I am trying to use the HC – SR04 with the board and have some similar questions. To clarify, are you saying that sensor can actually run fine at 3.3V? I hooked it up to the Sw5 pin and found code that says to power that at 5V write D22 high. Echo and Trig are hooked up to digital pins directly. Are you saying the return voltage from the echo pin will be too high? In addition, I added a DHT 22 temp/humidity sensor. I also directly connected a digital pin and 5v for power. So far nothing has blown up.