> First, begin with a smoke alarm. A tried and true design that you can buy for $10. Buy the exact parts that are already in use, and put them in your final product. The smoke sensor, the transistors, the through-hole resistors, the Fairchild IC, the 9V, the LED. Buy all of that and use it in your final product.
But why not piggy-back atop of the legacy hardware? Doing that would save time and let the software people solve higher-level (and presumably more interesting) problems.
Besides, software people I know are way to lazy to go sticking their fingers in sockets.
So... why didn't Nest do it this way?