Elenco Electronics EP-130, Руководство пользователя

Страница: 120 / 160

Here’s a timer you can use for taking timed tests or
simply for knowing when an amount of time has
passed. You can preset this timer for as much time
as you like, up to about 15 minutes. When the time
is up, it gives out a continuous buzzer sound until
you turn off the power or press the key to reset the
circuit.
After you build this project, set the control to position
2 on the dial and slide the switch to position A to
turn on the power. Hold a stop-watch and start it
when you press the key. The timer makes a buzzing
sound in about 30 seconds or more.
Now set the control to each division on the dial from
2 to 8, and note how long it takes the timer to
produce a sound. Setting the timer’s calibration - the
time that passes at each setting of the dial - requires
a lot of patience, but it is necessary for making sure
your timer works accurately. After you set the
calibration, make a graph showing each control
position and the time it takes for the buzzer to
sound. Now, your tester is ready for use.
Look at the schematic. The control changes the
reference voltage of the comparator (IC 1). The
timer setting is determined by the resistor R and the
capacitor C. When the voltage applied to the
positive (+) terminal of IC 1 exceeds the reference
voltage, the alarm sounds.
Since the dual operational amplifier has a high input
impedance (input resistance), its current loss is very
small, so you can use it to make a timer with a very
long setting. IC 2 works as an astable multivibrator
that produces the buzzer sound.
Notes:
EXPERIMENT #100: STUDY TIMER
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Wiring Sequence:
o
1-29
o
2-30
o
3-114
o
5-83-70-106-118-137-26-121
o
93-63-28-132
o
92-90-64-113
o
65-105-91
o
66-82-84-89
o
67-81
o
94-69-117-138
o
119-124
o
122-131
o
27-68
Schematic