What symbol is used for a detector?

• A. Thermometer in a circle
• B. Circle with a dot
• C. Rectangle with a line
• D. Triangle with a bolt

Answer: (A)

In alarm drawings, the symbol for a detector is a way to show a device that senses environmental changes. A thermometer inside a circle is the conventional way to indicate a temperature detector, i.e., a heat detector. That specific symbol communicates clearly that the device responds to temperature changes, which is exactly what a detector does. The other shapes don’t carry that same meaning in typical alarm diagrams, so they don’t represent a detector. For quick reading of diagrams, using the thermometer-in-a-circle helps distinguish temperature detectors from other components like manual stations or sirens.