Copyright Goodheart-Willcox Co., Inc.
244 Engineering Fundamentals
Electronic Circuit Simulation
Engineers and technicians can now simu-
late the performance of circuitry using computer
software without ever having to build an actual
circuit. Engineers lay out the components they
would use on the screen, and the software shows
them how it would work. Simulation software
saves time and money by speeding up the design
and development process and identifying prob-
lems early on.
Components in Use
Look at the circuit diagram of a continuity
tester in Figure 11-29. When there is continuity
between the two probes, the circuit is closed and
a sound is heard from the speaker.
The input components in this circuit are
the battery, the probes, and the conductors. The
battery causes electron fl ow through a chemical
process. The conductors and probes create a path
for current to fl ow.
The switch, capacitor, transistor, 555 timer IC,
and resistors are all control components. When
there is a closed circuit between the probes and
the switch is closed, the 555 timer IC generates an
audio signal, which is amplifi ed by the transis-
tor and played through the speaker. The resistors
limit current fl ow in the circuit.
Troubleshooting
Figure 11-30 shows a schematic drawing of
a penlight circuit. The two cells, wired in series,
provide about three volts. The diode drops
about 0.7 V, delivering about 2.3 V to the LED in
forward bias.
To troubleshoot this circuit, an electrical
engineer might start by testing the voltage at
the batteries (cells) to see that it is close to 3 V.
Then the engineer might go around the circuit
with a continuity tester or ohmmeter to make
sure there is continuity where there should
be. Ohmmeters are used to measure resistance.
Goodheart-Willcox Publisher
Figure 11-29. 
This diagram shows a continuity tester, which helps test for properly working circuits.
SPST Switch
555
IC
Test Leads
Speaker
LED
Goodheart-Willcox Publisher
Figure 11-30. 
This schematic shows a penlight circuit. Electrical
engineers begin troubleshooting by testing battery
voltage, then test the continuity throughout the circuit.
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Copyright Goodheart-Willcox Co., Inc.
244 Engineering Fundamentals
Electronic Circuit Simulation
Engineers and technicians can now simu-
late the performance of circuitry using computer
software without ever having to build an actual
circuit. Engineers lay out the components they
would use on the screen, and the software shows
them how it would work. Simulation software
saves time and money by speeding up the design
and development process and identifying prob-
lems early on.
Components in Use
Look at the circuit diagram of a continuity
tester in Figure 11-29. When there is continuity
between the two probes, the circuit is closed and
a sound is heard from the speaker.
The input components in this circuit are
the battery, the probes, and the conductors. The
battery causes electron fl ow through a chemical
process. The conductors and probes create a path
for current to fl ow.
The switch, capacitor, transistor, 555 timer IC,
and resistors are all control components. When
there is a closed circuit between the probes and
the switch is closed, the 555 timer IC generates an
audio signal, which is amplifi ed by the transis-
tor and played through the speaker. The resistors
limit current fl ow in the circuit.
Troubleshooting
Figure 11-30 shows a schematic drawing of
a penlight circuit. The two cells, wired in series,
provide about three volts. The diode drops
about 0.7 V, delivering about 2.3 V to the LED in
forward bias.
To troubleshoot this circuit, an electrical
engineer might start by testing the voltage at
the batteries (cells) to see that it is close to 3 V.
Then the engineer might go around the circuit
with a continuity tester or ohmmeter to make
sure there is continuity where there should
be. Ohmmeters are used to measure resistance.
Goodheart-Willcox Publisher
Figure 11-29. 
This diagram shows a continuity tester, which helps test for properly working circuits.
SPST Switch
555
IC
Test Leads
Speaker
LED
Goodheart-Willcox Publisher
Figure 11-30. 
This schematic shows a penlight circuit. Electrical
engineers begin troubleshooting by testing battery
voltage, then test the continuity throughout the circuit.

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