Patent GB-712740-A (GB)
Title: Improvements relating to electrical systems for automatically dimming automobile headlamps
IPC: H05B37/02
Filed: 1952-09-05
Abstract: 712,740. Photo-electric headlamps dimming system. GENERAL MOTORS CORPORATION. Sept. 5, 1952 [Sept. 27, 1951], No. 22322/52. Class 40 (3) In a system for dimming the headlamps of a vehicle in response to light from an oncoming vehicle in which a photo-electric detector feeds an amplifier, in the plate circuit of which is connected a relay for controlling filament switching means, this relay also controls a switch to shunt part of the biasing means in the grid circuit of the amplifier. Thus the sensitivity of the amplifier is increased so that if the lights of the oncoming vehicle are also dimmed the headiamps of the first vehicle are not restored to "bright." As shown, a photo multiplier tube 86 receiving light from the headlamps of the oncoming vehicle is supplied with a voltage derived from the secondary of transformer 14, rectified by tube 54, smoothed by condenser 48 and tapped from a potentiometer 62. The primary of the transformer is supplied from the vehic'e battery through a vibrator 16. The anode current of the multiplier tube passes through resistors 104, 114 and 116 which together form bias resistors for the grid of amplifying tube 102. When the grid is sufficiently negative an anode relay 112 drops out and its contact 142 connects line 2 through the right hand contact of a foot-operated switch 147 to a relay 146, which switches from the "bright" to " dim "filament. Normally relay 112 maintains its moving contact in the left hand position where resistors 114 and 116 are short-circuited. When the relay is de-energized, this short circuit is removed, increasing amplifier sensitivity so that the dim beam from the headlights of the oncoming vehicle is sufficient to maintain relay 146 energized. A switch 156 is provided to earth the fifth stage of the plate-multiplier tube thus reenergizing relay 112, de-energizing relay 146 and restoring the bright beam. Condenser 124 shunting resistors 104 and condenser 126 shunting resistors 114, 116 may be replaced by a single condenser shunting all three and the supply voltage for the photo-multiplier need not be rectified but condenser 160 must then be increased, Figs. 2 and 3 (not shown). So that the headlamps are not retained at "dim" by a bright evening sky the sensitivity of the system may be reduced by shunting the amplifier grid to earth through a resistor and switch, Fig. 5 (not shown). A further overriding control is described, Fig. 3 (not shown) in which re'ay 112 is connected across the supply by earthing the cathode of a rectifier tube the anode of which is connected to the plate of tube 136, the filament being connected to line 2. A still further overriding control is described, Fig. 4 (not shown) in which a switch is connected in the cathode circuit of rectifier 54. An arrangement is also described, Fig. 6 (not shown) in which the foot-operated switch is omitted from the line supplying relay 146. Instead a switch is inserted in supply line 2 which in one position connects the armature of relay 146 to the supply and in the other connects the supply direct to a foot-operated driver's switch rendering the automatic control inoperative.