Using a multimeter
What you’ll learn
- What each multimeter setting does
- How to measure voltage, resistance and current safely
- How to use continuity mode to find broken wires
A basic digital multimeter costs about $10–20 and will save you hours of guessing.
The parts of a multimeter
- Display: shows the reading.
- Dial: chooses what to measure: V⎓ (DC volts), Ω (ohms), A or mA (current), and a 🔊 continuity setting.
- Black probe → always in the COM socket.
- Red probe → in the VΩ socket for almost everything. Move it to the A / mA socket only when measuring current.
Measuring voltage (most common)
- Dial to V⎓ (DC). Pick a range above what you expect (e.g. 20 V), or use auto-range.
- Circuit powered on.
- Touch the probes across the part (in parallel): red on the + side, black on the − side.
- A negative reading just means the probes are swapped.
Use it to check: Is the battery really 6 V? Is 5 V reaching the sensor?
Measuring resistance
- Dial to Ω.
- Power off. Measuring resistance in a powered circuit gives wrong readings and can damage the meter.
- Touch the probes to each end of the resistor. Best done with the part out of the circuit.
Checking continuity (finding broken connections)
- Dial to the 🔊 symbol.
- Power off.
- Touch the two ends of a wire or connection. A beep means they’re connected.
This finds broken jumper wires, bad solder joints and wrong breadboard rows fast.
Measuring current (careful)
Current must flow through the meter, so it goes in series: break the circuit and put the meter in the gap.
- Move the red probe to the mA (or A) socket.
- Dial to A⎓ / mA⎓.
- Break the circuit and connect the meter across the gap.
Example: diagnosing a dead robot
| Measurement | Reading | Conclusion |
|---|---|---|
| Battery pack voltage | 4.1 V (should be ~6 V) | Batteries are flat |
| After new batteries | 6.2 V | Good |
| Voltage at Arduino 5V pin | 0 V | Power isn’t reaching the board |
| Continuity on the power jumper | No beep | Broken jumper wire found |
Example: checking a resistor’s value
Power off, meter on Ω, probes on each leg: reading 218 Ω for a “220 Ω” resistor is normal (resistors have a 1–5% tolerance).
Try it
This is last lesson’s LED circuit with two voltmeters connected across parts, exactly where you’d put a multimeter’s probes: one across the resistor, one across the LED. Start it and read both values. Do they add up to the 5 V supply?
Then double-click the battery and change it to 9 V. Which voltmeter’s reading changes more, and why?
Runs on Falstad Circuit Simulator, free, no account needed.
Read the metersEasy
In the simulator, write down both voltmeter readings. Do they add up to the supply? About 3.2 V across the resistor and 1.8 V across the LED: together 5 V.
Which setting?Medium
You suspect a jumper wire is broken inside. Which multimeter setting do you use, and should the power be on? Continuity (the beep symbol), with the power off.
Measure the current safelyHard
Describe step by step how to measure the current through the LED in a real circuit. Power off; move the red probe to the mA socket; set the dial to mA; break the circuit (pull out one end of the resistor); connect the meter across the gap so current flows through it; power on and read; afterwards, move the red probe back to VΩ.
Quick check
Finished this lesson?