Fig. 1

3D parts and wiring. a Photograph of a HOSI unit in the field, running from a smartphone. b 3D assembly. c Parts arranged for 3D printing. d Wiring diagram and required wire lengths. Ground pin on electronic board (GND) (ground) is the shared negative voltage; positive voltage input pin on electronic board (VIN) is the 5v output from the Arduino that is delivered via the USB cable. Note that the spectrometer should be connected to the Arduino’s regulated 5v output (as shown), not VIN that has a slightly lower voltage due to a protective diode in the Arduino