LED Necklace

[Scott S1]

I am in the process of designing a necklace with about  20 leds, give or take a few. Thats all fine and dandy. I am trying to design the necklace so tha the LEDs will light in response to music. The music wil come from an mp3 player, via a stereo cable. I am having trouble designing a circuit that dosen't take much space, and will accoplish the goal of blinking the leds in response to the music. It would be nice to have the leds get brighter as the music got louder, if possible.
The circuit will be eitther be powered by AA  batteries or 3v 2032 bats, not sure yet. Half of the LEDS will be white, and the other half will be blue. Also i was thinking of putting clear beads over the leds. Would this wash out too much of the color from the leds ?  One more thing, would it be better to use a cat5 ethernet cable or speaker wire ?
Can anyone help me design this necklace ?
Thanks
Scott

 

[Hero999]

I designed a simple circuit that will do this and posted it over on Electrotech Online but don't expect any help from me over there because the idiot administrator banned me for disagreeing with him. If you want help, ask here or on SiliconTronics.com.

Fortunately it can be found on the second Google hit:
http://www.google.co.uk/search?hl=en&client=opera&rls=en&q=electrotechonline+audio+light++Hero999&aq=f&aqi=&aql=&oq=&gs_rfai=
http://www.electro-tech-online.com/electronic-projects-design-ideas-reviews/106030-make-lights-flash-music-circuit-help-needed.html#post866658

It will need modifying though to flash 20 LEDs, i.e. the base resistor will need to be a lower value (try 22R) and the LEDs will need to be connected in parallel, each with its own resistor, for blue/white LEDs use 120R.

You'll need at least 4.5V to power blue/white LEDs but 6V (four AA cells) is recommended for decent battery life.

 

[Scott S1]

In your circuit, will the leds get brighter as the music gets louder ? I just thought of that idea, to me it might make it look better. How could I modify your circuit to adjust the brightness of the leds to the loudness of the music ?

thanks for your time

 

[Hero999]

Yes, that's the idea.

 

[Scott S1]

is there a similar transistor ? I can't seem to find that one locally, in North America.

once again, thanks for your time.

 

[Hero999]

You can use any NPN transistor which can handle 300mA or more, e.g. BC338, PN2222A, 2N2369 etc.

 

[Scott S1]

does voltage of the transistor matter ?

 

[Scott S1]

Also how bright of leds you you think I should use ?

Thank you for your time

 

[Hero999]

1) Think about it.

2) Only you know that.

 

[Scott S1]

Accordng to this schematic

where do the 3 individual wires from the audio input go ?

 

[Hero999]

You only need to use one channel, the 0V goes to 0V and the input can be connected to either the left or right channel. It's possible to use both channels but you'll need a resistor for each, for more information, look at the graphical drawing of the circuit I posted on Electrotech.

 

[Scott S1]

Would I be able to get half of the leds to respond to the right channel and the other half respond to the left channel, while powering the leds from the some power source ?

Again, thank you

 

[Scott S1]

Am I going to have to use 2 transistors since there's gonna be 20 led's in the circuit ?

 

[Hero999]

Yes, you can connect half the LEDs to one channel and half to the other channel but you'll need two transistors for that.

 

[Scott S1]

I found 2 transistors that are similar to the BC548 in your schematic, the 2N3904 and 2N4401. The only difference between the two, that I could find, is the collector current. The collector current of the 2N3904 is 200mA, while the collector current of the 2N4401 is 600mA. Would it be better to have a higher or lower collector current ? Another noob question: What is the importance of capicators ? 
Here is a schematic I made. http://www.flickr.com/photos/51872883@N08/4769178237/
Is this correct, or is there something i forgot to do ?

Thanks for helping

 

[audioguru2]

Please attach your schematic here with your post instead of over at Flickr.
Your LEDs will appear as a dim blur because the transistor circuit is missing a "peak detector" circuit. With a peak detector then each sound peak will be clearly visible.

Your extremely simple circuit uses the transistor as a rectifier so that the LEDs might be lighted for half the total time of a sound. If the sound is with a short duration then you will not see the LEDs light.

 

[Scott S1]

Thank you for pointing that out audioguru. I have attached the schematic. Can you explain how I can alter my circuit to include the peak detector circuit ?
Thanks

View attachment 41085

 

[Hero999]

The circuit will not work because the LEDs are connected backwards.

I don't know if the transistors are connected correctly because you haven't used the correct symbol for them.

Don't worry about the peak detector, although it will improve the circuit's performance, it's not essential. It's not an easy circuit to make and is extra complexity which is not really needed. The LEDs will still flash to the beat of the music without a peak detector. The bass component of the music will be strong enough and last for a long enough to turn the LEDs on for long enough to be visible. The LEDs will start to turn on when the voltage from the earphone socket it exceeds about 0.6V, adjusting the volume will alter the brightness of the LEDs.

Where are you goinh to get 55R resistors from?

I think they need to be lower, if you plan to stick with 4.5V, 47R should do.

Increasing the voltage to 6V would also prolong the battery life, even if it means using AAA cells rather than AA. The batteries may start life with a voltage of 1.5V per cell giving 4.5V in total but it will slowly drop to 1V per cell over their life, giving 1V per cell at the end of their life. The LEDs will typically have a forward voltage of 3.5V at their rated current, dropping to 3V at low currents when they'll be very dim. If you use 6V the LEDs will be brighter for longer and will still light when the voltage per cell drops below 1V per cell, 4V total.

 

[Scott S1]

I have attached an updated circuit. Is everything correct in the schematic ? Is there anything i can do to the circuit to have the leds blink at listening volume as they would blairing, as I would like to attach a splitter so I can listen to headphones while it blinks ? I found a few "simple" peak detector circuits. If I decide to use one, would I put it before the capacitor ? I would use 2 of them, correct ? Would the results of the peak detector be worth installing it ?
Also, would either the 2N3904 or the 2N4401 transistors work ?

Thank you for your time, I really appreciate it.

View attachment 41086