Troubleshooting Step by Step
How technicians find faults: a calm, logical method that beats guessing every time.
The question
Your circuit doesn’t work. Where do you even start?
What you’ll discover: a step-by-step method for finding faults, the most common mistakes, and the half-split trick professionals use.
In simple terms
BeginnerLook, think, then test
When something doesn’t work, the temptation is to start pulling parts out. Don’t. Work through it in order:
- Power first. Is the switch on? Are the batteries fresh and the right way round? Is the battery pack actually connected to the rails?
- Look carefully. Compare your build with the diagram, one connection at a time. Point at each wire as you check it.
- Check the usual suspects (below).
- Measure. Use your multimeter to find where the voltage stops being what it should be.
- Change one thing at a time, and test after each change.
The usual suspects
- An LED, diode, transistor or electrolytic capacitor in backwards.
- Both legs of a part in the same breadboard row.
- A breadboard power rail that’s split halfway along.
- A wire in the wrong row, or one row off.
- A pushbutton turned the wrong way.
- Flat or mixed batteries.
- A wrong resistor value: check the colour bands, or measure it.
Go deeper
IntermediateFollow the voltage
With the circuit switched on, put the black probe on the − rail and keep it there. Then touch the red probe to points along the circuit, starting at the battery and moving toward the far end, comparing each reading with what you expect:
- At the + rail: the battery voltage (about 4.5 V).
- After the resistor, at the LED’s long leg: about 2 V if the LED is lit.
- If you see 4.5 V where the resistor meets the LED and the LED is dark, current isn’t flowing: suspect the LED (backwards or faulty) or its connection to −.
- If you see 0 V right after the resistor, the resistor or its connection is open.
The fault is between the last good reading and the first bad one.
AdvancedThe half-split method
For a long chain of parts, testing every point in order is slow. Professionals use the half-split method: test in the middle of the chain. If the middle is good, the fault is in the second half; if not, it’s in the first half. Then split that half, and so on. With 16 possible fault locations, half-splitting finds the fault in at most 4 tests instead of up to 16.
Good troubleshooters also keep notes: what they measured, what they changed, what happened. That habit saves hours on harder problems, and it’s exactly the habit IT technicians use on computers and networks.
Quick check
What should you always check first?
Power: the switch, the batteries and their orientation, and the connection to the rails.
You read 4.5 V where the resistor meets a dark LED. What does that suggest?
No current is flowing through the LED: it may be backwards, faulty, or not connected to −.
How many tests does the half-split method need for 16 possible fault locations?
At most 4.
Words to know
- Fault
- Something that stops a circuit working as it should.
- Open circuit fault
- A break in the path, such as a loose wire.
- Half-split method
- Testing in the middle to halve the search area each time.
- Reference point
- The point the black probe stays on, usually the − rail.
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.