To get a better match between the output range of the motor and the input range of the analog-to-digital converter in the Imp (at least at sub-hurricane wind speeds), the motor is connected to the imp through a small op-amp circuit mounted in the tube that supports the motor.    It consists of an LM358 op-amp, two resistors and a pair of 0.1uf disk ceramic capacitors to help with noise suppression.  You can either build it on the pins of the op-amp or grab the Gerber files for a little printed circuit board from the same Github repository as the the WxImp code.

Cut one of the Grove Sensor Cables in half and solder the four wires to the output side of the circuit, matching the colors of the cables to the markings on the board or the schematic above.   The end of the cable with the connector connects to grove connector on the Imp Explorer nearest the USB connector.

The 7mm Uxcell motors in the parts list come are designed for small quad copters and so they come in “left” and “right” versions.  If the motor you use has a black wire and a white wire, connect the white wire to the + rotor input and the black wire to the – rotor input.   If the wires are red and blue, the blue wire goes to plus and the red wire to minus.