Which device is designed to control output devices based on input signals and a user-developed program?

• A. PLC
• B. Sensor
• C. Motor controller
• D. Relay module

Answer: (A)

Think of a device that reads signals from sensors and switches, runs software you write, and then turns motors, valves, or alarms on or off based on that software. That’s a Programmable Logic Controller. It sits between inputs and outputs, and the program you develop defines exactly how each input should influence the outputs, including timing and sequencing. The PLC continuously scans inputs, executes the logic, and updates the outputs in a fast, reliable loop, which is essential for industrial automation where precise, repeatable control is required.

Other options don’t fit as well. A sensor only measures and reports data, not control devices. A motor controller drives a motor but doesn’t provide the programmable logic to react to different inputs. A relay module can implement simple, fixed logic but isn’t programmable or flexible enough to handle complex control tasks.