Elenco Electronics EP-130, Manual

Page: 69 / 160

-69-
Here’s another type of one-shot circuit, but in this
project you hear the effects of the multivibrator. You
can see from the schematic that this project uses a
combination of simple components and digital
electronics. When you press the key, the 100
μ
F
capacitor is charged and lets the NPN translator in
the left corner of the schematic operate. You can see
that the collector of this transistor serves as both
inputs for the first NAND gate.
The digital portion of this circuit controls the
operation of the PNP transistor on the right side of
the schematic. Set the switch to A to turn the power
on. When the output of the first NAND is 1, the
multivibrator works, and you hear a sound from the
speaker.
The sound will continue until the 100
μ
F capacitor
discharges, keeping the first transistor from
operating. The output of the first NAND becomes 0,
stopping the multivibrator. The sound will last about
10 seconds with the 22k
Ω
resistor. Try replacing the
22k
Ω
with the 47k
Ω
or the 100k
Ω
resistor and find
out what happens.
After you’ve experimented with this project a bit, try
this: press the key and release it. When the sound
stops, find the wire running between terminals 52
and 54. Disconnect the wire from terminal 52. Does
anything happen? If something does happen, can
you explain why?
Notes:
EXPERIMENT #52: TRANSISTOR TIMER WITH TTL
Wiring Sequence:
o
1-29
o
2-30
o
3-41
o
5-59-60-62-48-116-121
o
40-82
o
79-49-42-131-138
o
46-86
o
47-50-51-80
o
52-54
o
53-77-111
o
55-57-56-75-78
o
58-76-81-112
o
85-115-137
o
119-132
Schematic