The Fix-It Challenge
Build a circuit, measure it, then hunt down a hidden fault like a real technician.
What you’ll do
This is the Stage 3 challenge, and it’s best done in pairs. First, build a circuit from its diagram and check it with your meter. Then your partner secretly adds one safe fault while you look away, and you have to find it using the troubleshooting method, not guesswork.
What you need
- A battery holder for 3 AA batteries, with a switch, and 3 AA batteries (4.5 V)
- A breadboard and jumper wires
- 2 red LEDs, 5 mm
- 2 resistors, 220 Ω (bands: red, red, brown; five-band: red, red, black, black, brown)
- A digital multimeter
- A pencil and paper for your readings
Safety numbers for this build
| Supply | 4.5 V (3 AA batteries) |
|---|---|
| Current | about 11 mA per LED, about 22 mA in total |
| Supervision | An adult nearby, especially for current measurements |
Part 1: build and measure
- Build My First LED Circuit and check it lights.
- Add a second branch: another 220 Ω resistor from the + rail to a new row, and a second LED from that row (long leg) to the − rail. Both LEDs should light.
- With the black probe on the − rail, measure and write down: the voltage at the + rail, and at each LED’s long leg. Expect about 4.5 V and about 2 V.
- Optional, with an adult: measure the current in one branch (Lesson 16). Expect about 11 mA.
Part 2: the fault-finding game
Switch off. While the builder looks away, the fault-maker picks one fault from this list:
- Turn one LED around.
- Move one LED leg into the same row as its other leg.
- Move the end of the jumper from one LED’s short-leg row into an empty row.
- Pull one jumper partway out so it doesn’t make contact.
- Swap one 220 Ω resistor for a 10 kΩ (brown, black, orange).
- Take one battery out and put a short jumper wire in its place (the pack then gives about 3 V).
Switch on. The builder now finds the fault using the troubleshooting method: power first, look, usual suspects, then measure, comparing with the readings from Part 1. Write down each test and what it showed. Then swap roles.
Answer key: symptoms and causes
Open after you’ve tried
| Symptom | Likely faults | What the meter shows |
|---|---|---|
| One LED dark, the other fine | That LED backwards, both its legs in one row, or a loose or misplaced jumper in that branch | Every one-LED-dark fault reads about 4.5 V at the row where the resistor meets the LED: no current flows in that branch, so there’s no drop across the resistor. The meter tells you the branch is broken; then look closely. Backwards LED: the flat side faces the wrong way. Both legs in one row: the LED is shorted to itself. Short-leg jumper moved: the LED’s short leg isn’t reaching −. |
| One LED very dim | 10 kΩ resistor in its branch | About 1.7 to 1.8 V at the LED: at about 0.25 mA it drops a little less than a brightly lit LED |
| Both LEDs dim or dark | A battery replaced by a jumper (dim), or a loose jumper between the battery and a rail (dark) | About 3 V at the + rail with a battery missing; 0 V at the + rail if its jumper is loose |
For teachers and parents
Open the teacher guide
Time: 45 to 60 minutes.
Learners will: build from a schematic; take and record reference measurements; apply a systematic troubleshooting method; explain how a measurement points to a fault.
Risk and supervision: low. Approve each fault before it’s made, and check that no one creates a short circuit. Supervise current measurements and make sure meters are returned to the voltage setting.
Discussion questions: Which readings were most useful, and why? Was it faster to look or to measure? How is this like finding a problem with a computer or a car?
Common misconceptions: “If the LED is dark, the LED must be broken” (usually it’s the wiring or orientation). “Measuring is only for experts” (a meter on volts with the black probe on − is simple and safe).
Assessment idea: collect each pair’s written test log. A strong log shows power checked first, readings compared with the reference values, and one change at a time.
Learn more
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.