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).

enter image description here


1 Answer 1


C1 passes audio from the microphone to Q1 base while preventing the DC supply to the microphone from interfering with the DC bias on the base of Q1.

The +9V (positive battery terminal) should be connected to the top of R1, R2, and R3, and there should be a resistor between the top of R3 and D1 to limit the current in the LEDs.

I expect the -9V label is the negative terminal of the battery - it would be better to call that terminal "Ground". The +9V and -9V labels imply an 18 volt supply.

  • \$\begingroup\$ Thanks @peter-bennett. I will appreciate if you can also clarify the other points. \$\endgroup\$
    – wajmo
    Nov 16, 2020 at 23:50

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