As most smoke dectors it is based on the MC145017 integrated detector chip. This chip and similar has an amplified analog output from the ionization chamber on pin 14.
The smoke detector can be powered with 5 volt but the build alarm will be on at all times (not a problem if you remove the horn).
The electronics on the Arduino shield are left overs from another project. Only GND, 5V and analog in are needed for this project.
Data out from sensor. Y axis are Arduino A/D values 5V reference.
Lit the corner of some writing paper at 50cm. distance and briefly wafted the smoke towards the sensor. After a couple of seconds blew on the sensor to clear out residual smoke.