Test Tools and Fault Finding

The multimeter

A multimeter is the single most useful tool in electronics. It measures voltage, current and resistance, and has a continuity beeper and a diode test. Even a cheap one will find most faults.

Function	How to use it	Watch out for
DC voltage	Probes across the part or between a point and ground. Power on.	Choose DC or AC. Red probe to the V socket, black to COM.
AC voltage	Across the AC source. Quotes RMS.	Cheap meters read correctly only for sine waves (true-RMS meters handle other shapes).
Current (A)	Break the circuit and put the meter in series. Move the red lead to the mA or A socket.	Never connect across a supply on a current range; it is a short circuit. The meter’s fuse blows. Set the lead back to the V socket afterwards.
Resistance (Ω)	Power off, part out of the circuit (or one leg lifted).	Other parts in parallel lower the reading. A live circuit gives wrong readings and can damage the meter.
Continuity	Beeps below about 30–50 Ω. Power off.	Checks a connection or finds a short between rails before power-up.
Diode test	Red on anode: reads the forward voltage (0.5–0.7 V for silicon).	Reads “OL” in reverse. Also checks transistor junctions and LED polarity.
Capacitance, frequency	Available on many meters.	Discharge capacitors before measuring them.

Things that mislead

• Meter loading. A typical meter has an input impedance of 10 MΩ. Measuring the middle of a 1 MΩ / 1 MΩ divider from 5 V, which should be 2.50 V, the meter in parallel with the lower resistor reads 2.38 V (4.8 % low). Measuring high-impedance nodes needs a better instrument or a buffer.
• Burden voltage. Current ranges put a small resistance in series. On a 200 mA range with about 1 Ω of shunt, the meter drops about 0.2 V, which can upset a low-voltage circuit.
• Auto-ranging and settling. Wait for the reading to settle, and note the units (mV, V).
• Safety category. Meters are rated CAT II, III or IV for the mains environments they can safely be used in. For anything connected to the mains, use a meter with a suitable CAT rating and good leads (see below).

The oscilloscope

A multimeter shows a number; an oscilloscope shows how a voltage changes over time. It reveals noise, ripple, glitches, timing, and whether a digital signal is really a clean square wave.

• Bandwidth. Choose a scope with a bandwidth of at least five times the signal frequency: a 1 MHz square wave needs 5 MHz. The edge speed also matters: BW ≈ 0.35 ÷ rise time, so edges of 10 ns need about 35 MHz.
• Probes. A 10× probe presents a 10 MΩ load with little capacitance, which disturbs the circuit least; set the scope’s channel to 10× to match. Use the 1× position only for tiny signals at low frequency. Adjust (compensate) the probe on the scope’s test output so a square wave looks square.
• Ground. Connect the probe ground clip (or a short spring ground) at the point you are measuring. A long ground lead picks up noise and adds ringing. The probe ground is connected to earth: never connect it to a mains-referenced circuit.
• Trigger. The trigger holds the picture steady. Set it on the signal’s rising edge and adjust the level until the trace is still.
• Measure: volts per division and time per division give the amplitude and period. The frequency is 1 ÷ period.
• Automatic measurements. Digital scopes also measure frequency, peak-to-peak and RMS automatically; USB scopes and low-cost bench scopes handle most hobby work.

Logic analyzers and other tools

Tool	Use	Tips
Logic analyzer	Records many digital lines at once and decodes I²C, SPI and UART	Sample at least 4× the signal rate, preferably 10× (10 MS/s for 1 MHz SPI). Low-cost 8-channel USB units are enough for most hobby work.
Bench power supply	Adjustable voltage with a current limit	Start with a low current limit; the display shows the load current for free.
Function generator	Provides test signals (sine, square, ramp)	Handy for testing filters and amplifiers.
LCR meter / component tester	Measures inductance, capacitance, ESR	Cheap testers identify unknown parts and test capacitors.
Thermal camera or a fingertip	Finds hot parts	A part too hot to touch for a few seconds is above about 60 °C. Take care with voltages.
Magnifier or microscope	Shows cracks, bridges and cold joints	See Soldering for what a good joint looks like.
Breadboard	Temporary circuits with no soldering	Fine for low-frequency, low-current work; loose contacts cause intermittent faults.

A method for finding faults

Most faults are simple and found by a disciplined approach, not by guessing. Work through the steps in order.

• Check before power-up: supply polarity, no short between supply and ground (use the continuity beeper), and chips the right way round.
• Believe the measurements, not the assumptions. A signal is wrong where it is measured to be wrong.
• Change one thing at a time and note the result. Changing several things makes it impossible to know which one worked.
• Know what normal looks like. Keep a known-good board or recording to compare with.
• Take a break. A fault that looks impossible usually is not; step away and look again at the basics.

