I'm a newbie in electronics. I watched a youtube video that showed how to activate LEDs using a microphone. It's project #3 at https://youtu.be/zsJ61yJhWVU?t=115
The guy built the circuit on camera but did not explain its working. He did flash a circuit diagram (which I'm attaching here too).
I roughly get the point of this circuit but want to understand it completely.
a. I understand that two back-to-back amplifiers are needed to achieve good amplification. Correct?
b. I understand that capacitor is used at the base of the first transistor because audio input is not a DC (constant) signal but a time-varying analog signal. But why is it necessary? What will happen if we remove the capacitor at the input.
c. Why he has used two different transistors? (One is C945 and other is BC547)
d. If I build this circuit with two 2N3904, will it work ?
e. Why isn't there some resistance at the collector-to-emitter path of the second transistor (to me it looks like a short circuit and the transistor will be damaged).
f. Is there a way to build a circuit that converts audio input to LED output just like this circuit, but with even fewer components. (I want to start with simple and then understand why we need more components so that I can fully grasp the concept and practice).