The new Pixelblaze has 8 pins on the board. What pins are required for the Sensor Expansion Board to attach to the Pixelblaze. All seven starting from ground? Certain ones only? If I just wanted to use pins and jumper wires, how many wires are required for the Sensor Expansion Board to work properly?
You just need 3 wires from the PB to the expansion board: GND, 3.3v, and Data. Note that the data pin is labeled TX on the expansion board and labeled RX on the PB. So counting up from the bottom in your photo, it’s pins 1, 3, and 4.
IO26 (and IO25) support digital output (setting the pin high to 3.3V or low to 0V), as well as analog and digital input. The additional IO pins exposed on the underside pads also support capacitive touch sensing. Just be mindful that you cannot source or sink much current through these pins. You’ll need a digital switch circuit to service higher currents.
For an example of how to use them as a digital output, check out this code.