Common faults and what they look like

Fault	Symptom	Check
Cold or cracked solder joint	Works only when touched or flexed; intermittent	Reflow suspect joints; magnify; tap the board with a pen
Reversed polarity	Nothing works; a part gets hot; the supply hits its current limit	Check the supply, LEDs, diodes, electrolytic capacitors, ICs
Wrong or missing ground	Noisy or odd readings; circuits that work only with the scope connected	Make sure all parts share one ground; measure the ground between the boards
Floating input	Random behavior, erratic readings	Add a pull-up or pull-down (see Resistors); tie off unused inputs
Brown-out	Resets when a motor, radio or LED switches on	Look at the supply with a scope; add bulk capacitance, or a thicker supply wire
Wrong pinout	A new part does not work or gets hot	Check the datasheet pin order for the exact package
Short between pins	Supply collapses; part hot	Continuity-test between adjacent pins; look for solder bridges
Wrong voltage level	3.3 V and 5 V parts that talk and fail	Measure the signal level; use a level shifter
Wrong value	Works but badly: off in gain, timing or brightness	Measure the parts; check the color code or marking (see Resistors)

Troubleshooting

Symptom	Likely cause	Try
Nothing happens	No power, wrong polarity, broken supply connection	Measure the supply at the board; check polarity and fuses
Part gets very hot	Wrong resistor value, short, reversed part, overloaded transistor	Switch off; check the value, orientation and current; use a current-limited supply
Intermittent behavior	Cold joint, loose breadboard contact, broken wire strands	Reflow suspect joints; wiggle test; replace the breadboard wiring with soldered wires
LED does not light	Backwards LED, no resistor path, wrong supply	Reverse the LED (the longer lead is the anode, +); check the circuit
Chip resets or misbehaves	Missing decoupling capacitor, noisy supply	Add 100 nF across the supply pins and a bulk capacitor on the rail
Relay or motor kills the microcontroller	Voltage spike with no flyback diode; shared supply sagging	Add a flyback diode; separate or filter the supplies
Readings jump around	Floating input, noise on a high-impedance node, poor ground	Add a pull-down; average several readings; shield or shorten the wire
Supply voltage collapses on connect	Short circuit, or a current limit set too low	Disconnect; check for shorts with the beeper; raise the limit slowly

Test safety

• Mains voltage kills. Do not probe live mains equipment unless you are trained. Use a meter and leads rated for the category (CAT II/III), with the probe tips guarded.
• Isolate mains-powered circuits through an isolation transformer before connecting an earthed oscilloscope, because the probe ground can short the circuit to earth.
• One hand rule. When working near high voltage, keep one hand in a pocket so that current cannot cross your chest.
• Discharge capacitors before touching a supply after it is switched off.
• Check the meter before and after: measure a known source to confirm that the meter works before trusting a “dead” reading.
• Eyes. Wear safety glasses when powering up a new or repaired circuit: a reversed capacitor or shorted battery can pop. See Workshop Safety.

Related

• Design, Testing and Troubleshooting: the overview of this guide.
• Circuit Basics: the quantities and laws that the measurements test.
• Batteries and Power Supplies: bench supplies and current limiting.
• Soldering: station setup, joints and safety.
• Sensor Wiring and Installation: field wiring and measurement.

FAQ

Do I need an oscilloscope?

Not at the start: a multimeter will find most problems. Get a scope when you work with timing, noise, PWM, communication buses or switching supplies. A basic 50–100 MHz two-channel bench scope or a USB scope is plenty for most hobby work.

Why does my multimeter show a different voltage from what I calculated?

The meter loads the circuit (10 MΩ is not infinite), the components have tolerances, and the real supply may differ from its label. For a divider of high-value resistors, the meter’s own resistance can cause a visible error.

Is it safe to measure resistance with the power on?

No. A live circuit gives a meaningless reading and can damage the meter. Switch off and discharge first, and lift one leg of the part if other parts are in parallel.

How do I find a short on a board?

With the power off, use the continuity or low-ohms range to check from the supply to ground. If it beeps, find the short by measuring at several points along the rail: the resistance is lowest closest to the short. A bench supply with a low current limit and a finger or thermal camera on the hot part also finds it.

What is the best first step when something does not work?

Measure the supply voltage at the part that is failing. A surprising share of faults are power or ground problems, not the circuit that was being tested.

References

• Horowitz, P. and Hill, W., The Art of Electronics, 3rd ed., Cambridge University Press, and The Art of Electronics: The x-Chapters.
• IEC 61010-1, Safety requirements for electrical equipment for measurement, control and laboratory use (CAT ratings).
• Tektronix, XYZs of Oscilloscopes (primer on scope use) and Keysight Oscilloscope Fundamentals application notes.
• Platt, C., Make: Electronics, 3rd ed., Make Community.

Last reviewed: October 2026. Values are typical: check the datasheets of the parts you use. Do not probe live mains equipment unless you are qualified.