Elenco Electronics EP-130 - RC Delay Timer

-
119
-
This circuit is a delayed timer that uses a
n
o
p
erational am
p
lifier and the RC time constant. R
C
s
tands for resistor/capacitor. A circuit that delays a
n
operation is a time constant
.
Throu
g
h resistors RA and RB the ne
g
ative
(
–
)
terminal of the o
p
erational am
p
lifier receives
a
volta
g
e of about 4.5V. This is the comparator’
s
reference voltage. Connected to capacitor C1 is th
e
p
ositive
(
+
)
terminal of the com
p
arator. This ca
p
acito
r
receives its charge through the series resistance o
f
R
2 and the control. The char
g
in
g
speed is slowe
r
when the resistance is large, and
f
aster when th
e
resistance is small. This charging speed set the delay
time
f
or the timer circuit
.
Now turn the control
f
ully clockwise to position 10.
Set the switch to position A to turn on the power. LED
1 li
g
hts first; LED 2 li
g
hts about 5 to 7 seconds later
.
This 5 to 7 second time di
ff
erence is the delay tim
e
that is set by the CR time constant
.
Now, turn o
ff
the power, set the control
f
ully counter
-
clockwise to
p
osition 1, and see what ha
pp
ens whe
n
you turn on t
h
e power aga
i
n.
LED
2
li
g
h
ts
l
ater t
h
a
n
LED 1 again, but how many seconds later
?
N
otes
:
EXPERIMENT #
99
: R
C
DELAY TIMER
Schematic
Wirin
g
S
equence
:

8
1-
3
1-
63
-27-1
3
1

28
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8
7

8
3-33-36-70-116-135-121

3
4-
6
7

68
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8
2-
8
4

88
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69
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11
5-
136

119
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124

1
22-132