Robotics & Arduino
Basics

Why HIGH and LOW?

Zero and one are names for signal levels—not whether the Arduino has power.

A water cylinder at 50 percent, with a graduated analog scale on the right and illustrative LOW below 25 percent and HIGH above 75 percent marks on the left.
The picture illustrates the idea. Its 25% / 75% marks are not the Uno’s actual thresholds; use the ranges alongside it.

Water level = signal voltage.

  • The Arduino stays powered while we measure one signal pin.
  • Empty = 0 V; full = 5 V on our Uno.
  • LOW (0) is a low range, not necessarily exactly 0 V. A pin can also be at 0 V while the board stays powered.
  • HIGH (1) is a high range, not necessarily exactly 5 V.

Digital: two answers.

Uno digital inputs, with a 5 V supply
LevelVoltageReading
0–30%0–1.5 VLOW / 0
Between 30–60%Between 1.5–3 VNot guaranteed
60–100%3–5 VHIGH / 1

The middle is not a third digital value: it may read LOW or HIGH.

Analog: measure the level.

analogRead() gives 0–1023.
Empty: 0 · Halfway: ≈512 · Full: 1023

Back to Blink: writing LOW sets the output near 0 V; writing HIGH sets it near 5 V. That turns our LED off and on.