Two-wire adapter for sensing and announcing abrupt changes in a current shunt monitor

摘要

Disclosed is an annunciator circuit for generating a signal in response to a change in the load in a remote sensing circuit. This has application in mass transit vehicles that have passenger presence sensing circuits associated with vehicle doors. The circuit comprises: a remote circuit including a voltage regulator and a sensing circuit; a two wire-adapter circuit; a power supply; and a two-wire electrical conduit. The two-wire adapter circuit is spaced from the remote circuit and comprises: a current shunt monitor which monitors the current in a parallel resistor; and a circuit for sensing an abrupt change in the output of the current shunt monitor. The two-wire electrical conduit extends between the remote circuit and the two-wire adapter circuit, and connects the power supply to the voltage regulator of the remote circuit via the resistor in parallel with the current shunt monitor. The sensing circuit in the remote circuit switches the output of the voltage regulator to a shunt resistor to place an additional drain on the voltage regulator upon detection of a sensed condition. The circuit for sensing the abrupt change in the output of the current shunt monitor due to the additional drain on the voltage regulator is configured to output a defined digital signal on sensing the abrupt change.