What type of device automatically activates the fire alarm?

The device that automatically activates the fire alarm is a smoke sensor. Smoke sensors detect the presence of smoke particles in the air, triggering the fire alarm system when smoke is detected. This automatic response is essential for early warning in case of a fire, allowing occupants to evacuate safely and alerting emergency services.

In contrast, a pull station requires manual activation. It is used by individuals to manually alert the fire alarm system, which is different from automatic devices. The heat detector, while also an automatic device, is specifically designed to activate based on changes in temperature rather than smoke detection, which differentiates it from smoke sensors. The alarm panel is the control unit of the fire alarm system that receives signals from detection devices, but it does not activate the alarm independently; it acts based on inputs from sensors such as smoke detectors or heat detectors. Thus, the correct choice regarding a device that automatically activates the fire alarm is the smoke sensor, as it plays a critical role in the system's early warning functionality.