Testing
Back to Electronics hub GlossaryTesting is worth marks in the practical activity and runs right through construction. You use test equipment carefully, work through a pre-power-up checklist, then test that each stage — input, process and output — actually works.
The multimeter
A multimeter measures voltage (V), resistance (R) and current (I). Choose the correct function and a range just above the value you expect, so the reading is accurate and the meter is not overloaded.
- Voltmeter — connect in parallel (across the component).
- Ammeter — connect in series (current flows through it).
- Resistance — measure with the power OFF and the component out of circuit.
Connecting the probes & choosing the range
The black probe always goes in the COM socket. Move the red probe between the VΩ socket and the mA/A socket depending on what you measure, then turn the dial to the function and a range just above the value you expect.
| Function | Black probe | Red probe | Dial / range |
|---|---|---|---|
| DC voltage (V⎓) | COM | VΩ | DCV, one range above expected (e.g. 20 V for a 9 V supply). Connect in parallel. |
| AC voltage (V~) | COM | VΩ | ACV, range above the expected a.c. value. Connect in parallel. |
| Current (A / mA) | COM | mA/A | DCA, range above expected. Connect in series — move the red probe back afterwards. |
| Resistance (Ω) | COM | VΩ | Ω, start on a high range. Power OFF, component out of circuit. |
| Continuity (🔊) | COM | VΩ | Continuity setting. Power OFF — it beeps when the path is complete. |
You expect about 6 V across a resistor and the meter has ranges 2 V, 20 V and 200 V. State the range you would choose and how you would connect the meter.
Answer
Logic probe & continuity tester
- A logic probe shows whether a point in a powered digital circuit is high (1) or low (0) — useful for checking gate inputs and outputs.
- Connect its power leads first: the red lead to V+ (the positive supply) and the black lead to 0 V (ground). The probe needs this to work.
- Touch the probe tip on a test point in the circuit: a logic 1 lights the "high" LED, a logic 0 lights the "low" LED.
- A continuity tester beeps when there is a complete (low-resistance) path — useful for checking a track, a joint or a wire is connected and not broken. Use it with the power off to find breaks, short circuits and dry joints.
The red lead goes to the .
The black lead goes to .
When the tip is on a test point, a logic 1 lights the light.
See a full answer
(a) State where the red and black leads of a logic probe are connected. (b) When the probe tip is on a test point and the "high" LED lights, what is the logic state there?
Answer
A soldered track might be cracked. State which instrument you would use (and the power setting), and what result tells you the track is good.
Answer
Pre-power-up checklist
Before switching on, check the build against the design. This catches faults before they can damage components.
- Supply voltage and polarity are correct (+ and 0 V the right way round).
- Every component matches the parts list (correct resistor values / codes).
- Polarised parts are the right way round — diodes, LEDs, electrolytic capacitors, transistors, ICs (pin 1).
- No short circuits, missing links or unintended bridges between tracks.
- All joints are clean and solid (no dry joints); tracks cut where the layout requires.
- Continuity of key connections checked with the power off.
Give two checks you would make on a stripboard build before connecting the power.
Answer
Functionality testing (input → process → output)
Once powered, test the circuit stage by stage so you know exactly where any fault is:
- Input — change the input (cover the LDR, warm the thermistor, press the switch) and check the input voltage changes as expected.
- Process — check the processing stage responds (e.g. the comparator output switches over, the transistor turns on).
- Output — check the output device works (lamp lights, buzzer sounds, motor turns, relay clicks).
Worked example — testing a light-sensing alarm
Equipment: a digital multimeter (probes in COM and VΩ), set to 20 V DC, with the black probe clipped to the 0 V rail throughout.
| Stage | What to do | Expected result |
|---|---|---|
| Input | Red probe on the LDR divider output. Cover the LDR, then shine a light on it. | Voltage swings clearly (e.g. ~1 V in light → ~4 V covered). |
| Process | Red probe on the comparator output pin. Cover the LDR again. | Output switches over — e.g. ~0 V → near the + supply (about 8–9 V). |
| Output | Red probe on the buzzer's + terminal with the LDR covered; also listen. | About the supply voltage across the buzzer and it sounds. |
If a stage's voltage is right but the next one is wrong, the fault is between them — check that joint, track or component before moving on.
A light-sensing alarm does not sound when the LDR is covered. The input voltage changes correctly and the comparator output switches, but the buzzer is silent. State where the fault is.
Answer
Testing demonstration videos
Watch each demonstration of pre-power-up and functionality testing on real circuits.
Fault Finder challenge
A circuit isn't behaving. Read each symptom and test reading, then pick the most likely fault. Score 8 / 10 to earn the 🔬 Fault Finder badge.
Check your understanding
Answer the multiple-choice questions, then mark yourself with the RAG self-check.
RAG self-check
Tap Red / Amber / Green for how confident you feel.