Infrared "Hot Spot" Scanning Device
Theta Engineering did optical, electrical and firmware design and development of a rugged and highly sensitive hand-held battery-powered infrared detector device designed for use by fire fighters. This project included elements of optics, acoustics (getting sufficient volume from speaker), motion control (precise control of speed and phase of chopper motor), low-noise circuit design, low-power techniques, and signal processing in firmware. An Atmel AVR microcontroller was used in this project along with a serial EPROM for the storage of configuration and calibration information.
This project was perhaps Theta Engineering's greatest engineering challenge as it involved extracting microvolt signals from a high-impedance optical transducer which was immediately adjacent to a vigorous noise source: the motor that turned the optical chopper disk. This low-cost (i.e., not shielded) DC motor tended to emit electrical interference in the form of hash from the brushes and noise from the PWM drive. Through use of a synchronous detector circuit and careful filtering of the signal spectrum to maximize noise rejection, the performance ultimately achieved far exceeded requirements. Fortunately, the design also included multiple points of digital calibration, allowing computer-assisted adjustment of sensitivity down to the desired level!