What is the use of R5, C4, C2, R6, R5, D1 in the below amplitude modulation circuit?

Question

the project is taken from this website for my college project

http://www.instructables.com/id/Modulating-Audio-on-a-LED/

I think I have figured out function of few components but still need confirmation on below points.

1. I think that the C2 is used to block DC current due to Vcc is it correct. also I made a assumption that the audio input creates the signal which can be allowed through this C2.

2. The diode will allow forward current but no backward current but what is the need and what will happen if i remove it?

3. Also they said if I will be using electrect microphone as input then R6 is required but in case I use mono audio cable with pc as input then it is not required, but why ?

4. why is both the output wires from the audio amplifier block is connected across the R3, I hope the transistor circuit in LED modulator block is used as potential divider mode.

5. what's the use of R4 and what will happen if I remove it?

6. What's the use of R5, and C4?

In the website they have used a simple receiver to receive the modulated light with solar panel with speaker but can i use the same circuit below as a receiver but instead of audio input put a photo diode and instead of LED put a small speaker or earphone? specially i was thinking of using earphone

Answer 1

D1, R5, and C4 form an envelope detector. You don't want L1 blinking on every individual cycle of the audio; you won't get the desired effect. The intent of this circuit is that L1 change in brightness a little bit with every pronounced note, and a bit more for every beat.

With the envelope detector, the voltage driving L1 represents the shape of the audio, its peaks and valleys in loudness that occur with each note and with each beat. How quickly L1 can respond to changes in the audio is set by the values of R5 and C4, called the time constant. The longer the time constant, the less responsive the circuit will be to individual notes and the more responsive it will be to the beat and other large changes in audio.

EDIT: I just went out to the link. Most Instructables circuits have problems ranging from frustrating to severe. This one is severe.

The circuit cannot do what the text says it does.

The corner frequency of the envelope detector (basically, an asymmetrical version of a low pass filter) is only 14 Hz. So by the time you get up to human voice frequencies or the middle of a piano, the audio is attenuated by almost 30 dB. That's a lot. Separate from that, a solar cell has a very slow response time, basically another lowpass filter.

Separate from that, the input hipass filter attenuates everything below 1500 Hz. Again, down at voice frequencies it is attenuating the audio by almost 12 dB. And that is in addition to the envelope detector attenuation.

Overall, the circuit functions as a notch filter. It passes very low and moderately high frequencies, but the stuff in the middle that makes up the vast majority of what we perceive as audio is greatly attenuated.

ak

Answer 2

This looks like a demodulation circuit.

1. & 5. C2 and R4 form a highpass filter. All frequencies above the cutoff will be passed through the filter. All frequencies below will be suppressed. If you remove R4, the circuit won't work.

2. & 6. If you remove this diode, you allow charge to pass from C4 into the op amp. The charge accumulates on C4. This is used to drive the transistor. So removing this diode will not turn on the LED on. R5 bleeds the charge off of C4.