Conductor or Insulator? Build a Tester
Turn your LED circuit into a tester, then hunt through your home for things that let electricity through.
What you’ll make
A tester that lights up when you touch it to something that conducts electricity. Then go on a hunt: which things in your home let electricity through, and which block it?
What you need
- Everything from My First LED Circuit
- 2 extra jumper wires, to use as test probes
- A collection of small, dry objects to test: a coin, a paper clip, aluminium foil, a key, a plastic spoon, a pencil, an eraser, a wooden craft stick, a piece of paper, a glass marble, a rubber band
Safety numbers for this build
| Supply | 4.5 V (3 AA batteries) |
|---|---|
| Maximum LED current | about 11 mA, when the probes touch metal |
| Supervision | An adult nearby for younger builders |
Build it, step by step
- Build My First LED Circuit and check it lights. Then switch the battery pack off.
- Remove the jumper wire that runs from the LED’s short-leg row to the blue (−) rail.
- Push one end of a new jumper wire into the LED’s short-leg row. Its free end is probe A.
- Push one end of another jumper wire into the blue (−) rail. Its free end is probe B.
- Switch on. Touch probe A to probe B: the LED should light. Your tester works.
- Now touch both probes to the same object, a little way apart.
In schematic terms, it’s the circuit from Project 1 with a gap between the LED and the battery’s − side. The object you test fills the gap.
What to expect
| Usually lights (conductors) | Usually stays dark (insulators) |
|---|---|
| Coins, paper clips, keys, aluminium foil: metals | Plastic, rubber, wood, paper, glass |
The surprise: pencil “lead” isn’t lead at all. It’s graphite, a form of carbon, and it conducts even though it isn’t a metal. Sharpen a pencil at both ends and touch the probes to the graphite: the LED lights! A pencil line drawn on paper conducts much more weakly, so the LED may glow only dimly, or not at all.
Watch out for coatings: a painted or plastic-coated paper clip may not conduct until you touch the bare metal at its tip.
Why it works
Your tester is a circuit with a gap. Metals, and graphite, close the gap. The object you test becomes part of the circuit. They have free electrons that can move easily, so current flows and the LED lights. In insulators, the electrons are held tightly by their atoms and can’t flow, so the gap stays open. That’s why wires are copper on the inside (a conductor) and plastic on the outside (an insulator).
It doesn’t light? Check these
- Nothing lights, even with the probes touching each other: go back and check your Project 1 circuit works, and that probe A is in the LED’s short-leg row and probe B is in the blue (−) rail.
- A metal object doesn’t light it: it may be painted or coated. Touch the probes to bare, shiny metal.
- Still nothing: check the batteries and the switch.
For teachers and parents
Open the teacher guide
Time: 30 to 45 minutes, including My First LED Circuit if it isn’t already built.
Learners will: classify materials as conductors or insulators by testing; make predictions and compare them with results; explain why wires use both copper and plastic.
Risk and supervision: low, with one firm rule: probes touch test objects only, never outlets, appliances or other batteries. Collect the test objects in a tray beforehand so students don’t wander with live probes.
Discussion questions: Why are wires covered in plastic? What do the conductors have in common? Why might a coated paper clip fool the tester?
Common misconceptions: “All shiny things conduct” (shiny plastic doesn’t). “Pencil lead is made of lead” (it’s graphite). “If the LED doesn’t light, nothing at all is flowing” (a weak conductor can pass a current too small to light the LED).
Worksheet with answers:
- Make a two-column table: prediction and result, for each object. (Results as in “What to expect”; accept dim or no glow for graphite.)
- What did most of the conductors have in common? (They were metals. Graphite is the exception: a form of carbon that conducts even though it isn’t a metal.)
- Why is a wire copper inside and plastic outside? (Copper conducts the current; plastic insulates, keeping the current in the wire and away from people and other wires.)
Stretch: as a teacher demonstration, Science World’s “Conducting Currents” activity compares plain water, salt and salt water.
Learn more
- Science World: Conducting Currents: a water and salt demonstration for teachers
- Science World: Current Electricity: includes a conductivity tester activity
- Lesson 1: What is electricity?
- Next build: Automatic Night Light
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.