
icantly. We call this type o
f
operation
a
“hysteresis loop.
”
Build the circuit, but don’t press the key yet. Th
e
operational ampli
f
ier serves as a comparator in thi
s
state. When you alternate the control, LEDs 1 and
2
take turns li
g
htin
g
at some point. Note that this poin
t
doesn’t alter whether you turn the control clockwis
e
or counterclockwise.
Now push the key and you have a Schmitt tri
gg
e
r
c
i
rcu
i
t, w
hi
c
h
ma
k
es a
h
ysteres
i
s
l
oop.
T
urn t
h
e
control and see how the circuit operation is differen
t
from
be
for
e
.
As the ratio of resistors RB
/
RA increases, the widt
h
o
f
hysteresis becomes narrower. Try using di
ff
eren
t
values for RA and RB
,
and notice how the widt
h
chan
g
es
.
N
otes:
EXPERIMENT #
8
2: INTR
O
D
UC
IN
G
THE
SC
HMITT TRI
GG
E
R
W
iring Sequence:

7
0
-
36
-2
6
-121

2
7-
83

6
3-28-130-131

34
-
33
-
6
7-
90

6
8-134

8
4-69-138

8
9-137

1
19-124-135

1
22-1
3
2

3
1-129
S
chemati
c