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Electrical Continuity Tester

As electrical circuits incorporate more components and become more complex, problems can become difficult to diagnose through visual inspection alone. A continuity tester provides a useful tool for identifying faults in a circuit.

When Edison constructed the Pearl Street power station in lower Manhattan in 1880 and developed the world’s first electrical network connecting homes and businesses, there were no electricians or electrical engineers. Edison soon discovered the need for electrical tools for his workers to prevent damage to the equipment. A continuity tester is a basic electrical tool.

Build Instructions

A commercial continuity tester can be purchased for about ten dollars.


Figure 15. A commercial continuity tester


The circuit that you created in the preceding project (LED, resistor, and battery) can be adapted for use as a continuity tester.  Test a circuit or an element of a circuit such as a test lead (consisting of a length of wire with an alligator clip on each end, as shown in the illustration below). Place one end of a probe connected to the LED onto one end of the test lead. Then place the other end of the LED circuit onto the other end of the test lead. If the test lead is in good working condition, the LED will light up when the circuit is completed.


Figure 16. Using a battery and a LED as a continuity tester


The point at which test leads are attached to the alligator clips can sometimes corrode or become detached. If this occurs, the circuit will not be completed and the circuit will not light up.


To convert the LED circuit for use as a continuity tester, the components must be housed in case of some kind. There are many ways to do this, limited only by your ingenuity. One of the simplest methods is to tape the components to a popsicle stick.


Figure 17. A “popsicle stick” continuity tester


Another approach is to roll cardstock into a cylinder to create a tube and mount the components in the tube created in this manner. Another method sometimes used is to remove the parts from the casing of a pen or marker that has a large plastic barrel and mount the components inside the barrel of the pen casing.


Figure 18. Using the casing of a pen to house the components of a continuity tester


Another, more advanced method is to create a 3D printed design that is custom-made to house the components of the pen.


Figure 19. CAD design (top) and 3D printed case bottom for a continuity tester

The design for a 3D printed housing for a continuity tester is available on the Make to Learn site.


Lab 2

Adjusting the Brightness of an LED

Guiding Question: How can you adjust the brightness of an LED with a resistor?

Guiding Question: What is the purpose of a resistor?

A resistor in a circuit dissipates the flow of current in the form of heat. Decreasing the flow of current decreases the brightness of the LED. Including a resistor in a circuit can also protect the LED. All LEDs have a maximum amount of current that they can safely handle without overheating. A resistor can be used to limit the amount of current to a safe amount for the LED.




For a given power source, such as a 3 volt battery, there is a safe operating range for an LED. If the value of a resistor in an LED circuit is too high, the LED will not light at all. If the value of the resistor is too low, excessive current may damage the LED. Imagine a water wheel turned by the flow of water in a Roman aqueduct. If the flow of current is too great, the water wheel may be damaged. Placing a baffle in the aqueduct can limit the flow of current and protect the water wheel and protect the circuit. A resistor in an electrical circuit serves the same function.




Edison’s light bulb consisted of a filament in a glass bulb that glowed when an electrical current passed through it. The electrical current also heated the filament, causing it to become hot. Excessive current caused the bulb to burn out. Edison spent months developing mechanisms to regulate the current so that the light bulb would not burn out. A modern LED is more efficient than Edison’s light bulb. However, there are still limits to the amount of current that an LED can handle.




Activity: Exploring Resistors and LEDs

In this project you will explore the effect of different values of resistors on the operation of an LED. Note that current handling capacity of an LED can vary by color, so the results for one color of LED may not be the same as the results for a different value of LED.




A circuit with two AA batteries, a resistor, and an LED


Explore. Try lighting up the LED with a variety of resistor values. What do you notice? Use the following chart or table to record the relationship between the value of a resistor and the brightness of an LED.


Resistance versus Brightness

Resistor Value

Brightness of LED



What is the highest value of resistor for which the LED is still visible?  


What is the effect of the lowest value of resistor on the LED?


What do you think might happen if you use a 9-volt battery instead of the 3-volt AA batteries?



Representing Your Circuit

Representing your circuit

The symbol for a resistor in an electrical circuit is a zigzag line. This indicates the flow of electrical current is impeded by an obstruction. The symbol for a battery and LED are also shown in the table below. The symbol for a battery represents one cell of a battery with the positive terminal shown on one side and the negative terminal shown on the other side. The symbol for an LED consists of an arrow in a circle. The arrow indicates that the electrical current can only flow in that direction when the LED is laced in a circuit.











