A resistor's value
- Setting
- OHM, the range just above the colour code value.
- How
- Power off. The resistor out of the circuit, or one leg lifted.
- Good
- Within its tolerance: a 10 kΩ ±5 % resistor reads 9.5 to 10.5 on the 20k range.
Lesson 1: Testing
Timers & comparatorsIn this lesson
- Set the multimeter for each job.
- Test a resistor, a switch, a track and a supply.
- Read a logic probe.
- Find a fault on a real board.
Booklet and slides
Lesson 1 is the first lesson in the booklet. Your tasks and exam questions are there.
Block 6 booklet (PDF) Lesson slidesThe multimeter
Key words
- Range
- the biggest value a dial setting can show.
- COM
- the common socket. The black lead goes here.
- Over range
- the value is too big for the range. The display shows 1 on the left.
This is the class multimeter. One meter measures voltage, resistance and current. The dial chooses what it measures.
| Part | What it does |
|---|---|
| display | Shows the reading. The unit comes from the range you chose. |
| dial | Chooses what you measure and the biggest value it can show. |
| COM socket | The black lead. Always. |
| V/Ω socket | The red lead for volts, ohms, continuity and diode check. |
| ON/OFF | On before you measure. Off when you finish. |
| 20A and A | Only for measuring current. |
Every setting
Tap a setting. The dial on the meter turns to it, and the red lead moves to the right socket.
Five rules for every reading
- The black lead always goes in COM.
- Choose the range just above what you expect: 20 V for a 5 V circuit.
- A 1 on the left of the display means over range. Turn up one range.
- A minus sign means the leads are swapped. The size is still right.
- Never measure resistance with the power on.
Across or in series
A voltmeter goes across a part. Its leads touch each end. Nothing is broken.
An ammeter goes in series. You break the circuit and put the meter in the gap, so the current flows through it.
Practice 1
1. State the dial setting and the socket for the red lead to measure the current through a buzzer. You expect about 15 mA.
Answer
DCA 20m (the range just above 15 mA). Red lead in A. Break the circuit and put the meter in series with the buzzer.
2. Explain this reading and state what to do. A 4.7 kΩ resistor shows 1 on the left of the display on the 2k range.
Answer
The meter is over range: 4.7 kΩ is bigger than 2 kΩ. Turn up one range to 20k. It should then read about 4.70.
You can now…
- name the parts of the multimeter.
- choose the setting, range and socket for a job.
- say whether a meter goes across a part or in series.
Testing jobs
Switch the power off before you measure resistance. The meter sends its own small current through the part. With the power on, the circuit's current adds to it. The reading is wrong, and the meter can be damaged.
Put the meter across the part. For the most accurate reading, lift one leg of the part out of the circuit.
Six testing jobs
These are the six tests you will do most. The first four are done with the power off.
A slide switch
- Setting
- Continuity.
- How
- Power off. One probe on the middle pin, the other on an outer pin. Slide the switch both ways. Then try the other outer pin.
- Good
- It beeps in one position and not the other, for each outer pin.
A track, link or joint
- Setting
- Continuity.
- How
- Power off. One probe on each end.
- Good
- A beep. No beep between two tracks next to each other.
No shorts across tracks
- Setting
- Continuity.
- How
- Power off. One probe on each of two tracks side by side.
- Good
- No beep. A beep means a solder bridge or a stray strand of wire.
The supply voltage
- Setting
- DCV 20.
- How
- Power on, flying leads from the supply. Black on 0 V, red on +V.
- Good
- Close to the set voltage, for example 5.0 V.
A logic level on a chip
- Tool
- The logic probe only, not the meter.
- How
- Power on. Red clip on +V, black clip on 0 V. Touch the tip on one pin.
- Good
- Red Hi LED: logic 1. Green Lo LED: logic 0. On a 7400 chip, pin 14 is always 1 and pin 7 is always 0.
Practice 2
1. Describe how to test a changeover switch with the multimeter.
Answer
Power off. Set the dial to continuity. Put one probe on the middle pin and the other on each outer pin in turn, switching between them. It should beep to one outer pin, then to the other.
