The 509r is an optical smoke detector designed for the 500r+.
There are two components to the smoke detector: a 509 Transmitter built into a plastic adaptor ring, and the detector body itself.
When smoke triggers the detector it sounds its siren. The microphone in the 509 transmitter detects the sound and causes the transmitter to send a signal to the 500r+ or 500r control unit.
If the battery of the smoke detector is low then the detector starts giving a series of short “beeps”. The transmitter also detects these sounds and sends a low battery signal to the control unit. The transmitter also sends a low battery signal if its own batteries are low.