The Mini Digital HCI was originally developed for use on sounding rockets for Sandia National Laboratories then modified to provide either an analog or a digital output for NASA’s Wallops Test Range. The sensor consists of a Lithium Tantalate pyroelectric detector mounted in an aluminum housing. The detector element is 0.75 mm square configured in a current mode circuit. The circuit consists of the pyroelectric crystal connected to an Op-Amp with a shaping circuit to limit low frequency circuit noise and a uP with A/D converter to digitize the signal and perform a rate of change calculation on the data.
The processing in the imbedded software produces a digital pulse output that corresponds to the center of the IR horizon. This technique provides for a consistent output that is independent of spin rate or temperature or detector output variations. The unit contains a narrow band IR filter designed for use at 15um. An A/R coated Germanium lens is incorporated to provide a 3 deg FOV.