In this chapter, you will learn about
electronic systems and about components in electronic circuits.
You will also learn about the following control devices:
switches, diodes and transistors. Finally, you will make a
simple transistor circuit. An electric circuit consists of an
energy source and conductors. Conductors connect components
such as input, output and process devices to create a path for
the electrons to flow to and from the source of energy.
Insulators are used to keep the components from
short-circuiting.
Switches
A switch controls the electric current
by closing or opening the circuit. There are various types of
switches that control the circuit in different ways. In this
lesson, you will learn about manual switches that a user can
turn on or off.
1. Think about
different switches that you use daily and list them here:
Push button
switch
Push button
switches are often used for doorbell circuits, as in Figure 2.
This simple doorbell circuit consists of cells in series, a
push button and a buzzer, all connected by conducting wire. A
person visiting the house presses the button for a short time
and then releases it.
2.
Draw the circuit diagram of the doorbell circuit in the
photograph. Use the correct circuit diagram symbols. Note that
the cells are in series.
Single-pole,
single-throw switch (SPST)
Figure 3: The symbol for an SPST
switch
Figure 4: A typical light circuit with
anenergy source, switch and lamp
Single-pole, double-throw switches
(SPDT)ice 1 in one position and device 2 in the other
position. There is no "off" position for this switch.
Figure 5: The symbol for an SPDT
switch
Figure 6: A circuit with a battery, two
lamps and an SPDT switch controlling two outputs
Figure 7: A circuit with two SPDT
switches is often used to control a lamp with one switch at
each end of a long passage. It is also used to control a lamp
with one switch at the bottom of a staircase, and the other
switch at the top of the staircase.
Figure 8
opposite direction. How can the
direction in which the motor turns be changed? The way to do
this is to change the direction of the current through the
electric motor. Double-pole, double-throw switches can be used
to reverse the direction of current through a circuit, so they
are useful in applications such as automatic car gates. The
circuit diagram below shows how a DPDT switch can change the
direction of current through an electric motor.
Figure 9: A circuit where a DPDT switch
controls the direction in which an electric motor turns
opposite direction when the
current passes through it from terminal M2 to M1., to 4, to 2 and back to the
negative of the battery.
from the
positive terminal of the battery,
through
the ON/OFF switch to 1,
through
the top part of the DPDT switch from 1 to 5,
through
the motor from M2 to M1,
to 6,
through
the bottom part of the DPDT switch from 6 to 2, and
to the
negative terminal of the battery.
Figure 10: A diode
Figure 11: Circuit symbol of a diode.
The current flow in a diode is in the direction of
the arrow head.
Figure 12: An LED. The longer of the
two wires coming out of the LED should be connected to the
positive terminal, and the shorter wire to the negative
terminal.
Figure 13: The circuit symbol for an
LED.
collector, base and emitter.
Figure 15: One type of transistor
Figure 16: The circuit symbol for an npn
transistor
npn transistor acts as if there
is a switch between the collector and the emitter. With a
positive potential on the base of the transistor, the switch is
closed. So it is an electrically controlled switch.
There are other types of
transistors, for example pnp transistors that work a bit
differently from npn transistors. But you will only work with
npn transistors in this term.
Figure 17: A simple touch-switch
circuit that will not work well
npn transistor is a
"control device," and the LED is the "output device".
A transistor uses a small
current circuit to switch on a larger current circuit. This
is why transistors are also used in music equipment to
"amplify" the sound.
Figure 18: Circuit using a transistor as
an electronic switch
Figure 19: The construction of a
touch-switch circuit with a transistor and an LED.