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LED Necklace

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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 ?

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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:

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.

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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

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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.

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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

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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.

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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.

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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.


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The transistors are now connected backwards.

Yes, the 2N3904 or 2N4401 will do, it really isn't critical.

The voltage is too low for two LEDs connected in series, 0.5V isn't enough headroom and if you'd read my previous post, you would have realised that the batteries won't last very long.

I suggest you build the simple circuit, then go for the peak detector if you're not satisfied with the performance. No offence intended but you seem to be having difficulty with getting the simple circuit right which indicates that you need to gain more experience before moving on to something more complicated.

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Wouldn't the 7.5 volt circuit last just as long as a 6 volt one ? The 7.5v draws 200mA, the 6v draws 400mA. 7.5v: 200mA drawn until 6v would last roughly 4 hours. 6v: 400mA drawn until 3v would also last roughly 4 hours. I'm not trying to start an arguement, just trying to point that out.

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The voltage is at 6 volts, and the main resistors are 22 ohms. The two capacitors arer also 10 uf, like you suggested. So, the olnly thing I have to change is the resistors in front of the leds ? Also do I have the ground connected correctly ? I wasn't sure if that was where it went or not.

Thanks for your time.

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