1.1.2.AK Investigating Basic Circuits (DLB)
Introduction
What is an electrical circuit? How can you manipulate components and design a circuit that
will do what you want it to? This guided activity will introduce you to some components, tools,
and concepts that are fundamental in electronics. It will allow you to investigate and discover:
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What are voltage, current, and resistance? How are they related to one another?
What are some of the basic components that make up simple circuits and what do
they do?
What are the important characteristics of a circuit and how do I measure different parts
of a circuit?
How do I measure voltage in a circuit?
How does the arrangement of components affect the characteristics of the circuit?
How do I work safely with circuits?
How can I use calculations to design circuits before I start creating one?
Equipment
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Digital Logic Board (DLB) or breadboard
(2) Light emitting diodes (LEDs)
(2) - 330 Ohm resistors (Orange/Orange/Brown/Gold or Silver)
Digital multimeter (DMM)
#22 Gauge solid wire
Procedure
Follow the instructions to create different types of circuits. As you create each circuit, you will
be asked to make measurements and record observations to gain understanding about the
circuit and its components. Let us begin by investigating two components that are commonly
used in circuits. They are resistors and Light Emitting Diodes (LEDs).
Part A: Creating a Circuit and Measuring a Circuit’s Properties
1. A Simple Circuit - Arrange the components according to the picture shown you will
need to supply 5V from the DC Power Supply located on the DLB or from an
appropriate power supply. Notice that the LED has a flat notch on one side. Make sure
the notch is initially oriented on the bottom as shown in the picture. In this
arrangement, the flow of conventional current is from the top (5V) to the bottom (0V)
through the resistor and the LED. What do you think the role is of the resistor in this
circuit?
The role is to limit the amount of current moving through the LED or reduce the voltage
across the LED to protect it from failure. (Note: This definition will be expanded later as
the relationship between voltage and current R=V/I.)
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 1
(1a) Components on a Breadboard
(1b) Circuit Diagram
In a circuit, the flow of conventional current can be described as a positive charge
moving through a complete circuit path (VCC to GND). Can you trace the flow of
conventional current in both of the above pictures?
Help students understand how a BB is layed out.
2. With the LED illuminated, flip the direction of the LED on the breadboard (notch on top
now), and then flip it back to its original position (notch on bottom). What does your
observation tell you about diodes (and LEDs)?
Good Answer: LEDs only light up with current flowing in one direction.
Best Answer: LEDs and diodes are polar components. Current is limited in one
direction.
3. Using the DMM to Measure Values - Making sure that the RED lead is plugged into
(V) and the black lead is plugged into (COM) on the Digital Multi-Meter (DMM), turn
the DMM on to direct current voltage range 0-600V or first click (see 3a). Place the
RED DMM lead on the top of the resistor, and the black lead on the bottom of the LED.
Note the reading on the DMM. Now switch the DMM leads.
(3a) Digital Multimeter DMM
(3b) Reading voltage across the resistor and LED
What did you notice about the value on the DMM when you switched the leads?
Value should switch from approximately +005V to -005V in this example.
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 2
4. Now place the DMM leads across the circuit as you did initially in picture (3b) and turn
the DMM to the next smallest voltage range (0-200V). You may need a classmate to
help you. Note the value on the DMM and then turn one more click to the next
smallest ranges (0-20V). What is happening on the display with each click, as the
range you are measuring gets smaller and smaller?
Value should switch from approximately:
005V to 04.7V
(1 significant figure to 2 significant figures)
04.7V to 4.70V
(2 significant figures to 3 significant figures)
These values will vary dependent upon the voltage supply used.
The DMM reading becomes more precise by a factor of ten each time the voltage
range is decreased.
5. Turn one last click of the DMM to read the range (0-2V). What happened and why?
What was the most accurate measurement were you able to make of the voltage
across the resistor and LED?
The value displayed on the DMM is 1.
This means the value you are trying to measure is not in the range of 0 to 2.
6. Set the DMM to the range that will give you the most accurate voltage measurement
and touch the lead across the two ends of the red wire. Note the reading on the DMM.
Now touch the leads across the two ends of the black wire. Note the reading.
