Ultrasonic Headtracker Mouse

I recently rebuilt a headset for a head tracker mouse for a paraplegic chap

This instructable contains a bit of information that might help anyone else trying to do the same.

The unit that this headset works with is the PRC Headmaster, Prentke HM-2P. Its a fairly old design but it is based around ultrasonic transducers and time of flight calculations.

A bit of info on the headmaster (from here:

The HeadMaster Plus, model HM-2P, is a headpointing mouse emulator system designed to replace a 
computer mouse and keyboard for people who cannot use their hands but have good head control. 
An ultrasonic transmitting unit sits atop the monitor and sends signals to a lightweight headset 
that the user wears. HeadMaster translates changes in the user's head position into changes in the 
cursor's position on the screen. This positioning is relative; i.e., the user does not have to center
his head directly in front of the screen. Lightly puffing into a tube connected to the headset is equivalent 
to pressing the mouse button. HeadMaster is sold as a separate component. The Headmaster system is compatible 
with the Macintosh mouse a two- or three-button IBM serial mouse or the IBM PS/2 mouse. 

COMPATIBILITY: For use on Macintosh and IBM and compatible PC. 

SYSTEM REQUIREMENTS: A 512K or Mac Plus (for Apple systems); hard disk and printer are options. 

OPTIONS: Remote adapter, laptop adapter, cable for Sun Workstation, and a set of 10 extra puff tubes. 

DIMENSIONS: 5.5 x 5.13 x 1.5 inches. WEIGHT: 5 pounds.