Light can carry sound !!

Discussion in 'General Electronics Chat' started by Michael George, Feb 15, 2015.

  1. Michael George

    Thread Starter Member

    Feb 8, 2015
    52
    2
    schemeit-project(2).png I'm a beginner in designing circuits. I've designed this circuit myself. It may has a lot of mistakes but it works. I find the idea of this circuit is amazing because light or Infrared rays can carry my favorite song :D :D
    I also have some questions and I wounder if you would mind helping me to improve this circuit :

    1- The sound that comes out from the speaker is noisy. There is not much noise. I mean the noise is acceptable but what if I want to improve it, should I add band pass filters ? or change base bias ? or do something else ?

    2- The IR LED must be 1 inch far from IR receiver so that the quality of the sound is almost good. If they are closer than 1 inch, Noise increases. If the distance between them is larger than 1 inch, Sound becomes lower and lower until it disappears.
    What can I do to increase the distance without disappearing the sound ? ( such as the distance of TV's remote control )

    3- Are C3 and R5 provide a High pass filter ? or the capacitor just passes AC signals and resistor is just a part of voltage divider ?

    4- There are to kinds of IR receiver black one and White ( transparent one ) what is the difference between them ?

    Thanks in advance, I hope I'm not bothering you due to my many questions :D :D god bless you :)
     
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    C3 used 470uf was too big, normally just using 1~10uf, if you need more distance then you can try the ir used in the TV remote control.

    If you want to know what will caused the noise then you better using the o'scope to measure the waveform.
     
  3. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    The circuit you have posted is a very, very basic IR communication circuit. You may want to try the circuit in a dark room to try to eliminate some of the noise created by ambient light. Circuits like this can be improved, here is a good example, albeit a very old circuit. Read the link to better understand Pulse Frequency Modulated. The linked circuit goes back to the early 1980s.
    The black IR transmitters and receivers use the colored plastic to act as filtering.

    <EDIT> I see Scott beat me by a moment or two. :) </EDIT>

    Ron
     
    Michael George likes this.
  4. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Connecting the speaker between the positive rail and the collector of the receiver transistor results in DC current through the speaker. Not a good thing. You're receiver must be redesigned. Using an integrated amplifier with output designed for a speaker might be the best way to go.
     
    Michael George likes this.
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Did you see that : 2- The IR LED must be 1 inch far from IR receiver.
    So do you think only 1 inch also can be affected by ambient light?
     
  6. #12

    Expert

    Nov 30, 2010
    16,261
    6,774
    My instinct is to double the resistance in R4. Most small LEDs are rated for a limit at 20 ma.
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,435
    3,360
    It is good to experiment with simple ideas and circuit designs such as what you have shown.
    However, there are many things wrong with your design.

    The idea is sound but the implementation is faulty.
    First of all, what is the distance over which you wish to transmit your sound?
    How much power do you want from the speaker?

    Your speaker circuit is not properly designed. For simplicity, look at the LM386 audio amplifier for a better solution. It is not very loud but better than what you have shown.
     
    Michael George likes this.
  8. #12

    Expert

    Nov 30, 2010
    16,261
    6,774
    The sound side looks like a decent Class A amplifier to me, but it's very feeble for output power.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    You'd probably get better results by modulating a carrier - AM will work, but FM is better.
     
  10. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    I don't know Scott? I think it should be tried in darkness and I think it can be improved using the link I provided. I would try it in darkness and see if the noise level decreases.

    Ron
     
    Michael George likes this.
  11. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    That is just a simple circuit, and I don't think that is a good way to do, because the distance is too short, so using the normal method as ian field mentioned is the right way, the problem of noise should be suspend until the better circuit is used.
     
  12. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    Scott, go back and read my post #3, especially read the link I put in. I suggested using a modulated carrier the same as Ian did. I mentioned it was a very, very basic circuit. I also suggested the dark simply so the original poster could see if the noise level dropped which would teach them the ambient light might be contributing to the noise. Did you look at the link I suggested or read where I wrote: "Read the link to better understand Pulse Frequency Modulated". I agree it's just a simple circuit and I said that? What am I missing here? :)

    Ron
     
  13. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    What you linked was good for the people who already know more about the ee designing theory, but the TS said that he is a beginner, I'm not sure how deep, but when I saw the Cross-linked capacitors used 470uf, oh, I see, it's a big problem here, so I think he should spend some time to study.
     
