What it does

A multimeter answers the question every stuck project comes down to: is this wire actually connected, and is the voltage where I think it is?

Without one you are guessing from symptoms. With one you can check a connection in two seconds and stop suspecting your code for something that turns out to be a wire half out of a breadboard hole.

It is the single most useful tool to own, and three of its settings cover almost everything you will do.

Before you begin

• A digital multimeter with two probes.
• A breadboard circuit, powered or not depending on the measurement.

The black probe goes in the socket marked COM. The red one goes in the socket marked for volts and ohms, usually labelled VΩmA or similar. Leave them there; the only time the red probe moves is for current, and that is covered at the end.

Continuity, the one you will use most

This tests whether two points are joined. The meter beeps if they are.

Set the dial to the symbol that looks like sound waves, often sharing a position with the diode symbol. Touch the two probes together: it should beep. That is also how you confirm the meter and its leads are working.

Power must be off. Continuity sends a small current of its own, and a powered circuit gives nonsense readings and can confuse the meter.

What it settles in seconds:

• Is this jumper wire broken inside its insulation?
• Does this breadboard rail run the whole length, or is it split in the middle?
• Are these two legs of the push button the pair that are already joined?
• Is the solder joint I just made actually connected?
• Have I accidentally shorted 5V to GND?

That last one is worth doing as a habit. Before plugging in any new circuit, touch one probe to the 5V rail and one to GND. If it beeps, you have a short, and finding it now is far better than finding it afterwards.

DC voltage

This measures the difference between two points, with the circuit powered.

Set the dial to V with a straight or dashed line, not the wavy one. The wavy symbol is AC, for mains, and is not what you want here. If your meter is not auto-ranging, choose the 20 V range.

Then put the black probe on ground and touch the red probe wherever you want to measure.

Useful checks:

• 5V pin against GND should read close to 5 V. If it reads 4.3 V, your USB supply is struggling.
• 3.3V against GND should read close to 3.3 V.
• A module’s VCC pin against GND confirms it is actually powered, which is the most common reason a sensor “does not work”.
• The junction of an LDR divider, against ground, shows the voltage the analog pin is reading.

Voltage is always between two points. “What is the voltage here” has no answer without a reference, and that reference is almost always ground.

Resistance

Set the dial to the ohm symbol. Power must be off, and the component should ideally be out of the circuit, because other paths around it change the reading.

This is how you read a resistor whose colour bands you cannot make out, which is most of them in poor light. It is also how to watch an LDR change: put the probes on its legs and cover it, and the number climbs.

A reading of 1 alone, or OL, means the resistance is higher than the range selected, which for a continuity path means not connected at all.

Current, and why it is different

Current is the one measurement that requires rethinking the circuit, and the one beginners break meters with.

Voltage and resistance are measured across things, with the circuit left alone. Current must be measured through the circuit, so the meter becomes part of it: you break the connection and put the meter in the gap, in series.

It also needs the red probe moved to a different socket, usually marked mA or 10A.

The mistake, and it is a common one, is leaving the red probe in the current socket and then measuring voltage. In that position the meter is nearly a short circuit between the two probes. Across a 5 V supply it will blow the meter’s internal fuse, and on something larger it can do a great deal worse.

So: move the probe back to the voltage socket the moment you have finished measuring current. Every time.

For most Arduino work you can skip current entirely. It earns its place when you need to know whether a project will run for a night on a battery.

A worked check

Your sensor reads nothing. Rather than editing code, spend a minute with the meter.

1. Power off. Continuity from the Arduino GND pin to the module GND pin. No beep means the ground wire is the fault, and nothing else matters until it is fixed.
2. Power off. Continuity from 5V to GND. A beep here means a short, so find it before powering up.
3. Power on. Black probe on GND, red on the module’s VCC. Near 5 V, or the module is not powered.
4. Power on. Red probe on the signal pin. A sensible changing voltage means the module is working and the fault is in your code or your pin number.

Four measurements, and you know which half of the problem to look in. That is the whole value of the tool.

Safety notes

• Everything in this library is low voltage and safe to probe.
• Mains is not. Do not measure it with a general-purpose meter unless you have been trained, and never on a breadboard.
• Check the probes are in the right sockets before each measurement, especially after using current.
• A reading that makes no sense is more often a probe in the wrong socket or a wrong dial position than a genuinely strange circuit.

Check yourself

1. You want to know if a jumper wire is broken. Which setting, and is the power on or off?
2. Black probe on ground, red on a module’s VCC, reading 0.1 V. What does that tell you?
3. Why must the circuit be broken to measure current?
4. The red probe is in the 10A socket and you measure across 5V and GND. What happens?

Answers in order: continuity, power off. The module is not being powered, so look at the supply wire before anything else. Because the current has to flow through the meter, which means the meter has to be part of the circuit. You short the supply through the meter and blow its fuse.

Troubleshooting

Next steps

• Easier first: Identifying Components, which this tool settles the doubts in.
• Related: Using a Breadboard, where the continuity test proves what is joined to what.
• Later: Troubleshooting by Symptom, which assumes you have a meter to hand.

Sources and attribution

All text in this tutorial is original Siratek work.

Socket labels, dial symbols and ranges vary between meters, so the descriptions here are of the common arrangement and should be checked against the instrument in hand. No third-party tutorial text, diagrams, photographs or code has been reproduced here.