(6a)
(6b)
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 3
Voltage (ΔV=Vf-Vi) is a description of a components potential to do work. (1 volt
means the component could do 1 joule of work for every coulomb of charge that
passes though it (1V= 1J/1C). In order for a component to do work, there must be a
difference in the potential across the component to do work (often called a potential
difference). Why do you think the reading was the same for both of these sections of
the circuit? What you have just observed is exactly why birds can land on high power
lines unharmed.
Answers will vary based on student’s prior knowledge. Any answer is acceptable as
this idea will be expanded upon in discussion and further APBs.
For each of these arrangements, the potential difference across the test points is zero.
(6a) ΔV = 5V-5V=0
(6b) ΔV = 0V-0V=0
Voltage (potential difference) must be measured across a component that is doing
work. One side must have a higher potential to do work than the other. This is why
birds can land on high voltage lines without doing work (being electrocuted).
(6a) ΔV = 20,000V-20,000V still equals zero.
7. In Step 3 you measured the voltage across the resistor and the LED combined. Now
touch the leads across both ends of the resistor. Note the reading on the DMM.
(a) Voltage across LED and Resistor
(b) Voltage across Resistor Only
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 4
Should be approximately 4.70V (5V)
Should be approximately 3.00V (3V)
Can you guess the reading you will see when you touch the DMM leads across the
LED only? Were you correct? Why did you guess that value?
Should be approximately 1.70V (roughly 2V)
Students should make the connection that:
the resistor voltage (3V) plus LED voltage (2V) should add up to terminal voltage (5V).
Once again, the actual values may vary slightly based on the components used and
voltage source.
8. You might be asking why we need a resistor in this circuit. Is it doing any work? Some
components have limitations on how much electrical current can pass through them or
how much voltage they should have across them. We can calculate the relationships
between Voltage, Current, and Resistance for a component using Ohm’ Law (V=IR).
Let’s assume the voltage you saw across the 330Ω resistor was roughly 3V when the
circuit was active. What is the conventional current (measured in Amps) traveling
through the resistor according to Ohm’s Law?
Answers will vary based on student’s prior knowledge. Any answer is acceptable as
this idea will be expanded upon in discussion and further APBs.
V=IR
I = V/R = 3V / 330 Ω = 0.00909 A = 9.1 mA
Most students should be able to find the value 0.0090909 A on their calculator, but
may have forgotten their SI prefixes. A great question to ask is “wouldn’t it be nice to
have a short cut for expressing such a small number? Is 9 mA really small?”
In this arrangement, the electrical current has only one path through the components
(top to bottom in the picture). These components can be described as being in series
with each other. The amount of current flowing through an LED must each be equal in
the same path.
Warning: Do not attempt to verify the current in the circuit at this time using the DMM.
While the DMM can handle this amount of current, you must learn how to use the
ammeter portion of the DMM properly to protect it from blowing a fuse. Shorting
voltage sources is dangerous and can damage the DMM.
Part B: Series and Parallel Circuits
9. These two identical 330Ω resistors are in series with each other. There is only one
path through the circuit from the power source to the ground. In this diagram, the
power source is a 9V battery. One end of the battery has 9V of potential to do work
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 5
(positive terminal-top). The other end of the battery has zero potential to do work
(negative terminal/ground-bottom).
(a) Based on your observations, what would you expect the voltage read if you
touched the DMM across both R1 and R2? 9V
(b) Due to the fact that R1 and R2 are identical, what would you guess is the voltage
across each resistor is individually? Why?
The voltage across both individually would be 4.5V.
Added together they would add up to the terminal voltage (total voltage of the circuit)
9V
(c) We can actually replace these two resistors with one resistor that would have the
same impact on the circuit. (This theoretical resistor is called an equivalent resistor).
What would the value of this equivalent resistor have to be in Ohms (Ω)?
One 660 Ω resistor would be the same as two 330 Ω resistors in series.
10. Create this series circuit on your breadboard. With both LEDs illuminated, remove one
of the LEDs from the circuit path. What happened and why?
The other LED goes out. The circuit path is broken.
11. You can try 3 LEDs in series but none will light up. Why do you think that is?
The combined resistance is too large, or the voltage is too small, or the current is too
small.
