# Few Questions about Sound on Light

Discussion in 'Electronics Homework Help' started by Electro132, Mar 21, 2013.

1. ### Electro132

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Feb 12, 2013
Hi,

I need some help on my assignment involving the EMF spectrum and sound. It is also about creating vibration from sound using LED. I'm stuck with these questions:

1) Explain how sound can be used on LED.
2) Briefly describe how different Frequencies can be used to create sound.
3) You are given the frequencies 40 hz and 3.1 Ghz. how often do each of these frequencies peak and trough per second?
4) You are given 2 statements:

1 - "The higher the resistor for the same capacitor makes the frequency lower (e.i. 1.5 ohms), but the lower the resistor for the same capacitor makes the frequency higher (e.i. 3.38 ohms)"

2 - "The higher the capacitor for the same resistor makes the frequency higher (e.i. 1.0 ohms), but the lower the capacitor for the same resistor makes the frequency lower (e.i. 0.48)"

Using these statements, find out why changing the value of the capacitors are different than changing the value for resistors.

5) Which of these can you use to replace a tuning capacitor and why:

i) A log pot ii) Linear pot iii) None

6) Explain how electrical current can be turned to sound with a diode after it has been picked up on the FM band. What happens when you amplify the converted sound and projected it out using LED? Does sound have the ability to be carried onto Light waves?

Thanks

2. ### Harald KappModeratorModerator

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Nov 17, 2011
Obviously we're missing some background information what this is all about.
Sound is a physical phenomenon. Light is an electromagnetic phenomenon. You need some kind of transducer (microfone, speaker etc.) and some electronics to convert one into the other (neglecting some strange effects like e.g. sonoluminescense).

1) You can talk to an LED - with no effect (o.k., just joking here). Or you can make the LED's brightness change in step with the loudness of a sound (that's called modulation). Or you can use multiple LEDs to build a bar graph display. or, or, or ...
You cannot make the LED emit sound.

2) That is not an electrotechnical question, it is physics. Look up wikipedia.

3) That's basically in the definition of frequency. One cyvle has one peak and one trough.

4) What is this about? About oscillators? Or filters? Without knowing the respective circuit these statements are meaningless.

5) That too depends on the circuit. Normally you can't replace a capacitor by a potentiometer 1-to-1. But you may be able to create a replacement circuit that uses a potentiometer instead of a capacitor. This circuit will look very unlike the original circuit that used the capacitor.

6)You cannot turn current into sound using a diode. oh. maybe once if you overload the diode until the diode goes kawooom
You can use a diode to demodulate AM so the high frequency spectrum is demodulated into the low frequency (sound) spectrum. You cannot do this for FM. And the result of the demodulation still is an electrical signal (voltage or current), not sound.
And of course you can modulate the intensity of an LED by this signal (see No. 1). But you will not be able to hear the sound. You will need a light receiver plus a demodulator, amplifier and speaker to make the sound audible.

You state that this is your assignment. Are there any accompanying drawings we might need to see for clarity?
May I assume that the difficulties in understanding the task(s) are due your post being a translation of a source in another language? I'd question the competence of your instructor if these questions were posed the way you ask them here.
Or may be it's my own competence

Last edited: Mar 21, 2013
3. ### BobK

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Jan 5, 2010
Clearly you have not tried hard enough!

Bob

4. ### CocaCola

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Apr 7, 2012
Yeah, no kidding I have had my fair share that have emitted sound, although it's short lived... Sometimes starts out as a fizzle other times it's a pop, or a combination of both...

Yep, IR transmission of sound is common practice in newer automobiles for audio, especially ones with rear seat DVD players and also in many new commercial airlines for passenger radio or movie audio...

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,497
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Jan 21, 2010
The question is whether the IR is actually carrying sound, or information contained in the modulation that we can interpret (and translate) into sound.

For example, air, water, and steel bars can all carry sound. No matter how loud you yell at a beam of light, it's not going to carry your screams anywhere.

And which interpretation is correct depends on the subject that this assignment is for.

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Nov 17, 2011

7. ### Electro132

261
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Feb 12, 2013
Thanks Harald Kapp. Yes Q4 is about oscillators.

Do you mean like a circuit with a capacitor and resistor to help filter how much current would be passed onto the potentiometer so the tuner capacitor is not needed at all?

No there is none. The teacher didn't give any. But i'll try to put together a schematic to show you and see if i'm heading in the right direction,

Cheers

8. ### davennModerator

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Sep 5, 2009

no you can use a device called a varicap diode ( sometimes called a varactor diode)

This varaiable capacitance diode becomes the "tuning capacitor" of an oscillator circuit
so when the voltage to the diode is varied with a pot, the tuning of the oscillator is achieved.
These varicap diodes are very often used in FM transmitters, where the audio modulation voltage is applied to the varicap diode, to produce a small variation in the oscillator resulting in a FM modulated transmitter....

like this.....

The varicap diode is D1 coming off the right side of the crystal. The modulation voltage is applied via R18

Dave

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9. ### dietermoreno

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Dec 30, 2012
Oh its this guy again who thinks he can create RF with light and he thinks he can create sound with RF.

Just curious, what class are you in, and what level of schooling are you in?

