Quad-sensor filament guide

This accessory guides (up to two) filaments 1.75mm from rolls into extruders. It has been designed specially for Makerbot/CTC-clone (extruder-steppers in printing head), but can be used without/with small adaptation in any FDM-printer.
PTFE-tubes OD/ID=6/4mm are used as guiding sleeves. Мicro-switches detect mechanically broken filament or tangled roll (by the strain on the guiding tube). Detected failures are indicated by sounding a buzzer. Indication persists until both filaments are present in guide's channels and both guiding tubes are not pulled shrunk by inlaid filament. Early alarm provides enough time for recovering the correct feeding of filament without interrupting the print.
Electronic parts, wiring diagram, printing settings and assembly-instructions - in attached XLS.