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How to build a circuit that can change the color of an LED with the sound being put out of an MP3 player. We will use a color organ and an amplifier.  

Step 1: Parts List

1 - 8 Ohm Transformer
1 - LM324 Integrated Circuit 
1 - RGB common cathode LED
3 - 1k Resistors
3 - 10k Resistors
1 - 220k Ohm Resistor
1 - 3.6k Ohm Resistor
1 - 6.8k Ohm Resistor
1 - 390 Ohm Resistor
1 - 75k Ohm Resistor
1 - 100k Potentiometer 
2 - .22uf Capacitor
2 - .1uf Capacitor
1 - .01uf Capacitor

Step 2: Build Input

First we have to build the input into the color circuit. 

Connect the transformers green wire to the 10k resistor, then connect that  to pin 2 of the LM324 and also one side of the comparator. Then feed back the comparator back onto itself and to pin 1. Connect the blue wire of the transformer to ground and to a 1k Resistor, then connect the resistor to pin 3. Finally connect pin 11 to the negative side of one nine volt battery and the positive side of the battery to ground. Then connect pin 4 to the positive side of a second nine volt battery and then connect the negative to ground.

Step 3: Build the High Circuit

Bring the output of pin 1 up to a different area on the board. Connect the .01uf capacitor to the output of pin 1 then connect the the comparator to one 10k resistor and pin 12. Then connect the 10k resistor to the ground. Now also connect the 6.8k resistor to the  pin 1 output. Next connect the resistor to pin 13 and the 75k resistor. Now connect the 75k resistor to pin 14.

Step 4: Build the Mid Circuit

Next connect the 390 ohm resistor to pin one output. After connect both the .22uf capacitors to the resistor and one 1k resistor. Take one .22uf capacitor and connect it to pin 9 and to a 10k resistor. Next connect the the other end of the resistor to the second .22uf capacitor and to pin 8. Finally connect the 1k resistor to ground and to pin 10.

Step 5: Build the Low Circuit

Next take the pin 1 output and bring down to the 3.6k resistor. Next connect both the .1uf capacitors and the last 1k resistor to the 3.6k resistor. connect one .1uf capacitor to pin 6 and to the 220k resistor, then to the other .1uf capacitor and pin 7. Then connect the 1k resistor to pin 5 and ground. 

Step 6: Connect the LED

Finally connect the output from pin 14 to the red cathode, connect the output from pin 8 to the green cathode, connect the output from pin 7 to the blue cathode. Then connect the ground pin to ground. 

Step 7: Add an Amplifier and Input.

Finally connect an audio jack to the red and blue wires on the transformer and connect an amplifier of your choosing onto the transformer output of the blue and green wires. This completes the project, have fun!

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