Are you in high school physics?

You don't use light to transmit sound. You can use light to transmit a modulated waveform. Light can not transmit sound in the way you might be thinking. You can have a microphone convert sound waves into amplitude modulated waveforms and you can have an electro-optical transducer convert the modulated waveforms into light.

Fiber optics does this. Fiber optics is theoretically immune to RF interference and fiber optic light truly travels at the speed of light, compared to I learned in my homework question that a signal travels much slower than the speed of light on a copper wire. So fiber optic internet is faster than DSL.

Last edited: Mar 23, 2013
10. ### Electro132

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Feb 12, 2013
Yes it is. No hard feelings chap. And i apologise if i said any of the sort but i was referring to converting sound to light and sending it out.

Haha No actually i work. This is my down time.

LED's are light emitters. Have you ever heard of Infra - Red LED's? They are invisible. So this is how my mind works:

Since in AM a diode is used to clip off one side of the wave so you could hear the DJ's voice on the speakers and in FM the diode turns the changes in frequency into sound, it tells me that if i have an antenna to receive the signals and then feed it back into the IC where the Tuner is connected to and then have the output connect to the amplifier which amplifies the signal, along with the 2 darlington pairs and then outputted out through an Infra - Red LED then it should work, yes?

I think it will. The objective is to send a signal out through an Infra - red LED which can resonant using a tuner and FM radio antenna feed along with all the other stuff you said.

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Well, that's not going to work.

The IR LED will not induce a signal in an antenna designed for a totally different wavelength.

Even if it did, the cable connecting it to the tuner will not be able to carry the signal without massive losses.

Even if it didn't, the tuner would be incapable of passing the signal.

Even if it could, the rest of the circuitry would not operate on signals of this frequency.

12. ### Harald KappModeratorModerator

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Nov 17, 2011
You will need a photodetector to convert the light from the (infrared) LED to an electrical signal - still no sound.
You then need to amplify the electrical signal and feed it to a loudspeaker. That's the point where sound is reconstructed.

And even as light is an electromagnetic wave as is the radio signal you can receive with your tuner, the frequencies are so far apart that entirely different physical phenomena are used to control either of them.

Last edited: Mar 26, 2013
13. ### Electro132

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Feb 12, 2013
True. That is why the sound is fed into the IC and mixed with the tuning capacitor and placed through the output. The diode helps achieve this

What Cable? there should be copper or silver trackings.

How? it was coverted and either it is fed into the E-Cap for battery life or mixed with the Tuner and then out through the output

There's only 2 signals used. The rest are basically running the current. One is for the AM/FM receiver and the other is for the Infra - Red LED.

14. ### Harald KappModeratorModerator

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Nov 17, 2011
I don't think you understand the basics.
AM typically uses a frequency range of 535 - 1605 KHz
FM typically uses a frequency range of 88 - 108 MHz

Light is in the range of 400-790 THz, that's an order of 10^6 higher!

Entirely different physical phenomena are at work.

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Electro132, take a look at this.

Note that you don't use a tuner. You use something that converts light intensity to a changing voltage.

16. ### Electro132

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Feb 12, 2013
Ok, i didn't mean using both at the same time. Just one, let's try AM. But i'm having difficulty with the output. I have 2 sources going into the IC and mixing. then the output where it gets amplified and shot through the LED. Its the LED part that i need help with. Since the AM is one part of the entire circuit, after it reaches the IC the frequency used for that part is no longer needed.

The output frequency is the one i needed but not sure when the amplifier gets placed in (before or after the LED circuit part?)

17. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
All you need to do is use the audio signal to modulate the intensity of the light.

The link I posted gives one way to do that.

Then you need something at the other end to change that varying light intensity to a varying voltage. They use a solar panel, it would be easier to use a battery, and a voltage divider made up of a normal resistor and a photo-transistor..

18. ### Harald KappModeratorModerator

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Nov 17, 2011
Wha IC are you talking about? And what has that to do with an LED?

19. ### Electro132

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Feb 12, 2013
Well the IC mixes the incoming signal (diode converts signal from AM, which strips the signal for voice or in the case of FM - sound) with the tuner signal coming from the tuner and then combines the 2 signals to go out of the 'output' and into another IC (not sure if the first one is 555 Timer or this one) which mixes the signal with the chosen level of sound from the amplifier and then comes out of that output.

this part i am having difficulty in. You said to convert the signal into light (hence the intensity would be provided by the previous circuits before it reaches this stage), i would need a photo-transistor thing? What ever is the component that converts sound signals into Light is what goes after the 555 Timer. Then that goes into the input of the Transmitter (which has the coils and caps tuned to the freq of Infra-red) and out through the LED.

I believe this is possible (the last part about the Infra- Red freq) as we use this Freq for remote controls and other uses.

Anyway, that is what the IC is for.

20. ### Electro132

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Feb 12, 2013
I think if the AM/FM signal received by the receiver is amplified by a transistor or 2 then it should provide enough to send back through to the input of the 1st circuit with the battery source. Then the only problem would be to limit the amount coming into the circuit with a voltage regulator. But i'm wondering if the diode should come before the voltage regulator? i think it would explode, won't it? Still not sure. Further testing is required, which i will do.