What it shows
A continuity tester tells you whether a path conducts: touch its two probes to the ends of a wire, a fuse or a switch and it beeps if current can flow. It is the first tool for finding broken wires and checking which materials conduct.
The science
The buzzer and the LED branch are connected in parallel, then in series with the battery and the two probes. With the probes apart the loop is open and nothing happens. Touching both probes to a conductor closes the loop, the buzzer sounds and the LED lights. The 470 Ω resistor limits the LED to about 15 mA from 9 V. A resistor of a few hundred ohms in the tested object still lets the buzzer sound faintly, so the tester also gives a rough idea of how well something conducts.
Materials
Cost: under Rs 100. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- An active buzzer rated 3 to 12 V
- One green LED and a 470 Ω resistor
- A 9 V battery with a snap connector
- Two probe wires with pointed bare ends (old pen refills make good holders)
- A small plastic box such as a mint box
- Tape and a slide switch if you want one
Safety
For this project: Test only unplugged things. Never touch the probes to anything connected to the mains or to a car battery.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Make a hole in each end of the box for the probe wires and one in the lid for the LED and the buzzer sound.
Connect the battery snap red wire to the buzzer red lead and also to the 470 Ω resistor.
Connect the resistor to the LED long leg; join the LED short leg to the buzzer black lead.
Connect that joined point to probe one, and probe two to the battery snap black wire.
Touch the probe tips together: the buzzer sounds and the LED lights. Separate them: silence.
Pack the parts into the box, LED poking through the lid, and tape the probe wires so they cannot pull out.
Test a steel spoon, a pencil lead, a rubber band, a coin, a key and a plastic ruler, and record which beep.
Test a long wire by touching one probe to each end to show it is unbroken.
What you should see
The tester beeps and lights for metals and pencil graphite, stays silent for plastic and rubber, and finds a broken wire.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the tester use a buzzer as well as an LED?
You can listen for the beep while watching the probes rather than the box.
2Why must the tested object be unpowered?
An outside voltage could push a damaging current through the tester or give a shock.
3Why does pencil lead make the buzzer sound?
Graphite is a form of carbon that conducts electricity.
4What is the resistor for?
It limits the current through the LED to a safe value.
Take it further
Add a 1 kΩ resistor across a second pair of sockets and compare how loudly the buzzer sounds through it.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Hardware & repair
Using a Digital Multimeter
Measure voltage, current, resistance and continuity safely
Robotics
Electronic Components Handbook
Resistors, capacitors, LEDs, diodes, transistors and more, with how to use each