Simple Simple Question

Forum 2005-2010 (read only) Hardware Interfacing
1

So first off, digital pins in the arduino when set on output will output 5 volts correct? arduino.cc says each i/o will give off 40 mA dc current (correct me if im wrong cause im entirely inexperienced with electronics).

So if i add for example a 1k resistor into the mix then what is the current now. Im not sure cause i know the formula V = I *R, but arent the current and voltage independent-fixed-values? or is the voltage 5v and is there an internal resistance in the arduino of .. 125 ohms cause 5/125 = 0.040A. thus would i just add on the 1k to that resistance (since its obviously in series) to make it 1125 and my result is 5/ 1125 = 0.004A.

What im trying to do is just set up a 2n2222A transistor (not rocket science, i know) but i read that i should aim for 0.6-0.7 volts into it. So i could do it by trial and error and a volt meter, but id just like to know how to do the calculation to know what resistance would be appropriate.

Any help would be sweeeeeeeeeeeeeeeeeeeet.

2

What are you trying to do with the transistor? Standard BJT npn transistors turn on by having current flow into the base, not by specific voltages. The 0.6 to 0.7 volts just means you have to have at least that much voltage to break down the PN junction. You need the resistor on the base to limit current into the base of the transistor, not limit voltage.

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Please don't cross post, it does annoy people.