A5366: Photoelectric Smoke Detector with Interconnect and Timer
pin 7 is I/O Pin : A connection at this pin allows multiple smoke detectors to be
interconnected. If any single unit detects smoke, its I/O pin is
driven high, and all connected units will sound their associated
The above data sheet has a nice typical circuit. Likely just what you have there.
The Horn appears to be a 3 pin Piezo. Where its feed back and pulsing will not be as easy, as just monitoring pin 7 of the chip.
Be-warned: this chip runs directly off of battery, up to 15V. Typically 9V battery. So the output of the IO will be that to. So it would be best to drive a resistor divider or clamping diode or something to ensure it does not exceed the VCC of the Arduino.
here is an example circuit to protect against the higher voltage of the A5366.
simulate this circuit – Schematic created using CircuitLab
D2 is optional. BAV99 is a nice little SOT23 that can do this.