Hi all,
I am embarking on an interesting project in school and wish to do this part of the project now. I have to get the output from the audio jack from my computer and amplify the signal and bias it to my LED ratings. However, I am stuck on a few issues.
1) What is the range of my output signal from my computer? And how can I measure it? The Oscilloscope did not really help me on this. Where can I find this information? I googled it and very few sources seem to come to a common census. I suspect I have to figure out something about my soundcard output but I'm just too new to this so I'm hoping that there are some of you guys out there who can help me on this.
2) After figuring out this small signal, say within the +/- 200mv range, I would like to amplify this small signal to be within the limits of say, +/-2V, how can I do so? What kind of circuits can help me achieve this?
3) Lastly, after increasing my audio signal range, how to I DC bias this signal? Would simply using an amplifier as an adder circuit suffice?
I am sure once these questions have been answered, I will be able to drive my LED to let this signal be transmitted across free space to be received at my photodiode.
Thank you all in advance for your help!
Joel
P.S my LED linearity range is yet to be figured out as the product delivery is being delayed.
I am embarking on an interesting project in school and wish to do this part of the project now. I have to get the output from the audio jack from my computer and amplify the signal and bias it to my LED ratings. However, I am stuck on a few issues.
1) What is the range of my output signal from my computer? And how can I measure it? The Oscilloscope did not really help me on this. Where can I find this information? I googled it and very few sources seem to come to a common census. I suspect I have to figure out something about my soundcard output but I'm just too new to this so I'm hoping that there are some of you guys out there who can help me on this.
2) After figuring out this small signal, say within the +/- 200mv range, I would like to amplify this small signal to be within the limits of say, +/-2V, how can I do so? What kind of circuits can help me achieve this?
3) Lastly, after increasing my audio signal range, how to I DC bias this signal? Would simply using an amplifier as an adder circuit suffice?
I am sure once these questions have been answered, I will be able to drive my LED to let this signal be transmitted across free space to be received at my photodiode.
Thank you all in advance for your help!
Joel
P.S my LED linearity range is yet to be figured out as the product delivery is being delayed.
Welcome to diyAudio Joel 🙂
1/ The oscilloscope is absolutely the correct way to measure this. What problems did you have ? The scope will show the peak to peak voltage in real time as it happens.
2/ You can use a just one transistor to amplify this voltage... and to get it to light an LED as well.
3/ Transmitting audio from the LED to a photodiode across a small distance should be easily do-able but it won't be perfect with simple circuitry. There will be some distortion and non linearities.
So firstly you need to use the scope and determine what signal level you have... we could guess and be pretty close but you will learn more by doing it properly 🙂
1/ The oscilloscope is absolutely the correct way to measure this. What problems did you have ? The scope will show the peak to peak voltage in real time as it happens.
2/ You can use a just one transistor to amplify this voltage... and to get it to light an LED as well.
3/ Transmitting audio from the LED to a photodiode across a small distance should be easily do-able but it won't be perfect with simple circuitry. There will be some distortion and non linearities.
So firstly you need to use the scope and determine what signal level you have... we could guess and be pretty close but you will learn more by doing it properly 🙂
Hi 🙂,
Thanks for your reply!
1) I think it might be due to the fact that I am not using a real oscilloscope. In fact, I am using a virtual oscilloscope by NI Elvis II. When I play audio tracks or stuff from youtube, the oscilloscope only displays a periodic signal which I think is surely a wrong reading. Is this because of different load impedence or something like that?
2) Once I have determined the small signal limits from the PC, I want to "stretch" this range by amplification as I have altered the linearity range of my LED. (ie. now offset at 5V, range of +/-2V.) This can help increase the intensity modulation which happens when the corresponding current drive the LED array. I'm not sure if you get what I mean, I can attach some images if you'd like.
3) The main aim of the power amplifier part is to ensure that I have enough power to drive the LED, this might be an optional stage, once I checked out my linear range of the LED (linear range changes with resistor value in series).
Thanks for your help so far!!! I really just need some help on the first 2 pointers! Would really appreciate it if you can help me further!
Joel
Thanks for your reply!
1) I think it might be due to the fact that I am not using a real oscilloscope. In fact, I am using a virtual oscilloscope by NI Elvis II. When I play audio tracks or stuff from youtube, the oscilloscope only displays a periodic signal which I think is surely a wrong reading. Is this because of different load impedence or something like that?
2) Once I have determined the small signal limits from the PC, I want to "stretch" this range by amplification as I have altered the linearity range of my LED. (ie. now offset at 5V, range of +/-2V.) This can help increase the intensity modulation which happens when the corresponding current drive the LED array. I'm not sure if you get what I mean, I can attach some images if you'd like.
