TWO- WIRE ADAPTER BACKGROUND OF THE INVENTION Field of the Invention

[0001] This invention relates to an electrical circuit for generating a defined digital signal in response to a change in the load in a remote sensing circuit and has application, for example, in mass transit vehicles that have passenger presence sensing circuits associated with vehicle doors. The electrical circuit generates a defined digital input to the vehicle control computer. Description of Related Art

[0002] This application relates to sensing of the presence of passengers at the doors of mass transit vehicles. ^' The prior art is described in our co-pending application No. 11/084.789 entitled "Capacitance Activated Switch Device" incorporated herein by reference. As explained therein, it is common for vehicle operators to be able to place certain doors in a condition enabling passengers to open such doors by contacting a switch mounted on the side of the door, such as a mechanically-operated touch bar. In our co-pending application, we describe a capacitance-activated switch device attached to the door that senses the presence of a passenger entering an alternating electric field. Electrical conduits supply power to the switch. The electrical conduits are used to signal the presence of a passenger at the door. The conduits are fed through a limited space along the hinged edge of the door. Space is also limited for circuitry attached to the door itself. It is an advantage, according to this invention, to address these space limitations by using the two-wire power leads to the door mounted or remote sensor circuit to signal the passenger presence at the door.

SUMMARY OF THE INVENTION

[0003] Briefly, according to one aspect of this invention, there is provided a two-wire adapter. A two- wire electrical conduit connects a power supply to a load via a current shunt monitor. A circuit is provided for sensing an abrupt change in the output of the current shunt monitor due to a change in the current drawn by the load. The circuit for sensing an abrupt change is configured to output a defined digital signal on a third wire upon sensing an abrupt change.

[0004] Briefly, according to another aspect of this invention, there is provided an annunciator circuit having a remote circuit including a voltage regulator and sensing circuit

and a two-wire adapter circuit comprising a current shunt monitor and a circuit for sensing an abrupt change in the output of the current shunt monitor. A two-wire electrical conduit connects a power supply to the voltage regulator of the remote circuit via a resistor in parallel with the current shunt monitor. The sensing circuit in the remote circuit connects 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 an 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 an abrupt change. Preferably, the annunciator circuit further comprises high-side and low-side drives connected to receive the defined digital output signal.

[0005] According to yet another aspect of this invention, an annunciator circuit comprises an electric field-sensing circuit comprising a remote circuit including a voltage regulator and circuit for sensing the change in capacitance in an ac electric field. A two-wire adapter circuit comprises a current shunt monitor and a circuit for sensing an abrupt change in the output of the current shunt monitor. A two-wire electrical conduit connects a power supply to the voltage regulator of the remote circuit via a resistor in parallel with the current shunt monitor. The sensing circuit in the remote circuit connects 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 an 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 a third wire upon sensing an abrupt change.

[0006] According to still another aspect of this invention, an annunciator circuit for generating an input to a control computer on a mass transit vehicle comprises an electric field-sensing circuit for sensing the presence of a passenger at a mass transit vehicle door, including a circuit haying a voltage regulator and circuit for sensing the change in capacitance in an ac electric field associated with the vehicle door. A two-wire adapter circuit is positioned above the vehicle door comprising a current shunt monitor and a circuit for sensing an abrupt change in the output of the current shunt monitor. A two- wire electrical conduit is connectable to a power supply and extends from above the door and to the voltage regulator of the sensing circuit via a resistor in parallel with the current shunt monitor. The sensing circuit connects the output of the voltage regulator to a shunt resistor to place an additional drain on the voltage regulator upon detection of a passenger at the vehicle door. The circuit for sensing an 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 a third wire upon sensing an abrupt change.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Further features and other objects and advantages will become apparent from the following detailed description made with reference to the drawings in which: [0008] Fig. 1 is a circuit diagram for a two-wire adapter circuit according to one embodiment of this invention;

