Copyright Goodheart-Willcox Co., Inc.
240 Engineering Fundamentals
One of the most common ICs is the 555
timer. It includes 23 transistors, two diodes, and
16 resistors. The 555 timer has literally thou-
sands of applications. The 555 can be connected
in a circuit in one of two ways: monostable or
astable. When used in monostable mode, it
creates an output for a given period of time.
Think of a seat belt indicator on a car’s dash-
board that turns on to tell the driver to use
the seat belt, but turns off after a given period
of time. 555 timers can also be used in astable
mode, where a repetitive pulse is created. An
example of this could be the repetitive buzzing
of an alarm clock. Figure  11-23 shows the 555
timer and its pin diagram.
Semiconductors
Semiconductors are materials with conduc-
tive capabilities between that of conductors and
insulators. Silicon is the most common semicon-
ductor. Semiconductors are used in items such as
transistors, diodes, solar panels, and integrated
circuits (ICs).
Sensors
Electronic sensors create an electrical signal
based on environmental conditions. The signal
changes as the environmental conditions change.
For example, electronic thermostats are used in
many homes to tell the heating system when the
house has cooled down and it is time to make
more heat. Inside the thermostat is an electronic
sensor. This sensor can tell what the temperature
is in the home and sends that information through
an electrical signal to the heating unit.
Control Systems
There are two types of control or feedback
systems. They are open systems and closed
systems. Closed systems include some sort of
built-in feedback. Think of the heating system
mentioned above. The heating system blows
heat into the house and increases the tempera-
ture. The sensor provides feedback to the heat-
ing unit and tells the unit when the house has
reached the desired temperature and it is time to
turn off. A dimmer switch on a lighting circuit is
an example of an open system. It does not have
a mechanism for feedback built-in and relies
on a person to manually adjust the light to the
desired level. The lights will stay at that level
until a person adjusts it.
Output Components
Output components are the parts that use
electricity to perform a desired task. For example,
a lamp uses electricity to create light and a motor
uses electricity to create rotary motion.
1. Ground
2. Trigger
3. Output
4. Reset
8. Vcc
7. Discharge
6. Threshold
5. Control voltage
555
Figure 11-23. 
This diagram shows
a 555 timer and pin
configuration.
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