Schematic symbols for resistor, battery, and LED


The symbols can be depicted in a schematic drawing to illustrate how the components of a circuit are connected. The schematic diagram on the left in the illustration below depicts the same relationships as the pictorial representation on the right. Electrical symbols represent a type of language that engineers use to communicate with one another. Therefore the ability to read and interpret schematic drawing is an important skill.

Create your own schematic. Use the electrical symbols shown above to create a schematic drawing of a circuit that you constructed for your project.


Lab 1

Basic Lighting Circuit

In this project, you will learn how to construct a basic circuit to light an LED with a battery.

Basic Circuit Components

An electrical circuit consists of three components:

1. A power source such as a battery.

Batteries are just one example of a power source. In future activities you will learn about different sources of electrical power. For now, think of batteries as storage containers for electrical power. The photo above shows an example of a battery found in electric circuits.

What do you notice that is similar in all the batteries?

2. An electrical component powered by the source such as a light.
An electrical component is anything to is powered by electricity, like the device you will use to do this activity. Electrical components are the things that we are interested in using to accomplish some other task. For example, electric motors are found in electric cars, which we use to get around from place to place.
Can you think of other electrical components?

3. Wires that connect the power source to the load. Electrical wires conduct electricity (electrical current) from the battery to the electrical component. Think of wires as the roads in which electrons travel on. The above photo shows a common electrical wire made out of copper. Wires can be made out of anything that conducts electricity.

Open and Closed Circuits

When the wires are all connected, the circuit is complete, and power is delivered to the electrical component. When one of the wires is disconnected, the circuit is incomplete, and the component does not receive any power.

An electric switch is a device that can stop or allow electricity to flow through an electrical circuit:

(1) open (incomplete circuit)

(2) closed (complete circuit)

When the switch is closed, it completes the circuit, allowing electrical current to flow. An alligator clip (wire) can be used to complete the circuit, serving as a rudimentary switch. Below is a diagram of a closed circuit used to light up an LED.

C:\Users\Glen\Documents\Dropbox (UVa Lab School)\Make to Learn\Invention Kits\1. Electricity\Images (Electricity)\Continuity Tester (without resistor).png


Figure 1. A circuit with LED, battery, and wires.


Resistance, Current, and Voltage

The flow of electricity in a wire is like the flow of water in a hose. There are three major factors that affect the flow of water in a hose. Each of these elements has an electrical counterpart.

  1. Water pressure is the amount of water in a bucket. The amount of water or water pressure affects the rate at which the water flows through the hose.

  2. Resistance to the flow of water is increased or decreased by adjusting the valve in the water faucet. The rate at which the water flows through the hose is adjusted by opening the valve (decreasing resistance) or closing the valve (increasing resistance).

  3. The amount of water that flows through the hose is increased by increasing the water pressure and decreased by increasing resistance.

The amount of water is adjusted to meet specific needs. The water valve can be opened completely in order to quickly fill a bucket. It can be partially opened to produce a more moderate flow to water plants.


C:\Users\Glen\Documents\Dropbox (UVa Lab School)\Make to Learn\Invention Kits\1. Electricity\Images\Water Valves.png

Figure 2. Decreasing resistance by opening a valve increases water pressure

When the valve is closed, the water pressure on the right-hand side of the valve is zero. When the valve is opened, the water pressure on the right-hand side of the valve increases. As the valve is opened, (1) the resistance to the flow of water decreases, and (2) the flow of water current increases. (Note: The height of the water affects the water pressure.)


Table 1. Water and Electrical Analogy




Water Pressure

Electrical Force



Electrical Resistance



Electrical Current



The electrical counterparts of water pressure, resistance, and amount of water flowing through a hose are summarized in the table above. Electrical force, measured in volts, is the equivalent of water pressure. Electrical resistance, measured in ohms, is the electrical counterpart of resistance to the flow of water in a hose. Electrical current, measured in amps, is the electrical counterpart to the amount of water flowing through a hose.


For a more indepth explanation of this analogy, check out: Crash Course: Electric Charge


Electric Light

Incandescent Lights and Light Emitting Diodes

Edison’s incandescent bulb created light by transmitting an electrical current through a filament. As the filament heated up, it glowed, producing light. Today, incandescent lights are being replaced by light emitting diodes (LEDs). LEDs do not heat up in the same way that incandescent lights do, and they are more energy efficient. However, they serve the same purpose in an electrical circuit – to create light.




An LED has a positive lead and a negative lead. The positive lead is connected to the positive terminal of the battery. The negative lead is connected to the negative terminal of the battery. The positive lead is longer than the negative lead. The rim of the LED is flat on the side with the short lead. This arrangement provides a way to determine the orientation of the LED when the leads are not visible because they have been inserted into a breadboard or socket.  


