Short version: Can the digital pins handle 5V input?
Longer version: I hooked my new Spark Core into my existing Arduino project by making my Arduino think the Spark Core was a button. I connected a Spark Core digital pin to an UNO pin configured as INPUT_PULLUP (an input pin with pullup resistor enabled, which supplies low amperage 5V). Sure enough, if I set the Spark pin to output/LOW, the Arduino thinks the button has been pressed. When I set it to HIGH, it thinks the button is released.
I was very happy to see this work, but then I realized I should make sure the digital pins on the Spark Core can handle the 5V I’m feeding it. Hopefully this question makes sense?.. Thanks. Loving the Spark Core, and am totally blown away by the cloud functionality out of the box.