>> Can a speaker act as input devices to an Voice Activation circuit ?
>> I want to built a Voice Activation circuit without using Mic.
>> Is this possible ?
>>
>> Any voltage frustration from the speaker that can trigger a transistor
>> ?

I didn't see the original message for some reason, but yes a speaker can
be used as a microphone.  As Alan mentioned, some intercom systems work on
this principal.

However, you won't get enough voltage directly from a speaker to turn on a
normal silicon bipolar transistor without physically banging on the
speaker.  You will need some amplification before it can "trigger a
transistor", no matter how frustrated the speaker gets.


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