Which of the following would be considered a digital device?

• A. Thermostat
• B. Relay
• C. Sensor
• D. Valve

Answer: (B)

In HVAC control, digital devices are those that operate with discrete, binary states. A relay fits this perfectly because it acts as an on/off switch: when the control signal energizes it, the contacts close and the load is powered; when not energized, the contacts open and the load is off. There’s no gradual or proportional output—just a simple, two-state operation—which is the essence of a digital device.

The other components aren’t inherently digital in the same way. A sensor provides a measurement that can vary continuously (temperature readings), and a valve or a thermostat typically modulates or processes signals rather than delivering a strict on/off binary output. So the relay best exemplifies a digital device in this context.