Home Project Ideas Project Guide Ask An Expert Blog Careers Teachers Parents Students

Is this connected to that? Use a homemade electronic tester to find out if electricity can flow between two objects.

Time Required Average (6-10 days)
Prerequisites None
Material Availability Readily available
Cost Average ($50 - $100)
Safety Adult supervision is required. All devices that are tested should be unplugged. Don't take any electrical appliances apart to test components inside. Before testing any device, you must make certain that you have removed all electrical power to the device. Do not go near the sockets in the wall with the circuit tester.


What do lamps, computers, televisions, printers, and kitchen appliances all have in common? They all need electricity to work. In order for electricity to provide power to these devices, it has to flow into and out of them. In this electronics science fair project, you will make your own simple circuit tester and use it to study how electricity flows through a lamp.


The objective of this science fair project is to build a simple circuit tester and use it to investigate how electricity flows through a household lamp.


David Whyte, PhD, Science Buddies

Share your story with Science Buddies!

I did this project I Did This Project! Please log in and let us know how things went.

Last edit date: 2013-08-26


Electricity is like water in a river, it flows. For example, when you turn on a lamp, electricity flows in through the power cord, then it flows through the lightbulb, and finally, it flows back out through the power cord. Electricity flows through conductors. Most metals are good conductors. Copper is an excellent conductor, so it is used in power cords. To keep the electricity from flowing where it is not supposed to go, conductors that carry electricity are surrounded by insulators. Plastic and rubber are good insulators, which is why they are used to coat power cords. If electricity is like water flowing down a river, the conductors are like the sides of the river—they keep it within certain areas.

Electricity has to flow into and out of an object to provide power, and the path of the electricity is called a circuit. A circuit is a circular journey. In an electrical circuit, the electricity makes a circular journey through the device it is powering. For example, when a lamp is turned on, a circuit is formed from the socket in the wall, through the lamp, and then back to the socket. If the path of the electricity is broken, then the flow of electricity is stopped and the power to the device is turned off. The role of switches is to break the flow of electricity. A common type of switch has two pieces of metal that touch to make a circuit, and separate to break the circuit. More complex switches are used in electronic devices, but the basic idea is the same, they interrupt the flow of electricity.

You can test whether two objects are connected in a circuit using a device called a circuit tester (also called a continuity tester). In this electronics science fair project, you will make your own circuit tester. To determine if there is a path for electricity through a lamp, you will unplug it and attach probes to the prongs of the plug. When it is plugged in, electricity flows into the lamp from one prong and out through the other prong. By attaching your circuit tester to the two prongs, you can determine if there is a closed circuit for the flow of electricity. You will also determine how the lamp switch and the type of lightbulb affect the flow of electrical current.

Terms and Concepts

  • Conductor
  • Insulator
  • Circuit
  • Switch
  • Circuit tester
  • Continuity tester
  • Closed circuit
  • Current
  • Circuit diagram
  • Alternating current


  • What word is used by scientists to describe the flow of electricity? Hint: Think of the word for "moving water."
  • How many types of switches can you find in your house?
  • What is the definition of an open circuit?
  • What is voltage?
  • How are current and voltage related?
  • What are some examples of good conductors and good insulators?


Materials and Equipment

  • AA battery holder
  • Snap connectors, 9-V
  • Wire strippers
  • Insulated test/jumper leads
  • Electrical tape
  • Buzzer
  • AA batteries, brand-new (2)
  • Plastic container, 16-ounce (oz.) with a snap-on top
  • Scissors
  • Hole punch or screwdriver
  • Lab notebook
  • Incandescent lightbulb, 60-watt (W), preferably with clear glass (1)
  • Energy-saving fluorescent lightbulb (1)

Share your story with Science Buddies!

I did this project I Did This Project! Please log in and let us know how things went.

Experimental Procedure

Note Before Beginning: This science fair project requires you to hook up one or more devices in an electrical circuit. Basic help can be found in the Electronics Primer. However, if you do not have experience in putting together electrical circuits you may find it helpful to have someone who can answer questions and help you troubleshoot if your project is not working. A science teacher or parent may be a good resource. If you need to find another mentor, try asking a local electrician, electrical engineer, or person whose hobbies involve building things like model airplanes, trains, or cars. You may also need to work your way up to this project by starting with an electronics project that has a lower level of difficulty.

