Connecting arduino to speaker using 2n3904 npn transistor

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hello studying electronics , using arduino create musical instrument project. want connect digital output pin 6 watt 8 ohm speaker , use tone() library play different notes! know need resistor connect speaker digital pin prevent drawing current pin. want use 2n3904 amplify current speaker. have connected following circuit digital output pin , working well:

https://www.circuitlab.com/circuit/fqbp6z/speaker-output/

however have know report of project how calculate resistor should use? speaker max power 7 watts used p = (i^2)*r p = 7 watts , r = 8 ohms max = 0.935 a. therefore ic (from transistor) has smaller 0.935 a. right?
also voltage out of digital output pin 5 volts , max output current these pins 40ma therefore resistor have use limit current drawn pin has bigger 125 ohms (using v = ir)? correct?  how calculate ic ib transistor? datasheet here:

http://www.fairchildsemi.com/ds/2n/2n3904.pdf

i want choose resistance not draw current pin while supplying current base of bjt gets amplified current less max allowed current speaker. ic has less 200 ma aswell think (max ic datasheet!). need show these calculations report of project. if can me appreciate it!

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hello studying electronics

did search "speaker transistor" etc. in forum?? study means research..
http://forum.arduino.cc/index.php?topic=171105.0


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