You sir, are confusing me!!! :o
I will be using 3 ordinary 74hc595 shift registers driving mosfets. However, I thought this was called multiplexing. Is it not? I based my assumption on this tutorial: http://www.instructables.com/id/Multiplexing-with-Arduino-and-the-74HC595/
The Arduino is not powering the solenoids, only powering the TIP121, which is why I talked about the TIP121 in the first post.
Forget the Mosfets, the solenoids, for a moment and think only of the Arduino pins.
My question is hyper simple. The Arduino has a limit of 40ma per pin (Is this not correct?). The entire Arduino itself is limited at 200ma (from the spec sheets I read). When I use a 1k resistor at the output of a pin it brings down to 5ma. But, 22 pin outs like this is 22 * 5ma, so over the 200ma limit. This is what worries me.
In other words, when you start adding outs to the Arduino, do you have to start worrying about the total current being augmented and perhaps too much to bear for the Arduino.