  14. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    I agree.

    Ron
     
  15. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Low power can do any experiments even the parts damaged also ok, and the TS dare to do is a good thing, if he could continuing to do, maybe a good Hobbyist will show up soon ... :)
     
  16. WBahn

    Moderator

    Mar 31, 2012
    17,725
    4,788

    First, welcome to the world of electronics and it's good to see you jumping in and playing with some circuits that do some neat things.

    With regard to question #3, the resistors R5 and R6 are a voltage divider to establish a bias voltage for the transistor. It will basically hold the average voltage on the base of the transistor at 2.7V which will put ~2V across R4 which will result in a quiescent current of about 20mA in the LED. This will then go up and down in response to the input audio signal. So you have a pretty strong background current (and therefore light output) that is flooding your receiver. That is probably why you have such a narrow distance range that it will work over.

    The input is also high-pass filtered via C3 and the parallel combination of R5 and R6. The time constant is therefore about 0.57 sec which corresponds to a cutoff frequency of about 0.28 Hz. So your audio signal is getting heavily attenuated. For decent musical audio you want a cutoff frequency in the 15kHz to 20kHz range, so if you use a 10uF cap that will raise your cutoff frequency to about 13kHz, which would be a huge improvement. If you reduce the resistors to about half their current size that will get you into the 20kHz range.

    You might then increase the value of R4 to get the quiescent current down to about 10mA (so try a 220Ω resistor).

    To get a real rough sizing for C2, let's ask what the impedance is at, say 20Hz. A 100uF capacitor has an impedance of 80Ω. We want it to be low, particularly relative to R4. This is the impedance at the beginning of the audio spectrum and it will get lower from here. So this is probably reasonable for starters.

    Make those changes and see what happens.
     
    Michael George and ScottWang like this.
  17. darrough

    Member

    Jan 18, 2015
    86
    19
    Forrest Mims gives a couple of different circuits for lightwave communications in his book "Science & Communication Circuits & Projects". Actually he wrote a four volumes series titled "Engineers Mini Notebooks". These are awesome if you like learning by experimentation.
     
  18. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    The circuit I linked to in post #3 was one of Forrest Mims III circuits which appeared in several of his publications going back to the early 80s. He wrote some really good stuff for the electronics home enthusiast and hobbyist. He also includes some history about communication over light.

    Ron
     
  19. Michael George

    Thread Starter Member

    Feb 8, 2015
    52
    2
    Thank you all for your answers, I appreciate your help.
    You gave me solutions and I've tried some of them.

    It is a very good Idea to try the circuit in a dark room. I have two bulbs in my room: The first one is neon lamb and the other is yellow ( or orange ) bulb. when the neon is on, there is some noise. when the yellow bulb is on, I can not hear any sound from the speaker due to much noise. when room is dark the sound is very clear. Also, sun light affects the noise.

    For the receiver circuit, I used LM386 which is a power amplifier rather than the transistor. The gain is 200 and now I can increase the distance between the receiver and transmitter to about 60 inches. When the distance is about one or two inches, the sound is VERY BIG and noisy.

    I've tried to change the emitter resistors of receiver of both receiver and emitter but there is no sound, I've tried 470 ohm and 330 ohm because I don't have 220 ohm. maybe, I have to change the values of base bias resistors after changing the emitter resistor.

    I also changed the 470 uF capacitor to 10 uF but the sound is not as louder as the previous.

    Thank you again for your help,
     
  20. Reloadron

    Active Member

    Jan 15, 2015
    963
    232
    Pretty cool as you are getting interesting results by changing different things. On a side note if you parallel a pair of 470 Ohm resistors that will get you about 235 Ohms. You will find it interesting what changes in ambient light contribute to the noise levels.

    Ron
     
    Michael George likes this.
Loading...