i want a detail explanation of below circuit like how the bjts and capacitor worksThere's an intercom circuit here:
http://ee.old.no/library/EE1003-en.pdf
You can build from scratch as shown in the manual, or you can buy a ready assembled audio amplifier module and add the changeover switching and speakers. Or for less assembly work than the EE circuit - you could build your own audio module with one of the many audio chips that are available.
The one in the Philips kit uses the speakers to double as the microphone, the amplifier uses a common base input stage for its low input impedance to match better to the speaker. This is a neat trick that you could add on to any of the other options.
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