An electrical signal is a voltage or current that varies to transport energy or information. Depending on how it changes over time, we will talk about continuous, alternating, pulsating, or digital signals.
If we connect an oscilloscope to different points in a circuit, we will see that the voltage does not always behave the same. Sometimes it is a straight line, sometimes it ripples smoothly, and at other times it jumps between two values.
In this post, we are going to classify the types of electrical signals you will constantly encounter when working with circuits.
Main types of signals
| Type | Waveform | Classic Example | Main Use |
|---|---|---|---|
| DC | Flat line | AA battery (1.5V) | Circuit power supply |
| AC | Sine wave | Wall outlet (230V) | Power transmission |
| Pulsating | “Humps” or pulses | Rectifier output | Intermediate AC/DC steps |
| PWM | Variable square wave | Arduino analogWrite | Motor and LED control |
| Data | Irregular square wave | I2C, USB | Information transmission |
Direct Current (DC)
Direct Current (DC) is the usual form of power supply in digital electronics and microcontrollers.
It is characterized by electrons always flowing in the same direction and, in the ideal case of a power supply, maintaining a constant voltage over time.
If we plot it (voltage over time), we see a flat horizontal line.
- Where do we find it? Batteries, power supply outputs, PC power supplies, USB chargers (5V), and the power supply for your Arduino (3.3V or 5V).
- Its function: Providing a stable energy base for chips to operate.
Although we call it “Continuous,” in reality it is never perfectly flat. It always has a small ripple or noise, something we will learn to clean up with capacitors later on.
Alternating Current (AC)
Alternating Current (AC) is the common form of distributing electrical energy. In this case, the current periodically reverses its direction.
The most common waveform is sinusoidal. The voltage rises to a positive peak, falls through zero, reaches a negative peak, and rises again.
To define it, we need two key parameters:
- Amplitude (
): the maximum value the wave reaches. - Frequency (
): how many times per second the complete cycle repeats (measured in hertz, Hz).
- Where do we find it? In your home wall outlet (230V at 50Hz in Europe, or 120V at 60Hz in America).
- Its function: Efficiently transporting large amounts of energy over long distances.
Pulsating signals and PWM
What happens if we have a signal that varies but never changes polarity (never goes below 0V)?
We are talking about a pulsating signal.
A classic example is the output of a rectifier (when we convert AC to DC but have not yet filtered it): it looks like consecutive positive “humps”.
Digital data signals
Finally, we have signals that transport complex information. Technically, they are square waves (switching between 0 and 1), but unlike a constant clock or a fixed PWM, the pattern is irregular.
These signals encode messages (bytes) using communication protocols.
- Characteristics: They have very fast rising and falling edges (going from 0V to 5V in nanoseconds).
- Where do we find them?
- UART/Serial: When a microcontroller sends text to a computer.
- I2C / SPI: When you read a digital temperature sensor or write to an OLED screen.
The “purity” of these signals is critical. If the edges become too rounded (due to long cables or parasitic capacitances), the receiver will not understand if it is a 0 or a 1, and the communication will fail.
