An electrical signal is a time-varying voltage or current that carries information, rather than just delivering raw power, from one point in a circuit to another. While a power rail exists to deliver joules of energy to run a motor or light an LED, a signal exists to tell that motor when to spin, how fast to turn, or to report the temperature of the room back to a microcontroller. The most common mistake hobbyists and junior technicians make is confusing a power source with a control signal—for example, wiring a 12V DC power supply directly into a 5V-tolerant microcontroller input pin, instantly frying the silicon because they treated an information pathway as a power pathway.
The Core Concept: Information vs. Raw Power
To understand signals, you have to look at the intent of the electron flow. In a power circuit, the load (like a heating element or a DC motor) draws as much current as its resistance allows to do physical work. The voltage is ideally constant, and the current is a byproduct of the load's demand.
In a signal circuit, the voltage or current is the message. The physical work done is negligible. A 4-20mA current loop from an industrial pressure sensor might only deliver a few milliwatts of power to the receiving PLC (Programmable Logic Controller). The PLC doesn't use that power to run its internal processors; it measures the exact current level to determine the pressure in a pipe. If the current is 12mA, the pressure is exactly 50% of the sensor's scale.
What a Signal Actually Changes in a Circuit
When a signal arrives at its destination, it forces a state change. Depending on the receiving component, an electrical signal alters the circuit's behavior in one of three fundamental ways:
- Logic State Switching: A digital signal crosses a specific voltage threshold (e.g., 2.0V for a 3.3V CMOS logic gate), flipping a transistor from off to on. This changes a binary 0 to a 1.
- Analog Measurement: A varying voltage is sampled by an Analog-to-Digital Converter (ADC). The signal's exact amplitude at that microsecond is converted into a binary number for software processing.
- Timing and Frequency: The signal's frequency or pulse width (like a PWM wave or an I2C clock line) dictates the speed of data transfer or the duty cycle applied to a motor driver.
Where You Meet Signals in Practice
Signals take different physical forms depending on the environment. In a noisy industrial factory, you won't use a fragile 3.3V digital signal; you'll use a robust current loop. On a quiet PCB, you'll use high-speed digital buses.
| Signal Type | Physical Medium | Typical Use Case | Noise Immunity |
|---|---|---|---|
| I2C / SPI | 3.3V or 5V Digital | On-board sensors, EEPROM, displays | Low (short traces only) |
| 4-20mA Loop | Current (mA) | Industrial pressure, flow, temp sensors | Very High (immune to voltage drop) |
| 0-10V Analog | Voltage (DC) | HVAC damper control, VFD speed reference | Medium (susceptible to EMI) |
| RS-485 | Differential Voltage | Long-distance serial comms, DMX lighting | High (rejects common-mode noise) |
Worked Numeric Example: Sizing a 4-20mA Shunt Resistor
Let's look at the most common industrial signal: the 4-20mA current loop. A PLC's ADC usually reads voltage (e.g., 1V to 5V), not current. To convert the current signal into a voltage signal, we insert a precision shunt resistor across the input terminals.
The Goal: Convert 4-20mA into a 1V to 5V signal for a PLC analog input card.
- Calculate Resistance (Ohm's Law): We need 20mA (0.020A) to produce exactly 5V.
R = V / I → R = 5V / 0.020A = 250Ω.
Let's verify the bottom of the scale: 4mA (0.004A) × 250Ω = 1V. Perfect. - Calculate Power Dissipation: The resistor will get warm. We must ensure it doesn't burn out or drift in value.
P = I² × R → P = (0.020A)² × 250Ω = 0.0004 × 250 = 0.1 Watts. - Select the Component: A standard 1/4W (0.25W) resistor handles 0.1W easily. However, for signal accuracy, we must use a 1% tolerance metal film resistor. A standard 5% carbon film resistor could read 262Ω, turning our 20mA signal into 5.25V, potentially clipping the PLC's ADC and giving false high-pressure readings.
Bench Scenario: When a 5V Sensor Signal Destroys a 3.3V ADC
Theory is clean; the workbench is messy. Here is a real-world scenario demonstrating how a perfectly calculated signal can fail in the field due to environmental factors.
