Skip to content

The Multimeter

A multimeter measures what your eyes can't: voltage, current, resistance, and whether two points connect. A $20 one answers 95% of beginner questions. Learn four modes and you're set.

Voltage (V): "how hard is it pushing here?"

Probes across the two points (parallel — the meter sips almost no current). Red to positive, black to ground.

Meter across a AA battery → 1.5V (fresh) down to ~1.0V (dead)
Meter across your LED's resistor → ~3V (from the LED circuit math)

Check: your computed values from The LED Circuit should roughly match the meter. "Roughly" — real components vary ±5–10%.

Careful: never measure voltage with the probes in the current jacks (see below) — that's a short circuit across your probes.

Resistance (Ω): "what is this thing?"

Power off, component out of circuit (or at least one leg lifted — in-circuit resistors read low because of parallel paths). Probes across the component.

Red-red-brown resistor → ~220Ω on the meter

This is how you settle color-code arguments and identify mystery parts. (From Reading Resistors: when bands lie to your eyes, the meter tells the truth.)

Continuity: "are these connected?"

The beep mode (diode symbol •)))). Probes on two points: beep = connected, silence = not. Uses:

  • Did my solder bridge those pins? (beep = yes, wick it)
  • Is this trace actually cut? (silence = yes)
  • Which wire in this bundle is which? (beep when you find it)

The most-used mode in troubleshooting. Before powering a new board, continuity-check power rails for shorts (5V-to-GND beep = don't plug it in).

Current (A): "how much is flowing?"

The meter goes in series — it becomes part of the loop (break the circuit, insert the meter). Start on the highest range and work down.

Careful — the blown-fuse classic: measuring current means the full circuit current flows through the meter. Exceed the jack's rating (often 200mA on the small jack, 10A on the big one) and you blow the meter's fuse — or the meter. And leaving the probes in the current jacks, then measuring voltage next time, shorts the circuit through the meter. Move the red probe back to the voltage jack when done. Every multimeter owner learns this; learn it from reading.

Buying advice

$20–$40 gets auto-ranging (you don't pick the range — it does), continuity beep, and diode test. That's all a beginner needs. Expensive meters add precision and safety ratings you don't need yet — spend the difference on components.

Next: Power Supplies