Abstract: An indicator device for providing a high-intensity visual indication of an occurrence comprises a flash tube which is fed with a high voltage from a capacitor, the capacitor being charged by means energized from an intrinsically safe supply. Trigger pulses are applied to the flash tube in response to the occurrence, so that the tube emits flashes in response thereto. An annunciator can comprise a plurality of the devices housed in a single unit and fed from a single power supply.

Abstract: The invention is a communications and tone control system for use between safe and hazardous areas. The invention's field terminals operate in hazardous areas and are limited to low, intrinsically safe power levels by the use of zener barriers and current limiting resistors. The limited D.C. electrical power supplied to the field terminal's amplifier is modulated by field initiated communication and control signals. The resulting duplex transmission allows a strong information carrying signal and amplifier power to pass through a single twisted pair at intrinsically safe power levels. All field stations are connected in parallel to a central control system in a safe area allowing the communication system to operate redundantly. The field stations can be selectably connected to a plurality of output devices by tone controlled switching circuits.