Patent GB-1473166-A (GB)
IPC: F16D66/02
Filed: 1974-07-23
Abstract: 1473166 Brake lining wear warning systems NIPPONDENSO CO Ltd and TOYOTA JIDOSHA KOGYO KK 23 July 1974 [28 July 1973] 32506/74 Heading F2E A vehicle brake lining wear warning apparatus include probes 60, 61 which each become electrically non-conductive when an associated brake lining wears to a critical degree, a holding circuit 200 with a flip-flop and reset means, the flip-flop being connected to the probes and being switched into a set condition when a probe becomes non-conductive and retaining the set condition when the probe becomes conductive again, reset means which resets the flip-flop on connection of a power supply irrespective of the probe condition and warning means 300 connected to the flip-flop. As shown in Fig. 3, probes 60, 61 are broken when the wear of respective brake linings reaches a critical value. A warning means 300 includes a warning lamp 7 and a switching transistor 41 for operating it. A warning holding circuit 200 includes transistors 42 and 43, with feedback from transistor 42 to transistor 43 via a resistor 50 and diode 54; 40 is a car engine ignition switch. In operation, when switch 40 is closed, the potential developed at the base of transistor 43 remains low so transistor 43 remains non- conductive; thereby, a voltage derived from the battery 8 and effective between resistors 45 and 47 is applied via diode 52 to the base of transistor 42 and is high enough to render transistor 42 conductive. The output of the latter is now so close to ground potential that transistor 41 remains non-conductive so lamp 7 remains unlit and the feedback to 43 is zero. The warning holding circuit 200 is now in "reset" condition. If either of the probes 60, 61 is broken or a fault occurs in their connections, capacitor 55 charges up until the base-emitter threshold voltage of transistor 43 is exceeded, whereupon 43 becomes conductive, its output falls to near-ground potential, and consequently transistor 42 is rendered non-conductive. In turn transistor 41 is rendered conductive, so lamp 7 is energized. Circuit 200 is now "set". If, during a subsequent brake application, conduction through the broken probe is restored by contact with the brake disc, thus grounding the bias current through resistor 49, transistor 43 is maintained conductive by the feedback connection through the resistor 50. Circuit 200 is thus maintained "set" and lamp 7 remains energized. Fig. 4 shows an "integrated circuit" arrangement, wherein the warning holding circuit comprises a NOT gate 73, and two NAND gates 71, 72, interconnected by feedback circuits to constitute a flip-flop; the power circuit 300 comprises a NOT gate 70 and a light-emitting diode 7<SP>1</SP>. In operation, when the engine key switch is closed, voltage +Vcc is applied. Because of capacitor 74, the output of NAND gate 71 goes to a high level, so the output of NAND gate 72 goes to a low level, and consequently the output of NOT gate 70 goes to a high level or +Vcc. Thus no current flows through the LED 7<SP>1</SP> and circuit 200 is in its "reset" condition. On breakage of a probe, the output of NOT gate 73 changes from high to low level and the flip-flop changes its state. Consequently the output of NAND gate 72 changes to low level and that of NAND gate 71 to high level. The light emitting diode 7<SP>1</SP> is then energized. Even if the output of NOT gate 73 goes back to the high level, the flip-flop does not change its state and the warning is maintained. This condition continues until the engine key switch is opened. In Fig. 5 (not shown) a three-phase generator provided for the vehicle, and a relay, are so connected to the circuitry of Fig. 3 that upon closing of the engine key switch the lamp 7 is automatically tested, but when the engine is eventually started the auxiliary connections are automatically nullified, and the circuit functions as in Fig. 3. Fig. 6 (not shown) depicts similar provision for the circuit of Fig. 4.