Important Safety Notes Before You Begin:

  • All devices that are tested should be disconnected from a power source.
  • Don't take any electrical appliances apart to test components inside.
  • Do not go near the sockets in the wall with the circuit tester.
Continuity tester.
Figure 1. The partially assembled circuit tester. The connections have not been wrapped with electrical tape yet. The tester will buzz if current can flow between the two probes. The probes can be stored in the container when not in use.

Setting Up the Circuit

  1. Attach the 9-V snap connector to the AA battery holder.
    1. Even though the name says "9 V," you will use the connector to hook up the AA battery holder.
  2. Strip about 2 cm from the end of the red wire from the 9-V snap connector.
  3. Strip about 2 cm from the end of the red wire from the buzzer.
  4. Twist the ends of the red wires together from the snap connector and the buzzer.
  5. Wrap electrical tape around the exposed twisted red wires.
  6. Attach a black alligator clip to the black wire from the buzzer.
  7. Wrap electrical tape around the black alligator clip and the black wire from the buzzer.
    1. Even though the alligator clip is insulated, there is a chance the bare wire might touch another bare wire. The electrical tape ensures the bare wire is insulated.
  8. Attach a red alligator clip to the black wire from the battery pack.
    1. Wrap electrical tape around the red alligator clip and the black wire from the battery holder to insulate the bare wire.
  9. Place the batteries in the battery holder.
  10. Touch the red and the black probes together. You should hear a buzz from the buzzer. If you don't, check the batteries and your connections and try again. See Figure 2.
Continuity tester.
Figure 2. Diagram for the circuit tester. When the two probes are connected to a conductor, current flows through the circuit, activating the buzzer.

Housing the Tester

  1. Thoroughly clean and dry a 16-oz. plastic container and its snap-on top.
    1. Figures 1 and 3 show a salsa container. You can use whatever type of container you choose.
  2. Use the scissors to cut two notches in the top rim of the base of the plastic container. Refer back to Figure 1, above. Have an adult help you with this step.
  3. Place the buzzer, the battery pack and the wires inside the plastic base.
  4. Rest the wire for the red probe in one of the notches, so that the probe is on the outside of the container.
  5. Rest the wire for the black probe in the other notch, so that the probe is on the outside of the container.
  6. Punch five holes in the lid, using the hole punch or a screwdriver. This will make it easier to hear the buzzer.
  7. Place the lid on top of the container.

Testing the Flow of Electricity

Important Note: The circuit tester can be used to determine whether two things are electrically connected to each other. If two things are electrically connected, current can flow between them. The tester will buzz if current can flow between the two probes. Caution: Make sure anything you choose to test is not plugged into a wall socket or powered by batteries.

  1. Check to make sure an incandescent lightbulb is in the lamp.
  2. Plug the lamp in and turn it on to test that the lightbulb works. If it does not light, replace the lightbulb with a new incandescent lightbulb.
  3. Turn the lamp off.
  4. Unplug the lamp.
  5. Attach one probe to each prong of the plug. See Figure 3.
Continuity tester.
Figure 3. Circuit tester attached to lamp plug. The two kinds of lightbulbs are shown: incandescent bulb (left) and energy-saving fluorescent bulb (right).
  1. Does the buzzer make a noise? Record your observations in your lab notebook in a data table, like the one below.

Lamp (Unplugged)
Lightbulb Switch Buzzer (On/Off)
Incandescent On  
Incandescent Off  
Fluorescent On  
Fluorescent Off  

  1. Turn the lamp switch ON. Note: The lamp should remain unplugged at all times.
  2. Does the buzzer make a noise now?
  3. Remove the incandescent lightbulb.
  4. What happened to the circuit when the lightbulb was removed?
  5. Replace the incandescent lightbulb with the energy-saving fluorescent lightbulb.
  6. What happens to the buzzer?
  7. Turn the switch on the lamp OFF.
  8. What happens to the buzzer now?
  9. Explain your results.
    1. Look inside the incandescent lightbulb. Do you see why there is a circuit?
    2. For the energy-saving lightbulb, how would you explain your results? Hint: What is needed in this kind of lightbulb to make electricity flow?
  10. Repeat steps 1–15 with two different lamps.

Share your story with Science Buddies!

I did this project I Did This Project! Please log in and let us know how things went.


