What does an analog input do in a building automation system?

• A. A device that stores sensor data for later retrieval.
• B. A device that senses a variable such as temperature, pressure, or humidity and causes a proportional electrical signal change at the controller.
• C. A device that powers sensors.
• D. A device that communicates with the internet.

Answer: (B)

An analog input is the way a building automation controller reads a continuous physical measurement. A sensor such as a thermometer, pressure sensor, or humidity sensor outputs a voltage or current that changes smoothly in proportion to the measured variable. The analog input channel on the controller captures that signal and provides a value that the controller uses in its control logic. Because the signal is continuous and proportional to the measured value, the controller can respond with fine-grained adjustments to maintain the desired condition.

This isn’t about storing data, powering sensors, or chatting with the internet—that’s outside the role of an analog input. Its purpose is to translate a real-world variable into an electrical signal the controller can interpret and use to drive actuators in a closed-loop control.