I think the primary issue I had was trying to figure out the beta, or gain as others called it, when I asked. One person told me to assume a gain of 100, another person told me to assume a gain of 10, and I was unclear on how anyone comes to the conclusion of a good number, both seem to be guesses, and radically different guesses at that.
I looked over the datasheet to try to figure it out. There are several graphs and several tables of data, but I couldnt figure out which one is the correct figure. If you do the calculations with each figure, you get very different results, and I couldnt rely on any of those calculations to be even close.
It seems to me that the part has a specific beta, you shouldnt have to guess if its 10, 50,100, or whatever. I do understand that at different temperatures, the beta will vary, but at or near room temperature, they should be should be pretty consistent.
Your first calcluation, with the 50 beta estimate is for the base resistor correct? That resistor goes between the arduino, and the base on the transistor, correct?
The second calculation you posted you said was for the collector resistor, Im not sure how/what/why that is about. Is that a resistor between the collector, and ground?
With the base resistor, I basically made a guess, then made some calculations, then tried a few sizes of resistors, making notes along the way. Because I was cascading them, I ended up testing them in tandem, as well as single.
It seems to me that I should have been able to get it a lot closer with the calculations, but having to guess with the beta made the calculations seem pointless.
I should look for my voltmeter, I havnt seen it in like 20 years. Measuring was very difficult, since my minimum was 20ma, and my max was 240 ma, I couldnt use one setting to measure all conditions, and that made measuring relatively inaccurate.
In your second calculations you have 9v -.3, should that be 9v-.7 (saturation voltage)?
When I was doing my testing, I only measured current, but I measured it at a few places.
It would also be good to understand why we need a resistor. its my understanding that he base resistor can be used to limit the current to the collector, but its primary function is to limit the current on the microcontroller, without it, you may draw too much current and break the microcontroller. Is that a fair statement? is there more too it?