circuit diagram

the project is taken from this website for my college project


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

  • \$\begingroup\$ D1, R5, C4 form an en.wikipedia.org/wiki/Envelope_detector \$\endgroup\$
    – sstobbe
    Jul 28, 2017 at 17:45
  • \$\begingroup\$ Circuits are more about the actions of combinations of components, rather than trying to determine what each single component does. \$\endgroup\$
    – Chu
    Jul 28, 2017 at 18:33

2 Answers 2


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.


  • \$\begingroup\$ so I'll be using a photo diode or other light sensitive device as reciever will it be okay? and what about the reciever circuit will it also work as reciever circuit? \$\endgroup\$
    – Pi314
    Jul 28, 2017 at 18:22
  • \$\begingroup\$ Not well enough to be worth using (at either end of the link). Like most of the stuff on that site, it has major issues, build something else! I would start by removing D1, R5, and placing C4 in series with the audio where D1 used to be, with suitable component value changes that may just work then. Those microphones require a DC bias voltage to work which is supplied by R6. \$\endgroup\$
    – Dan Mills
    Jul 28, 2017 at 22:16
  • \$\begingroup\$ so I researched about this circuit more on internet and i found something similar to this circuit and also it have modifications you gave me but the thing is, it is not a led modulation, but a amplifying circuit only with a speaker, so what i was asking is can I replace speaeker with the led modulation shown in the above circuit? **I'll like to try this out and see if it works or not :) \$\endgroup\$
    – Pi314
    Jul 29, 2017 at 16:16

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.

  • \$\begingroup\$ It demodulates your audio input into a light level. \$\endgroup\$
    – klamb
    Jul 28, 2017 at 17:07
  • \$\begingroup\$ thanks for your clarification for function of diode and other components. and as you said that it demodulates audio into light level , I thought it modulates audio into light ,isn't it? \$\endgroup\$
    – Pi314
    Jul 28, 2017 at 18:26
  • \$\begingroup\$ It first demodulates the audio to prevent high frequency flickering. Then it uses the detected amplitude of the audio to vary the brightness. \$\endgroup\$
    – Oskar Skog
    Jul 29, 2017 at 21:20

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