How Do Voltage Levels Represent Digital Signals?
- Starter
- Article
- Ages 10+
- 5 min read
A digital signal uses voltage in two regions: the upper one is called HIGH and the lower one LOW. In the positive-logic convention used here, a circuit reads HIGH as TRUE and LOW as FALSE. It does not need the exact voltage, only which of the two regions the voltage sits in.
Contents
- High voltage region: HIGH
- Low voltage region: LOW
- In-between zone, outside both valid regions
- Signal jumping between the two regions
What you will learn
- Describe the two levels, HIGH and LOW, of a digital signal.
- Explain how a logic threshold decides whether a voltage counts as HIGH or LOW.
- Retell how a voltage-controlled switch inverts a logic level.
Before you start
You only need to know that voltage is measured in volts. No electronics background is needed.
Why does a digital signal need only two levels?
Because only two states must be told apart, a digital circuit looks at a chip pin and asks one question: is the voltage here in the HIGH region or the LOW region?
A digital electronics lecture from the University of Wisconsin–Madison puts it this way: when there are only two possible voltage levels, a signal can be treated as being in one of two logic states, HIGH or LOW. Read HIGH as TRUE and LOW as FALSE, and logic gates can process the signal.
On the hardware side, a lecture from San José State University says a bit can be represented as a voltage. A bit is a binary digit with the value zero or one. The voltage is the carrier, and the bit is what it is used to say.
- HIGH region
- LOW region
- Read as TRUE (filled circle with a tick)
- Read as FALSE (empty ring)
- Logic gate taking both values
What voltage counts as HIGH?
The logic family decides. Each logic family defines its own thresholds for valid high and low levels, so the same voltage can be read differently in two families. The exact numbers must be looked up for the family of the chip in use.
The Wisconsin–Madison lecture lists four thresholds, two for inputs and two for outputs:
| Symbol | Applies to | Meaning |
|---|---|---|
| VIL | Input | Voltage region in which the gate treats the input as LOW, or FALSE |
| VIH | Input | Voltage region in which the gate treats the input as HIGH, or TRUE |
| VOL | Output | Maximum voltage for an output at LOW |
| VOH | Output | Minimum voltage for an output at HIGH |
Picture two people on a phone call. The speaker must be clear enough, and the listener has a minimum level they can hear. The output is the speaker, the input is the listener, and each side has its own standard.
- VIH: input region read as HIGH
- VIL: input region read as LOW
- VOH: minimum output level for HIGH
- VOL: maximum output level for LOW
How does a circuit produce a HIGH or LOW level?
A switch controlled by a voltage is enough to produce a HIGH or LOW at an output. A transistor plays that switch inside a logic gate. The Wisconsin–Madison lecture explains it with an ideal switch.
The switch is open when the control voltage is below a threshold voltage and closed when it is above. Two cases follow:
- Input HIGH: the switch closes, the output is grounded and becomes LOW.
- Input LOW: the switch opens, the output is pulled up through a resistor and becomes HIGH.
The output is always the opposite of the input, so this circuit is a NOT gate. Many such gates combine, and sequences of HIGH and LOW levels represent numbers in the binary system.
- Input that controls the switch
- Switch
- Supply line
- Pull-up resistor
- Output
- Ground
Real-world example
Look at a light switch on the wall. It has two positions, on and off. You do not care exactly how many degrees the lever tilts, only which side it is on. A digital circuit does the same with voltage and only needs to know which region the voltage is in.
In an illustrative game, agree that a lamp lights only when two cards both show their bright side. Each card supplies a state; the game rule determines the result. This illustrates combining states, rather than describing household wiring.
Common misunderstandings
HIGH is not one single voltage. It is a region of voltages, and the logic family sets that region.
LOW does not mean the signal is lost. LOW is a valid state, equal in standing to HIGH.
Not every chip shares the same thresholds. Each logic family has its own set, so use the table of the right family.
Frequently asked questions
Are HIGH and LOW the same as one and zero?
Not quite. HIGH and LOW are voltage regions. Reading them as TRUE and FALSE, or one and zero, is meaning that we assign to the voltage.
How do VIH and VOH differ?
VIH is about an input: the voltage region read as HIGH. VOH is about an output: the minimum voltage a HIGH output must reach.
What do many bits make together?
Sequences of HIGH and LOW states represent numbers in the binary system.
Summary
- A digital signal is read from voltage in one of two regions, HIGH or LOW.
- Each logic family sets its own thresholds for inputs and outputs.
- A voltage-controlled switch is enough to invert a logic level.
Terms in this article
English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.
- digital signaltín hiệu số
- A signal that a circuit only needs to sort into one of two logic states, HIGH or LOW.
- logic thresholdngưỡng logic
- A voltage limit that a logic family uses to count a level as valid high or valid low.
- logic familyhọ logic
- A group of logic circuits that share one set of voltage thresholds.
- bit
- A binary digit, carrying one of two values.
- NOT gatecổng NOT
- A logic gate whose output is the inverse of its input.
- binary systemhệ nhị phân
- A way to write numbers as sequences of HIGH and LOW states.
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Sources
These sources are written for older readers. If they feel hard, read them with an adult.
- Digital Electronics Part I: Combinational Circuits(opens in a new tab)
University of Wisconsin–MadisonTier A: government, standards or research bodiesRead on November 3, 2026
- Bits Bytes and Words(opens in a new tab)
San José State UniversityTier A: government, standards or research bodiesRead on November 3, 2026