[0009] Fig. 2 is a circuit diagram for a remote capacitance-sensing circuit according to one embodiment of this invention; and

[0010] Fig. 3 is a simplified flow diagram of the program stored in the microcontroller U13 of Fig. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Referring to Fig. 2, there is illustrated a remote capacitance touch sensor circuit based upon an electric field imaging device that senses objects entering a low level electric field and measures the capacitive loading of the field caused by objects moving into the field. A two-wire input at Pl and P2 supplies power to the remote circuit via voltage controller U21. Associated with integrated circuit U21 are capacitors C21, C24 and C25 and resistors R25 and R26. An integrated circuit U22 (MC33794 sold by Freescale) provides most of the function of the touch sensor circuit. A low frequency sine wave is generated by the integrated circuit U22 which is applied to a volume by electrodes El and E2. Associated with integrated circuit U22 are capacitors C211, C212, C213, C214 and C23 and resistor R21. The digitized capacitance detected by U22 is processed by microcontroller- U23 to detect an abrupt change in capacitance. Associated with integrated circuit U23 are capacitors C26 and C29 and inductor L21. Up to this point, the circuit of Fig. 2 is basically as described in our co-pending application identified above. When the presence of an object is detected, the output of the voltage regulator U21 is shunted through resistor R23. The resistance of the resistor R23 is chosen to provide a small but significant increased load on the voltage regulator. The presence of resistor R23 is a feature new to this application. In the specific embodiment described with reference to Fig. 2, the resistance of resistor R23 is set at 475 ohms so that the increased load will be about 20 mA.

[0012] Referring now to Fig. I ₅ there is shown an embodiment of a two-wire adapter circuit according to this invention. A power supply is indicated at Vbat. The power supply

feeds a voltage regulator UIl that powers microcontroller Ul 3 and shunt monitor IC Ul 2. Associated with voltage regulator UI l are capacitors CIl and Cl 3. Associated with microcontroller Ul 3 are capacitors Cl 5 and Cl 6 and inductor LIl. Associated with shunt monitor U12 are capacitors C17 and C18. A current shunt monitor U12 is connected to each side of a small resistor R13. The power supply is connected to terminals TBl+ and TB2+, for example, which are then connected through a two-wire conduit to respective voltage regulators of the sensing circuits shown in Fig. 2. The terminals TBl+ and TBl- correspond to a sensing circuit for one side of a double swinging door of a transit vehicle, for example. The terminals TB2+ and TB2- correspond to the other door.

[0013] In the preferred embodiment, Ul 6 is placed between Rl 3 and TBl+ and TB2+. If the current draw through Rl 3 is excessive due to an accidental short at TBl+ or TB2+, the microcontroller Ul 3 can disconnect Rl 3 by pulling up enable pin 2 EN or U16 preventing Rl 3 from being destroyed.

[0014] As indicated in Fig. 1, the output voltage of the current shunt monitor in this particular embodiment is twenty times the voltage drop sensed across the resistor Rl 3. For a 20 mA change in current through the resistor Rl 3, the output voltage would change 400 mV. [0015] As shown in Fig. 3, the analog output of the shunt monitor is repeatedly digitized by microcontroller U13 and stored in a rolling memory so that the trend over a short period can be observed and abrupt changes can be recognized. Gradual changes in the current through resistor Rl 3 due to various factors, such as drift in the battery voltage, the temperature of the resistor R13, or the effects of temperature on the circuit shown in Fig. 2 will not result in an output from the microcontroller. When the microcontroller does produce an output, it activates both the high-side and low-side drivers U14 and Ul 5. Typically, only one of those will be connected to the vehicle computer (depending on the vehicle computer's logical input requirements).

[0016] We have described the two-wire adapter circuit in the setting of a capacitance- activated switch used with doors on a transit vehicle. However, it should be clear that any device (proximity switch, Hall switch, etc.) may use the two-wire adapter circuit. [0017] Having thus defined our invention in the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.