Is it possible to drive a tip31 from the signal generated from an mp3 player?

I have been reading through the many many instructables about driving leds from an audio signal, but so far my experiments  have not worked out.  I'm trying to drive one led from the audio signal coming from a pair of earbud style headphones.

Here are some videos demonstrating the circuit.

(I can't get the editor to embed videos today, so please follow the links to view the youtube videos

Proof that the led works

Proof that the transistor works

Hooking up the audio signal - I got nothin

Proof that there is an audio signal coming into the circuit

I'm basing this circuit on the numerous diagrams on instructables for syncing an led to music.  Here are just a couple of them...

Sync LED to music

LED Music Sync Headphones


These and many other instructables show the same circuit and they all seem to be working, so what am I doing wrong?

I hooked the base of the transistor up to the signal coming from one half of a earbud type headphone wire, hooked the ground of said wire to the emitter, and then hooked up the collector/emitter to a circuit with one standard led and 3V worth of batteries. 

I checked the output of the headphone wire with an oscilloscope, and there is definitely audio coming out of it, but it's a very tiny signal, on the order of a few millivolts.  How does such a small audio signal generate enough sound to be heard?  How does anybody drive a transistor off such a tiny signal?   Doesn't it need at leat 0.7 volts to get over the diodey hump of the base-emitter junction. ( I don't know, that was a pidooma, it's been way too long since my last electronics class. )



thanks for any thoughts...

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Got to say, you've done a great job of describing what you've done. The other designs don't work as well as yours - because YOU have made an amplifier, the others basically rely on the switching of the transistor.

Well done.

We could apply a lot more science to the design of the amplifier, but I won't bore you with the details.

Steve
foobear (author)  steveastrouk6 years ago
cool thanks! I want this to be part of an instructable, so I may ask you some more questions! But I'll see how far I can get on my own. Thanks for helping me figure it out!
You're thinking digitally ;-)

You bias the transistor to be in conduction, and the AC sound signal modulates the transistor.

And earphones are very sensitive, they can turn a low voltage (moderate current) into useful noise at the ears.

Steve
foobear (author)  steveastrouk6 years ago
Okay, I've pondered your reply and I still don't get it. I'm updating the question with videos to really demonstrate my problem. Maybe you can just point to the error?
Try sticking a 10K from +supply to the base, and a similar one from base to ground. Put a capacitor between your audio input and the base, a 10uF should be good.

Steve
foobear (author)  steveastrouk6 years ago
Okay, so I've attached the capacitor inbetween the base and the audio signal.  I've biased the transistor with a voltage divider made of two 10K resistors.  See the circuit in the photo.  With the two resistors in parallel to the circuit, the LED is now on in a dim way.  

So, now it seems that all the current is flowing through the two 10 k resistors and abandoning the transistor in the middle.   why should it bother with the transistor when it can go that way?  

So, as you can see in this video.  The signal goes through the capacitor to the base, but it's having no effect at all on the transistor.  It's like the current isn't even bothering to go through the transistor at all.

I don't know why the circuit that works for everyone else isn't working for me..

thanks for any other ideas about it....

here's a video of the circuit
IMG_0831.JPG
foobear (author)  foobear6 years ago
Okay, got it working. It turns out (I think) that the voltage was getting shorted through the speaker of the headphone, so there was no voltage being applied to the base of the transistor.

Here's the video of the working arrangement.

(What I still don't understand is why I have to do all this and nobody else does.)

Thank you!
ijibang6 years ago
Actually the simple schematic as per link https://www.instructables.com/file/FTM8RUEGE4013XS/ works fine.
You first failed because your input signal is too weak.
If your input comes from an amplified autio line, the LED will syn.
I too initially could not get it working using my MP3 input line. When I swith the input line from my PC and crank up the volume, it works beautifully.
Hope this explains your doubt.