NE555 & BD140 SIREN

In this project, I will show you how to design a simple Police Siren Circuit using NE555 Timer IC. I will also share with you how the circuit works and what is the underlying concept of the project.The concept behind the working of this project is Multivibrator. So, before going into the details of the project, let us understand the concept of a Multivibrator.

The schematic is wrong, the left 555 timer's output pin 3 should be going to the BD139, not pin 2+6.
2SD882 is fine for a substitute, this is only a 5V siren. Best to have a diode or snubber across the speaker if you want more power like at 12V operation.

diode across the speaker you meant parallel to the speaker; one lead to Neg - of the Spkr and another to Pos+

Which circuit or project you are following?Kindly attach the link