Belt Gizmo

The Belt Circuit or "Belt Gizmo" is the motherboard from a Sanyo ICC-808D, a four function desktop calculator from 1972.
The display is comprised of eight nixie-tubes and a letter box. The motherboard connects to a smaller "daughterboard" via a black coil cord. It sits in a modified tape measure holder worn on the pistol belt.
While the true purpose of this device has not been revealed, it is commonly thought by fans to be some sort of warning or read-out system for the Proton Pack. Featured in both films and the 2009 video game, it is nonetheless an integral part of the Ghostbusters' equipment. NOTE: Not every Sanyo ICC-808D circuit board is identical. For some time, the company frequently changed the chipset and other electronics of the calculator without modifying the outer case. However, due to the rarity of nixie-tube calculators from this era, most fans opt to build their own replicas from scratch or existing components.