Elenco 130-in-1 Electronics Playground - Manual (Page 40)

-121- -40-
This project shows how to control the LED display
through the use of transistors.
This circuit is similar to the one in Project 18
(Transistor Action). The differences between these
two are the position of the switch as well as the value
of the resistor. In this project we use the base circuit
of the NPN transistor as a switch, in order to control
the cathode of the LED. Project 18 controlled the
LED from the anode (positive side).
The transistors in this circuit act as switches. The
PNP transistor is always on, allowing the current to
flow from the collector to the emitter, because a
sufficient amount of the negative voltage is applied
to its base through one of the 10k
Ω resistors. When
you press the key the NPN transistor turns on,
thereby applying sufficient positive voltage to its
base, through the use of another 10k
Ω resistor.
When you close the key, then current can flow from
the PNP’s emitter to its collector.
Here are some important basic principles for you to
remember:
• When negative voltage is applied to its base, a
PNP transistor turns on; the current flows from
the collector to the emitter.
• When positive voltage is applied to its base, a
NPN transistor turns on; the current flows from
the emitter to the collector.
Current can now flow through the NPN transistor,
thus current can now travel a complete path - from
the negative batteries side, to the NPN transistor, to
the common cathode terminal of the display, to the
PNP transistor, to the positive side of the batteries –
thus lighting the display.
Turning on the LED with either of the transistors may
not see important to you now. But, to people who
design computer circuits that are complicated, it is
an easy way to control the circuits.
Have you noticed that transistors switch on and off
as fast as you press the key? These quick switching
allows operations to be performed quickly by
computers. Transistors are many times faster than
hand operated switches or relays. Later you will see
how to delay this fast switching by using other
components.
Notes:
EXPERIMENT #26: SWITCHING THE LED DISPLAY USING TRANSISTOR CONTROL
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
This is a pulse frequency multiplier with one
transistor. It doubles the frequency of the input signal,
so it is also called a pulse frequency doubler.
The operational amplifier IC acts as a square-wave
oscillator. The output from the oscillator is an AC
signal of about 500Hz.
When you finish the wiring, set the switch to position
A to turn on the power. Connect the earphone to
terminals 93 and 134 and press the key to listen to
the oscillating sound of 500Hz. Note the pitch of the
tone.
Now, connect the earphone to terminals 13 and 14
and press the key. Listen through the earphone; this
time you hear a sound that is an octave higher than
the previous sound. This means the frequency is
doubled to 1,000Hz.
How does this work? The operational amplifier is
configured as an oscillator. Transistor Q1 receives a
signal from the operational amplifier through the
transistor’s base; the base voltage changes with the
oscillations. This result is that opposite phase signals
appear at the transistor’s collector and emitter - when
one signal is at a wave maximum, the other is at the
wave minimum. The two outputs from transistor Q1
are applied to diodes Da and Db. The diodes pass
through only the positive portion of the waves, so
these two signals combine together to produce a
doubled frequency.
Notes:
EXPERIMENT #101: PULSE FREQUENCY MULTIPLIER
Wiring Sequence:
o 125-127-91-13-EARPHONE
o 134-110-92-80-83-76-14-EARPHONE
o 32-63-87-131
o 33-47-107
o 35-48-105
o 89-36-70-121
o 88-90-103-46
o 81-86-67-137
o 85-68-109
o 69-82-84
o 75-77
o 78-106-128
o 79-108-126
o 94-104-138
o 119-124-135
o 122-132
Schematic