help with Led music sync with tip31C,usb powered

[captianxawsome]

I have read through the forums and i am still confused. i make my circuit like this...
oblivously there are problem because it does work, does anyone have any suggestions since i have seen this posted here but not powered via usb. the audio source is a 3.5mm jack.

 

[MrAl]

Hi,

You need a high enough audio signal to drive the transistor directly. You also should have a base resistor.
How high is your audio drive signal? It should be at least 1v or more.

 

[captianxawsome]

Thank you for commenting,
i am confused about where the base resistor goes. and the audio signal is only powered by an ipod.
should i add a capacitor to increase the signal power?

 

[MrAl]

The base resistor would go in series with the base, but you may need another much higher value base resistor too from the base to ground.

The transistor turns on around 0.7 volts so you need the audio to have at least a 0.7v peak. I dont know what the ipod voltage output is but if it's less than that it wont turn the transistor on.

You could also try a large value base resistor from base to +Vcc as a bias resistor. Then a capacitor could be used to couple in the signal to the base. You'd have to experiment with the resistor value though starting from say 100k and working down to maybe 10k.
The idea is to get the transistor where it almost turns on with the resistor to +Vcc, then when the audio input goes positive it turns the transistor on. This may require a pot. Not sure how reliable this setup would be however, it may need adjustment now and then.

 

[captianxawsome]

if i changed my power input from 5v to 15v would that simplify anything in the circut?

 

[MrAl]

Hi,

Probably not. That's because the power supply voltage is not an issue here. It's the bias of the transistor. It has to be biased unless you have a larger audio input like 2v or at least more than 1v.
You might want to look into bias schemes for transistors.

Im not sure this will work the way you expect it too anyway. Most likely the audio will turn on the transistor when audio is present, and off when not present. It would take some good adjustment to get it to turn on according to the amplitude of the incoming audio in a linear fashion.

 

[captianxawsome]

So is the issue is only in the audio signal?

 

[Reloadron]

The following should be the highest signal output levels from an Ipod:

Ipod headphone jack 2.84 V
Ipod line out jack 2.94 V
Apple Universal Dock "Line Out" 2.01 V
As compared to:
Typical component CD player Line Out 5.76 V

Exactly where are you getting your signal from? Note the ipod signal levels are maximum and that is information I found on the web. That does not make it accurate.

Ron

 

[captianxawsome]

i am getting my signal from an ipod with a 3.5mm audio cord boubght off ebay.

 

[Reloadron]

You can try touching the transistor base lead to the +5 volts. The LED should light assuming things are correctly connected. That is without the ipod connected. I am assuming things are like the picture.

Ron

 

[MrAl]

You can try touching the transistor base lead to the +5 volts. The LED should light assuming things are correctly connected. That is without the ipod connected. I am assuming things are like the picture.

Ron

Ron:
He's got the emitter connected to ground so touching the base lead to +5v might put a lot of current through the base emitter diode right? That might not be good for the transistor.
Do you know if those voltage levels are peak or rms?

captian:
Do you have any smaller transistors to try this with? With the audio control all the way up i would think you would see some small LED activity.

 

[Reloadron]

Good point MrAl, I had a quick momentary in mind but should have suggested using a base resistor. Just to see if the transistor would turn on. Looking at the drawing, it looks to be correctly connected. The only logical explanation I can think of is not enough audio signal?

Ron

 

[MrAl]

Hi Ron,

Yes that's what i thought too. From the numbers you posted it sounds like there might be enough drive too though, so i started to suspect the transistor itself as being faulty or something.
I did a quick test and the TIP31 seems to turn on ok with only a small base current. 24ma output with 1v input and a 1k series base resistor. Turned on ok. So i suspect the transistor or the connections are not right or something simple.

 

[captianxawsome]

i created a darlington pair and everytime i touch the tip of the 3.5mm jack i can faintly see the light on.

 

[Reloadron]

That says (assuming things wired correctly) the transistor(s) are not being driven into saturation. Can you post an actual circuit diagram?

Ron

 

[Blacklockdown]

Perhaps you could do this with your project https://www.youtube.com/watch?v=7s6Dz4HDHn4

 

[captianxawsome]

So just an update on where i am in my project and if anyone can provide assistance...thanks

 

[captianxawsome]

i think the problem is that there is not enough amperage coming out of the audio source

 

[audioguru]

You can try touching the transistor base lead to the +5 volts. The LED should light assuming things are correctly connected.

Why do you want him to burn out the base-emitter of the transistor? It does not have an important series base resistor.

I am confused. In the first post he says, "oblivously there are problem because it does work" so if it works then why fix it?
Engrish translation problem?

 

[audioguru]

i think the problem is that there is not enough amperage coming out of the audio source

The base of a transistor needs 0.001A or less for the transistor to light an LED or a few in series. The output resistance of an ipod or any audio product can easily supply much more current than only 0.001A.

The problem is that a transistor without any biasing needs an input voltage of 0.7V to turn on and an audio source voltage is frequently less.