3) The main aim of the power amplifier part is to ensure that I have enough power to drive the LED, this might be an optional stage, once I checked out my linear range of the LED (linear range changes with resistor value in series).
Thanks for your help so far!!! I really just need some help on the first 2 pointers! Would really appreciate it if you can help me further!
Joel
The oscilloscope is really just a voltmeter (X axis) against time (Y axis). If you look at music on a scope you would see something like this (post #162, last couple of pics)
http://www.diyaudio.com/forums/mult...-power-do-your-speakers-need.html#post2883059
An LED won't light at all until around 1.8 volt is present across it, so yes, it has to be biased into conduction and then have the audio modulate it above and below that point. Such a simple scheme will never be totally linear (in a true HiFi sense) but will certainly pass music OK between LED and photdiode. You won't need more than around 20ma at most through the LED and so a simple transistor amp should be fine.
http://www.diyaudio.com/forums/mult...-power-do-your-speakers-need.html#post2883059
An LED won't light at all until around 1.8 volt is present across it, so yes, it has to be biased into conduction and then have the audio modulate it above and below that point. Such a simple scheme will never be totally linear (in a true HiFi sense) but will certainly pass music OK between LED and photdiode. You won't need more than around 20ma at most through the LED and so a simple transistor amp should be fine.
Thanks for your help! I have successfully transmitted and received the audio signal across free space and now the receiver side receives the exact audio signal, though I have already intentionally offset the voltage transmitted to +5V. So now its about recovering the original audio signal by removing this dc offset. What would be an appropriate and effective method of doing this?
A simple series cap (and resistor to tie the cap output to ground) would remove all the DC and leave just the audio. Try a 4.7uf and 470k.
Thanks Mooly! It works! Not the best of quality but, the signal's offset has been removed. I tried using 100uF and 22ohm and it did not work for the received audio signal (alot of clipping of the filtered signal), but worked for my Fgen. Are you able to tell me the reason for this (is there a formula?) I may want to play around with the offset bias voltage and the capacitor/resistor values to see if I can get a cleaner signal.
Another question is, if I'm using an 8ohm, 3-5watt speakers, what should be a good resistor/capacitor used for this extra RC circuit which brings down the offset voltage to 0v?
Thanks in advance! 😀
Its difficult to advise without seeing the whole circuit details. The deciding factor is the output impedance of your circuit. If its just an opamp or transistor stage then 22 ohms would be a near short circuit, the reason why had a lot of clipping.
To drive a speaker you need to determine the coupling cap according to the lowest frequency you want to reproduce.
For example, if the -3db point you want is say 100Hz then C would be around 200uf (220uf common value)
The formula is R * C equals 1/ω which is the same as 1/ 2pi (F*C)
Rearrange that and you get C= 1/2pi (F*R) where F is 100Hz and R is 8 ohm.
To drive a speaker you need to determine the coupling cap according to the lowest frequency you want to reproduce.
For example, if the -3db point you want is say 100Hz then C would be around 200uf (220uf common value)
The formula is R * C equals 1/ω which is the same as 1/ 2pi (F*C)
Rearrange that and you get C= 1/2pi (F*R) where F is 100Hz and R is 8 ohm.
Thank you again Mooly! I'm beginning to learn more about these cap applications. However, now that my receiver and speakers are working well, there is a humming sound played over the speakers when there is no music played. I measured and there is a 100hz and about 88mV(just after power amplifier) sound inherently heard. I would like to ask if I use a notch filter, what capacitor values should I be using, without of course, clipping the audio signal?
When it comes to hum, you are really best eradicating the source of the problem rather than fiddling it 🙂
You need to find where in the chain the hum is being introduced, and if using an open optical transmitter and receiver then light from CFL and fluorescent lighting could be responsible.
A notch filter isn't really applicable to a speaker feed and to add one to say the power amplifier input would require considerable circuitry (opamp based) to generate a deep enough notch. You will never eliminate hum with simple capacitor filters, you need attenuation of perhaps 50 or 60 db to make it inaudible.
You need to find where in the chain the hum is being introduced, and if using an open optical transmitter and receiver then light from CFL and fluorescent lighting could be responsible.
A notch filter isn't really applicable to a speaker feed and to add one to say the power amplifier input would require considerable circuitry (opamp based) to generate a deep enough notch. You will never eliminate hum with simple capacitor filters, you need attenuation of perhaps 50 or 60 db to make it inaudible.
Go to images.google.com and do a search for "optical communication circuit". Tons of stuff will pop up.
- Status
- Not open for further replies.