Elenco Electronics EP-130 - Timer

-
127
-
Here’s a timer you can use for takin
g
timed tests o
r
simply for knowin
g
when an amount of time has
p
assed. You can
p
reset this timer for u
p
to
a
pproximatel
y
15 minutes. When the time is up, it
g
i
ves out a cont
i
nuous
b
uzzer soun
d
unt
il
you turn
off the power or press the key to reset the circuit.
After you build this experiment, set the control t
o
p
osition 2 on the dial and slide the switch to
p
ositio
n
A
to turn on t
h
e power.
H
o
ld
a stopwatc
h
an
d
start
it
when you press the key. The timer makes a buzzin
g
sound in about 30 or more seconds.
S
et the control to each division on the dial from 2 t
o
8
, and note how lon
g
it takes the timer to produce
a
sound. Settin
g
the timer’s calibration - the time tha
t
passes at each settin
g
o
f
the dial - requires a lot o
f
patience, but it is necessary
f
or making sure you
r
timer works accurately. After you set the calibration
,
you nee
d
to ma
k
e a grap
h
s
h
ow
i
ng eac
h
contro
l
p
osition and the time it takes for the buzzer to sound
.
Then your tester is ready
f
or use
.
S
can the schematic. The control changes th
e
reference voltage of the comparator (I
C
1). Th
e
resistor R and the ca
p
acitor C determine the time
r
setting. When the voltage applied to the positive
(
+
)
terminal of I
C
1 exceeds the reference volta
g
e, th
e
al
arm soun
d
s.
The operational ampli
f
ier has hi
g
h input impedance
(
input resistance
)
, so its current loss is very small
,
a
n
d
you can use
i
t to ma
k
e a t
i
mer w
i
t
h
a very
l
on
g
settin
g
. IC 2 works as an astable mulitivibrator that
pro
d
uces t
h
e
b
uzzer soun
d
.
No
t
es
:
EXPERIMENT #1
0
7: TIME
R
Wiring Sequence:

1
-2
9

2
-30

3
-
114

5
-
83
-7
0
-
106
-
118
-
13
7-
26
-
121

93-63-28-132

92-90-64-113

6
5-105-91

66
-
8
2-
8
4-
89

6
7-
81

94-69-117-138

119
-
124

122
-
131
Schematic