What type of device responds to stimuli such as heat or smoke in a circuit?

A device that responds to stimuli such as heat or smoke in a circuit is best identified as a detector. Detectors are specifically designed to identify hazardous conditions and provide alerts when those conditions are present. This includes smoke detectors, which sense the presence of smoke particles in the air, and heat detectors, which react to changes in temperature that exceed a certain threshold.

In the context of safety and alarm systems, detectors play a critical role in monitoring environments for fire or other hazardous situations. They work by using various sensing technologies to recognize a specific stimulus and signal an alert system which can warn occupants or activate alarm systems to ensure safety.

While sensors could also be included in similar discussions, as they generally respond to a variety of stimuli including physical and environmental changes, the term "detector" is more precise when referring to devices specifically aimed at identifying smoke or heat for safety purposes. Hence, in the specific context of responding to stimuli like heat or smoke, a detector is the correct choice.