This might offer a solution for switching the Si570. http://www.amazon.com/Velleman-USB-EXPERIMENT-INTERFACE-BOARD/dp/B0006HJRW4/ref=sr_1_9?ie=UTF8&s=hi&qid=1212283743&sr=8-9
It's a USB Experiment interface board kit offered by Velleman. It has 8 digital outputs, 5 digital inputs, 2 analog inputs and outputs. It may be a little bit over kill, but it's a thought. 73, Tom kz8zz