  • Test a toaster. Unplug the toaster, attach the probes to the prongs of the plug. What happens when you push the toaster lever down?
  • Test other appliances around your house.
  • Test the connections in a USB cable (that is free at both ends). Attach the alligator clips to a small piece of wire; for example, a paper clip, so that you can make contact with the small wires in the cable. A helper can hold one of the probes. Make a diagram showing your results. Try other kinds of cables or cords.
  • Test to see if the cold water pipes in your house are electrically connected. Use wire to extend the lengths of your probes. Attach the extended probes to the copper pipes that carry cold water under the kitchen and bathroom sinks.
  • Try different kinds of buzzers in your tester. You may need to increase the voltage.
  • What happens if the circuit is closed and electricity can flow, but there is resistance to flow? This could happen if the metal in a switch was corroded, for example. Model this by placing 1-, 10-, and 1,000-ohm resistors (available at RadioShack) across the probes.

Share your story with Science Buddies!

I did this project I Did This Project! Please log in and let us know how things went.

Frequently Asked Questions (FAQ)

If you are having trouble with this project, please read the FAQ below. You may find the answer to your question.
Q: My multimeter is giving me negative readouts. What do I do?
A: You should first check to make sure the leads are in the proper terminal (you might be getting a negative reading if your leads have been switched). The positive node of the battery should be attached to the red wire (+) from the multimeter. Otherwise, try switching the channel to see if perhaps the multimeter was on the wrong setting.
Q: Why am I getting a reading of 0 from the multimeter?
A: There are several issues that may result in a reading of 0 from the multimeter:
One or more of your connections could be faulty. Make sure that all wires are attached securely. Double-check all the connections. The following suggestions may help you troubleshoot:

  • Your multimeter is not set to a sensitive enough scale. The currents flowing through the liquids in this experiment are very small, so your multimeter must be set at a high sensitivity. A current range of 200uA (microamps) seems to work well for most experiments.
  • Your battery might be dead, meaning that the experiment is without power. You can check whether or not your battery still works by putting your multimeter in the 10 volts range and connecting the leads to the battery. If the reading is 9 or above, your battery is still working.
  • Your electrode may have become compromised in some way. There should be no material collected on the electrode; if there is anything collected on the electrode, clean and rinse it well and try again.
Q: Why are my readings going up and down?
A: There are two possibilities for why your readings are fluctuating; you can tell what is happening in your experiment by how much the readings are changing. If the fluctuations are very small (i.e., the reading stays around the same number but increases or decreases slightly), you have nothing to worry about. In these types of experiments with multimeters, it can be very difficult to get an entirely stable voltage, and slight fluctuation is normal.

If your measurements are decreasing more quickly, you may have encountered a problem with electrolysis. Electrolysis is when water is broken up into hydrogen and oxygen gas by an electrical current. You can tell if electrolysis is occurring because there will be little gas bubbles of hydrogen and oxygen collecting on the electrode. The electrolysis will cause there to be a smaller surface area on the electrode, and your readings will decrease.
Q: What if I still need help using a multimeter?
A: Science Buddies has a guide to using a multimeter, which can be found here: http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_primer-multimeter.shtml
Q: I'm not very familiar with electronics in general. Where can I find help with the basics?
A: This particular project is one of the more challenging electronics projects in the Science Buddies library of Project Ideas. We have a basic electronics primer that is found here:

As always, if you need help, we encourage you to seek assistance from parents or science teachers. In addition, you can try one of our easier electronics projects before beginning with this one, as this one requires you to put together multiple components of a circuit.

Some electronics projects with a lower level of difficulty can be found here:

Ask an Expert

Related Links

If you like this project, you might enjoy exploring these related careers:

Two electricians working.


Electricians are the people who bring electricity to our homes, schools, businesses, public spaces, and streets—lighting up our world, keeping the indoor temperature comfortable, and powering TVs, computers, and all sorts of machines that make life better. Electricians install and maintain the wiring and equipment that carries electricity, and they also fix electrical machines. Read more
electrical engineer aligning laser

Electrical & Electronics Engineer

Just as a potter forms clay, or a steel worker molds molten steel, electrical and electronics engineers gather and shape electricity and use it to make products that transmit power or transmit information. Electrical and electronics engineers may specialize in one of the millions of products that make or use electricity, like cell phones, electric motors, microwaves, medical instruments, airline navigation system, or handheld games. Read more
Electrical Engineering Tech - old img019

Electrical Engineering Technician

Electrical engineering technicians help design, test, and manufacture electrical and electronic equipment. These people are part of the team of engineers and research scientists that keep our high-tech world going and moving forward. Read more