The Setup: You are using an ESP32 DevKit v1 to read an industrial 5V pressure transducer. The transducer outputs a linear analog signal from 0.5V (0 PSI) to 4.5V (100 PSI). The ESP32's ADC pins (like GPIO 34) are strictly 3.3V tolerant. Feeding 4.5V into GPIO 34 will permanently damage the silicon.
The Numbers: You design a voltage divider to step the 4.5V signal down to a safe 3.3V level. You choose R1 = 10kΩ and R2 = 22kΩ.
Vout = Vin × (R2 / (R1 + R2))
Vout = 4.5V × (22 / 32) = 3.09V.
This is safely under the 3.3V absolute maximum. On your desk, using 6-inch breadboard jumper wires, the ESP32 reads a perfectly stable 12-bit ADC value ranging from 410 to 3700.
The Outcome: You mount the ESP32 in a control cabinet and run 15 feet of untwisted 22 AWG wire from the sensor to the voltage divider. You power up the cabinet, and the ESP32 immediately starts throwing brownout reset errors. The ADC readings, when they do appear, jitter wildly by ±400 counts.
What Went Wrong: The 15-foot untwisted wire ran parallel to the output cables of a Variable Frequency Drive (VFD) powering a 3-phase motor. VFDs use high-frequency PWM (often 10kHz to 20kHz) to synthesize AC waveforms, generating massive electromagnetic interference (EMI). Because your voltage divider used high-value resistors (10k and 22k), the node at GPIO 34 had a high Thevenin equivalent impedance. This high-impedance node acted like an antenna, absorbing the VFD's high-frequency noise. The noise superimposed onto your 3.09V DC signal, creating transient voltage spikes that pushed the pin to 3.8V, triggering the ESP32's internal protection diodes and causing the brownout resets.
The Fix:
- Lower the Impedance: Swap the 10k/22k resistors for 1k/2.2k resistors. This drops the source impedance, making it much harder for induced noise to develop a significant voltage.
- Add a Bypass Capacitor: Solder a 100nF X7R ceramic capacitor directly between GPIO 34 and GND. This creates a low-pass filter that shorts the high-frequency VFD noise to ground while letting the slow-moving DC pressure signal pass to the ADC.
- Use Twisted Pair: Replace the untwisted 22 AWG wire with a shielded twisted-pair cable. Twisting the wires ensures that any magnetic interference induces equal and opposite voltages in adjacent twists, canceling the noise out (common-mode rejection).
Frequently Asked Questions
Can a signal deliver power?
Technically, any voltage across a load delivers some power (P=VI), but in engineering terms, a signal's power delivery is incidental and usually negligible. For example, an I2C data line might source a few milliamps at 3.3V (under 10mW), which is useless for powering a load but perfect for switching a logic gate. If a wire's primary job is to deliver watts to do physical work, it is a power line, not a signal line.
Why do we use current signals (4-20mA) instead of voltage signals in industry?
Voltage signals (like 0-10V) suffer from voltage drop over long wire runs. If you send 10V over 500 feet of thin wire, the resistance of the copper might drop the voltage to 9.2V by the time it reaches the PLC, causing a massive measurement error. Current signals, however, are constant throughout a series circuit. A 4-20mA loop will push exactly 12mA through the entire loop, regardless of whether the wire is 10 feet or 1,000 feet long, provided the power supply has enough voltage compliance to overcome the wire resistance.
What is the difference between a digital and an analog signal?
An analog signal is continuous; it can take on any infinite voltage value within its range (e.g., 2.451V, 2.452V) and directly represents a physical quantity like sound or temperature. A digital signal is discrete; it is constrained to specific, predefined voltage bands (usually just two: HIGH and LOW) to represent binary data. Digital signals are vastly superior for long-distance communication because the receiver only needs to determine 'is it high or low?' rather than measuring an exact, easily corrupted voltage.
For deeper reading on microcontroller signal conditioning, refer to the Espressif ESP32 ADC API documentation, which details the specific non-linearities and impedance requirements of their internal signal sampling hardware.