Figure 3. An LED has a positive and a negative lead


Activity: Light up an LED

For this activity, you will need:






Using these three components, see if you can make the LED light up with a battery.

Need help getting it to work?

Place two AA batteries in a battery case similar to the one shown in the illustration below.   

Use an alligator clip and connecting wire to connect positive lead of the LED to the positive terminal of the battery. Then connect the negative lead of the LED to the negative lead of the battery. When the circuit is complete, the LED should light up.


Figure 6. A circuit with AA batteries and an LED


Caution. Excessive current can damage an LED. When this occurs the LED can become hot or even explode. Take appropriate safety precautions before proceeding. These can include: (1) wearing safety glasses, (2) placing the LED under a Pyrex container, and (3) allowing the LED to cool off before handling it if it becomes overheated. This is more likely to occur with higher voltages (for example, if using a nine volt battery rather than two AA batteries).

Diagnosing Problems

Diagnosing and fixing problems is an important part of engineering. If the LED does not light up, you will need to develop a strategy to identify the problem and fix it.

What do you think some of the reasons that an LED might not light up?


Develop a list of some of the causes of the light failing to work. List the potential problems in the order that you believe they are most likely to occur. Then develop a method for determining how to fix the problem.


Potential Problems with a Defective Circuit


  1. ________________________________________________


  1. ________________________________________________


  1. ________________________________________________



Were you able to light up the LED successfully on your first try? Do you think that inventors like Edison were successful on their first try? Why or why not?



What did you have to do to make the LED light up?



What did you learn about circuits in this activity?




What personal characteristics do successful engineers have to have?





Electrical Innovations

The dawn of the electrical age began in 1800, the year that an Italian scientist named Volta invented the electrical battery. The electrical current provided by batteries led to a series of discoveries. These discoveries culminated in the invention of three great electromagnetic networks: the telegraph network, the telephone network, and the power grid.

The electrical battery ignited a series of discoveries during the first four decades of the century (1800 –- 1840). During the next two decades (1840 –- 1860) the telegraph network was established. Samuel Morse, an American inventor, developed a way to send coded messages through the telegraph network. The code uses two signals: a long and short click. Using this system, message can be sent through the network and decoded at the receiving end. At the end of this time, more than 15,000 miles of telegraph wire crossed the United States. The telegraph system changed our culture and society in multiple ways. It made national news and weather forecasting possible through the news wire service. It influenced the course of the Civil War by allowing generals to communicate faster across long distances instead of relying on couriers traveling by horseback (feel free to change this!).

Even more importantly, the telegraph system led to the invention of two other nineteenth century networks:  the telephone network and the power grid. Invention of these networks then led to the invention of modern electronics, making the  invention of computers possible. Inventors like Edison who worked in the telegraph system learned about electrical current, circuits, and switches. Edison’s laboratory in Menlo Park was the culmination of a journey that began with experiments designed to improve the telegraph system.

By following in the footsteps of these inventors and reconstructing their electrical devices you will gain skills and experience that will allow you to design your own inventions: electronic games, innovative toys, electronic wearables and other electronic inventions.



The Electricity Invention Kit

The Electricity Invention Kit explores the foundations of electricity and circuits through a historical and engineering lens. Edison’s inventions and those of many other inventors were made possible by discoveries about electrical relationships. Many researchers contributed to these discoveries. Georg Ohm conducted experiments that established basic electrical relationships among electrical resistance, current, and voltage. These findings are today known as Ohm’s Law. Student projects will emphasize the underlying scientific concepts of Ohm’s Law, a foundational relationship in circuits. What are the essential properties of circuits? What are the relationships between these properties? Students will explore these questions and then apply this knowledge to complete a design challenge.


Teacher Guide

The teacher guide contains lesson plans for each kit and activity. In addition, materials, sourcing, tools, and CAD files for fabricating the kits are also included.

Go to Teacher Guide

Unit Sequence

Activities Essential Questions
Lab 1: Basic Lighting Circuits How is a basic circuit constructed using a battery and wires to light an LED?
Lab 2: Adjusting the Brightness of an LED How can you adjust the brightness of an LED with a resistor?
Make: Electrical Continuity Tester
Lab 3: Electrical Meters How do you measure resistance, current, and voltage?
Lab 4: Electrical Light Dimmers What is the relationship between voltage, resistance, and current? (Ohm’s Law)
Invent: Electric Art

Key Concepts & Skills


  • Conductivity
  • Electricity
  • Open/Closed Circuits
  • Resistance
  • Current
  • Voltage


  • Computer-Aided Design (CAD)
  • Computer-Aided Manufacturing (CAM)
  • Soldering
  • Scientific Observations
  • Journaling