2. State what this tells you. Your meter beeps between the +V and 0 V test points with the power off.
Answer
There is a short circuit across the supply, for example a solder bridge between two tracks. Do not switch on. Find and remove it first.
3. State what this tells you. Your meter beeps between two tracks side by side, with the power off.
Answer
The two tracks are joined when they should not be: a solder bridge or a stray strand of wire. Remove it before you switch on.
You can now…
- test a resistor, a switch, a track and a joint.
- check for a short before you switch on.
- measure the supply and a logic level.
The logic probe
Key words
- Logic 1
- high: close to the supply voltage.
- Logic 0
- low: close to 0 V.
- Pulsing
- the logic level keeps changing between 1 and 0.
A logic probe shows whether a point is at logic 1 or logic 0. It takes its power from the circuit you are testing.
| Part | What to do |
|---|---|
| red wire | Clip to the +V rail of the circuit you are testing. |
| black wire | Clip to the 0 V rail. |
| TTL / CMOS | Set to TTL for 74 series chips and our 555 circuits. |
| MEM / Pulse | Set to Pulse to catch a changing signal. |
| metal probe | Touch it on the point you are testing. |
What you see, and what it means
Red LED (Hi) on: the point is at logic 1 (high).
Green LED (Lo) on: the point is at logic 0 (low).
An LED flashing: the point is pulsing. The logic level keeps changing.
Neither LED on: the point is not joined to anything, or the probe has no supply.
Common mistakes with the logic probe
| Mistake | What you see | Do this |
|---|---|---|
| Clips on a different supply, not the circuit you are testing | Readings that make no sense | Clip to the same circuit's +V and 0 V. |
| Red and black clips swapped | No LEDs at all | Red on +V, black on 0 V. |
| A clip on the insulation, not bare metal | No LEDs at all | Clip onto a test pin or bare wire. |
| MEM switched on | An LED stays lit after you move the probe | Switch to Pulse. |
| Touching the chip body, or two pins at once | Wrong or flickering readings; risk of a short | Touch one pin. Count from pin 1 at the notch. |
| Probing an input that is not connected | Neither LED, or a flicker | That is a fault in the circuit: the input needs a pull-up or pull-down. |
| Thinking a flashing LED is a fault | Flashing on a 555 astable output | Flashing is right there. |
Practice 3
1. State the logic level. The probe tip is on an input of a 7400 chip, and the green LED is on.
Answer
Logic 0 (low).
2. Give two possible reasons. You clip on the probe and neither LED lights on any pin.
Answer
Any two: the red and black clips are swapped; a clip is on insulation, not bare metal; the circuit's power is off; the clips are on a different supply.
You can now…
- connect a logic probe to the circuit you are testing.
- say what each LED pattern means.
Test bench
The 555 astable practice board
Tap a test point with the meter or the probe. The black lead stays on T2, 0 V.
Predict what the logic probe shows on T3, the output of the 555 (pin 3).
Tap a test point
Tool
0.00
Dial
Red lead in
Power
Learn to Test
Nine guided tasks, one for each kind of test.
Practise Testing
Find the fault on ten boards.
In Probe Detective, each board has one hidden fault. Take readings, then choose the fault. Find 4 of 5 on your first try to earn the Probe Detective badge.
Before you switch on
Check the board against the layout first. This finds faults before they can damage a part.
- Every part matches the parts list. Check resistor colour codes.
- LEDs, diodes, electrolytic capacitors and chips are the right way round.
- Every track cut is made. No solder bridges.
- No beep between +V and 0 V on continuity.
- The supply is set to the right voltage, + to +V.
Practice 4
Give two checks you would make on a stripboard before you connect the power.
Answer
Any two: resistor values match the parts list; polarised parts the right way round; track cuts made; no solder bridges; no beep between +V and 0 V; supply set to the right voltage.
Test stage by stage
With the power on, test the input, then the process, then the output.
If one stage is right but the next is wrong, the fault is between them. You only need to check that part of the board.
