Elenco Electronics EP-130, Руководство пользователя

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Now we’re getting down into the field of electronics.
The explanations from now on will become a bit
more difficult, and more interesting! This project
shows how to control the LED display with
transistors.
This circuit is very much like the one in Project 20
(Transistor Circuit Action). The only differences are
the position of the switch and the value of the
resistor. This project uses the base circuit of the
NPN transistor as a switch, to control the cathode of
the LED. In project 20 we controlled the LED from
the anode (positive side).
Both transistors in this project act as switches. The
PNP transistor is always on, allowing current to flow
from collector to emitter, because a sufficient
amount of negative voltage is applied to its base
through one of the 10k
Ω
resistors. The NPN
transistor turns on when you close the key, thereby
applying sufficient positive voltage to its base,
through another 10k
Ω
resistor. So, the current can
flow from emitter to collector only when you close
the key.
The following basic principles are important for you
to remember:
• A PNP transistor turns on when negative voltage
is applied to its base; the current flows from
collector to emitter.
• An NPN transistor turns on when positive
voltage is applied to its base, the current flows
from emitter to collector.
Now that the current can flow through the NPN
transistor, it can travel a complete path – from the
negative side of the batteries, to the NPN transistor,
to the common cathode terminal of the display, to b
and c anode terminals of the display, to the PNP
transistor, to the positive side of the batteries – thus
the display lights.
Turning on the LED with either transistor might not
seem important now. But to people who design
complicated computer circuits, it is a handy way to
control circuits.
Have you noticed that the transistors switch on and
off as fast as you press the key? This speedy
switching allows computers to perform operations
very quickly. Transistors are many times faster than
relays or hand operated switches. Later we will
show you how to delay this fast switching by using
other components.
Notes:
EXPERIMENT #25: TRANSISTOR CONTROL SWITCHING OF THE LED DISPLAY
Wiring Sequence:
o
21-23-41
o
25-47
o
40-82
o
119-42-137
o
46-84
o
124-48-81
o
83-138
o
121-122
Schematic