Patent GB-1374993-A (GB)
Title: Vehicle brake indicating circuits
IPC: B60T17/22
Filed: 1971-04-29
Abstract: 1374993 Indicating circuit JOSEPH LUCAS (INDUSTRIES) Ltd 19 April 1972 [29 April 1971] 12250/71 Heading F2E An indicating circuit for use with vehicle dual braking systems 14, 15 arranged in parallel and interconnected by a valve 18 whereby a substantial difference in pressure between the systems 14, 15 causes the valve to isolate the systems from each other, comprises a switch 26 operable by the valve 18 to energize a warning lamp 10 when the systems are isolated from each other and a testing circuit operable when the vehicle starting switch 9 is moved to a position to start the vehicle, the testing circuit when operable serving to energize the warning lamp 10. The braking systems 14, 15 operate the brakes of the wheels 16 of the vehicle and are connected to a brake pedal 17 of the vehicle. The vehicle is provided with a fluid reservoir (not shown), a battery 8 having its negative terminal connected to a line 21 which is earthed and having its positive terminal connected to an ignition switch 9. The latter is movable from its off position to a terminal 13 and can, in addition, contact a terminal 32. The terminal 13 connects to the ingition-controlled leads of the vehicle and the terminal 32 is connected to the starter. The terminals 13 and 32 are connected respectively to the anodes of a pair of diodes 12 and 31. The cathode of the diode 12 is connected to the line 21 through the warning device 10 and a pair of switches 19 and 20 in parallel, the switches 19 and 20 being open until the levels in the reservoirs feeding the systems 14, 15 fall below a predetermined point. The cathode of the diode 12 is also connected to the line 21 through a warning lamp 11 and a switch 27, the latter being closed when the hand brake is actuated. The lamp 10 and the switches 19, 20 are connected to the anode of a thyristor 22, the cathode of which is connected to the line 21, and the anode and cathode of which are bridged by a capacitor 33. The gate of the thyristor 22 is connected to the line 21 through a resistor 37, and is further connected through a resistor 23 to the collector of a P-N-P transistor 24, the emitter of which is connected to the cathode of the diode 12, and the base of which is connected through a resistor 25 and the switch 26 in series to the line 21. The cathode of the diode 31 is connected through a resistor 30 to the bases of a pair of N-P-N transistors 29, 28. The transistor 29 has its collector connected to the lamp 10 and to the switches 19, 20. The transistor 28 has its collector connected to the lamp 11 and to the switch 27. The emitters of the transistors 28, 29 are connected to the line 21. In use when the switch 9 engages the contact 32 to start the engine, the lamps 10, 11 are illuminated for testing purposes. The lamp 10 is off when the engine has been started but the lamp 11 remains illuminated until the handbrake is released. The lamp 10 is illuminated when the levels in the reservoirs feeding the systems 14, 15 fall below pre-determined position in addition to when the valve 18 isolates the systems 14, 15 from one another. In a modification (not shown), the switches 19, 20 are connected to the switch 26 and to the resistor 25. The switches 19, 20 and 26 are therefore in parallel and if any are closed, the lamp 10 remains illuminated until the ignition switch 9 is moved to its off position.