Worked example: a light-sensing alarm
Meter on DCV 20. The black lead stays on 0 V the whole time.
| Stage | What to do | A good result |
|---|---|---|
| Input | Red lead on the LDR divider output. Cover the LDR, then shine a light on it. | The voltage changes clearly, for example about 1 V in light and about 4 V covered. |
| Process | Red lead on the comparator output. Cover the LDR again. | The output switches, from about 0 V to close to the supply. |
| Output | Red lead on the buzzer's + terminal, LDR covered. Listen too. | Close to the supply voltage, and the buzzer sounds. |
Practice 5
The alarm is silent when the LDR is covered. The input voltage changes and the comparator output switches. State where the fault is.
Answer
In the output stage. The input and process work, so check the buzzer and its joints: it may be the wrong way round, faulty, or on a dry joint.
You can now…
- check a board before you switch it on.
- test input, process and output in order to find a fault.
Check your understanding
Pick an answer to see feedback on it. You can then try the others. Your score counts first tries only.
Fill the gaps
Turn the dial to the range just what you expect. A 1 on the left of the display means . Test continuity with the power . A logic probe takes its power from .
RAG self-check
Tap Red, Amber or Green for how sure you feel.
Booklet answers: Lesson 1
Try the task in your booklet first. Then tap to check.
Task 1: Predict the reading
| Measuring | Dial | Red lead | Reading |
|---|---|---|---|
| A 9 V battery | DCV 20 | V/Ω | about 9 |
| A 10 kΩ resistor, out of the circuit | OHM 20k | V/Ω | about 10.0 (9.5 to 10.5) |
| A 470 Ω resistor, out of the circuit | OHM 2k | V/Ω | about 0.470 (0.447 to 0.494) |
| A push-to-make switch, held pressed | continuity | V/Ω | a beep (about 0) |
| A red LED, red probe on the long leg | diode check | V/Ω | a number, about 1800 |
| The same LED, probes swapped | diode check | V/Ω | 1 |
| The current through an LED (about 11 mA) | DCA 20m | A | about 11.0 |
Task 2: Probe Detective
- (a) No supply reaching the board: the power supply is off, or the supply leads are not connected.
- (b) LED2 the wrong way round (or R4 open, or a dry joint). Check: diode check on LED2, or continuity across R4's joints.
- (c) Pin 2 and C1 held at 0 V: C1 shorted, or a solder bridge to 0 V. T4 should rise and fall between about 1.7 V and 3.3 V.
Exam question 1 (2019 Q5)
- (a) Error 1: the meter is on the wrong range, a current range. It should be on DCV. Error 2: probe 1 is in the wrong socket. It should be in VΩmA, the middle socket.
- (b) Probe 1 to VΩmA, probe 2 to COM (1). Set to Ω (1). Start on the highest range and work down until the display reads 1, then go up one range (1).
Exam question 2 (2026 Q5(b)(iii))
One probe at test point E (1). The other probe at test point F (1). Dial turned to the continuity (buzzer) symbol (1).
Key words
COM · V/Ω · across · series · continuity · pulsing
Picture glossary
Multimeter
One meter that measures voltage, resistance and current. The dial chooses which.
Logic probe
Shows whether a point is at logic 1 (red LED) or logic 0 (green LED).
Test point
A pin on the board where you put a probe to take a reading.
Range
The biggest value a dial setting can show. Choose the one just above what you expect.
Continuity
A complete path between two points. The meter beeps when it finds one.
COM
The common socket on the meter. The black lead always goes here.
Solder bridge
Solder that joins two tracks that should be apart. It makes a short circuit, so the meter beeps between the tracks.
Dry joint
A dull, lumpy joint where the solder has not joined the leg to the track. It makes a break, so no current flows.
Sources & credits: logic probe picture from National 5 Practical Electronics question paper 2025, Q7, © Qualifications Scotland (SQA). Booklet answers to 2019 Q5 and 2026 Q5(b)(iii) follow the marking instructions, © Qualifications Scotland (SQA). Text, drawings, simulator, practice board and booklet by R Stewart. R Stewart - October 2026.