12. Components in a circuit can also be arranged in parallel. Create this parallel circuit on
your breadboard. With both LEDs illuminated, what happens if you remove one of the
LEDS from the circuit path? The other LED stays light. They are both illuminated with
the same brightness as if it were one LED in the circuit alone. All LEDs have their own
path to the battery (same voltage source).
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 6
13. Using the diagram shown to you in Step 9 for series circuit as a reference, can you
draw a circuit diagram showing 2 resistors in parallel? (Draw circuit below.)
14. In a series circuit, all component have the same current (Amps) flowing through them
(even if the resistors have different values. That is not the case for components in
parallel with each other. What do components in parallel share in common?
Voltage. All resistors have their own path from and to the battery.
(voltage source to ground)
15. Based on your observations and what you have learned about parallel circuits, use
Ohm’s Law (V=IR) to calculate the current in each of the 3 resistors?
For R1 V=IR
I = V/R = 9V / 330 Ω = 0.027 A = 2.7 mA
For R2 V=IR
I = V/R = 9V / 660 Ω = 0.014 A = 1.4 mA
For R3 V=IR
I = V/R = 9V / 165 Ω = 0.054 A = 5.4 mA
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 7
Imagine you add more resistor (R4 and and R5) in parallel. For each new path to the
battery you create, what do you think that does to the total amount of current going into
and out of the battery?
As more additional paths are created in the circuit, more curent flows into and out of the
battery.
Conclusion
1. Describe the proper way to place the DMM leads and the steps you use to attain the
most precise measurement value for voltage across components using a Digital
Multimeter (DMM).
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In order to measure voltage the circuit must be on (active)
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Voltage measurements must be made across a component.
(DMM must be in parallel with the component)
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Make sure the DMM leads are plugged in correctly for voltage (Red=V
Black=COM)
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To get the most precise measurement, start with the largest range and keep
reducing it until you no longer get a reading (out of range). Go back to the last one
that gave you a value. That is the most precise voltage measurement.
2. If the value on the DMM is negative, what does that tell you about the orientation of the
DMM leads in relation to the flow of conventional current?
If the voltage is reading negative on the DMM, the flow of conventional current is in the
opposite direction than the orientation you have the leads placed. The conventional
current is flowing from the black lead to the red lead in your circuit between these two
test points. (Opposite of how you guessed)
3. LEDs and resistors transfer electrical energy into light and thermal energy. What is an
important characteristic about LEDs (and diodes) that make it unique compared to a
resistor? LEDs and diodes are polar components. Current is limited in one direction.
4. In your own words, describe what it means for components to be in series with each
other. What characteristic do components in a series always share in common?
(Voltage, Current, or Resistance)
In series circuits:
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Components are wired from end to end completing one circuit path.
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All components share the same current. Remove a component and current
stops for the entire circuit.
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For resistors in series, the total resistance in the circuit increases with each
resistor you add to the circuit.
5. In your own words, describe what it means for components to be in parallel with each
other. What characteristic do components in parallel always share in common?
(Voltage, Current, or Resistance)
In parallel circuits:
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 8
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Components are wired so that each has its own path to the voltage source and
ground.
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All components share the same voltage. Remove a component and current
stops only for that section of the circuit.
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For resistors in parallel, as more paths are created to the voltage source, more
current flows from the voltage source.
(The total resistance of the circuit decreases as you add resistors in parallel.)
Going Further
6. In this investigation you were introduced to the idea of equivalent resistance (replacing
multiple resistors in series with one that does the same job. Equivalent resistance for
series circuits can be shown as a simple mathematical expression. How would you
express this relationship to the total resistance in the circuit mathematically?
Req = R1 + R2 + R3
Students most likely will be able to identify this
relationship.
7. A theoretical equivalent resistor can be placed in parallel circuits and shown
mathematically as well. This relationship is a little less straight forward than equivalent
resistance in series circuits. With a little research, can you determine how the
equivalent resistance for this parallel circuit would be expressed mathematically? We
will expand our understanding on these concepts in later activities.
1/Req =1/R1 + 1/R2 + 1/R3
Students most likely will need to look this up.
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Digital Electronics ANSWER KEY 1.1.2 Investigating Basic Circuits (DLB) – Page 9