US2935646A - Automatic headlamp control circuit - Google Patents

Description

y 1960 E. G. MATKINS 2,935,646

AUTOMATIC HEADLAMP CONTROL CIRCUIT Filed June 23. 1958 3 Sheets-Sheet 1 IN VEN TOR.

I iaw A T TOR/V5 Y May 3, 1960 E. G. MATKINS AUTOMATIC HEADLAMP. CONTROL CIRCUIT Filed June 23, 1958 3 Sheets-Sheet 2 m w m m ATTORNEY NQ h m w Q M u Wk v Q I \N w w W l Q United States Patent AUTOMATIC HEADLAMP CONTROL CIRCUIT Eugene G. Matkins, Anderson, Ind., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application June 23, 1958, Serial No. 743,781

17 Claims. (Cl. 31583) This invention relates to automatic control systems and more particularly to light responsive systems for automatic control of vehicle headlamps.

Automatic control systems for vehicle headlamps are commonly used for switching the headlamps from the upper beam to the lower beam when an oncoming vehicle is encountered and vice versa when the vehicle has passed. In such systems, it is desirable to employ circuit components which may be energized at the low voltage level of the vehicle battery to obviate the need for a high voltage p'ower supply. In the control circuit, this is accomplished by utilizing low voltage electron tubes in conjunction with a transistor as a switching device for a control relay. In order to obtain positive switching action, the amplifier stages are connected in bistable or flip-flop circuit fashion and regenerative feedback is derived from the coil of the control relay itself to assure positive switching action. 1

In the light sensing or pick-up device, low voltage operation is achieved by the use of a photoconductive cell in lieu of the more commonly used photomultiplier tube which requires high voltage energization. The photoconductive cell, such as a broad area cadmium-selenide cell, has the disadvantage of a long time constant or slow response to changes in light intensity. In order to achieve the fast response necessary for automatic headlamp control systems, the photoconductive cell is connectedwith the input circuit of a self-biased amplifier stage in such a manner that the self-bias voltage change is delayed with reference to the signal voltage. This is preferably accomplished by utilizing a self-biased amplifier stage having a condenser connected in parallel with the cathode resistor to prevent rapid change of the cathode voltage. Since this arrangement would permit flashes of light of less intensity than the dimming level to cause dimming action, the condenser is switched out by the control relay when it is in the upper beam position or alternatively, an

additional condenser may be connected across the input circuit from grid to ground when the control relayisin the upper beam position.

In such automatic headlamp control systems, the sensitivity of the system is caused to change from a dim sensitivity control value to a hold sensitivity control value when the headlamps are switched from upper beam to lower beam. This sensitivity change prevents the control system from returning the headlamps to upper beam in response to the decreased light when the headlamps of an oncoming vehicle are switched from upper to lower beam. In the inventive system, the change in sensitivity is obtained either by changing the voltage across the photoconductive cell or by changing the load resistance of the cell as an incident to operation of the control relay. The overall system sensitivity is changed by a bias adjustment of the amplifier which changes the dim control sensitivity and the hold control sensitivity values proportionally.

A more complete understanding of this invention may "ice be had from the detailed description which follows taken with the accompanying drawings in which:

Figure 1 is a schematic diagram of one embodiment of the invention; and

Figures 2 and 3 are schematic diagrams showing modifications of the system of Figure 1.

Referring now to the drawings, there are shown illustrative embodiments of the control system especially adapted for automatic control of the headlamp circuit 10 of an automotive vehicle. In general, the system comprises a pick-up unit 12 including a photoconductive cell 14 and which is mounted on the vehicle in a suitable location to intercept the light beam from the headlamps of an oncoming vehicle. An automatic control unit 16 for the vehicle headlamps is connected with the pick-up unit through a connector device 17 and is suitably located within either the passenger or the engine compartment. The control unit 16 comprises a direct coupled amplifier which includes electron tube stages 20 and 22 and transistor stage 24. The transistor stage 24 energizes a control relay '26 which in turn controls the energization of a power relay 228 for switching the headlamp circuit 10'. A manual control unit 18 is located for convenient operation by the driver of the vehicle. The entire system is energized directly from a low voltage source or battery 30.

The photoconductive cell 14 is suitably of the type known as a broad area cell which may comprise a photosensitive material of cadmium-selenide. A suitable broad area cell of this type is manufactured by the Radio Cor poration of America and identified as type C-72l8. The photoconductive cell exhibits a variation in electrical resistance in response to incident light and thus a current variation corresponding to light intensity is developed when a voltage is impressed across the cell. The supply voltage circuit for the cell 14 includes the battery 30 which has one terminal connected to a point of reference potential or ground and the other terminal connected through a conductor 32, terminal 34, conductor 36, resistor 38, and a potentiometer resistor 40 which is. con nected to ground through a sensitivity switching circuit including a conductor 42 and the control relay 26. To apply an adjustable value of voltage across the cell 14, a movable contact 44 on potentiometer resistor 40 is connected through terminal 4-6 to cell 14 and thence through a terminal 48, current limiting resistor 50 and a variable load resistor '52 to ground. Accordingly, a signal voltage corresponding to incident light intensity is developed across the load resistor 52 and in the absence of light the signal voltage is substantially Zero.

The response of the photoconductive cell to light intensity is a function of the temperature of the cell and it is desirable to stabilize the temperature of the cell at some value greater than the ambient temperature. This temperature stabilization is provided by a cell oven 54 which suitably surrounds the cell and includes a resistance heater element 56 and a temperature responsive switch 58 which are serially connected through a conductor 60 and the conductor 32 to the battery 30.

In the control circuit, a direct-coupled amplifier controls the energization of the control relay 26. The control relay includes an energizing coil 88, a pair of fixed contacts 72 and 73, and a movable contact 74 which is spring biased into engagement with fixed contact 72. The first amplifier stage 20 comprises a triode electron tube adapted for low voltage energization and having its plate connected through a plate load resistor 62 to the supply voltage conductor '36. The cathode is connected to ground through a cathode bias or self-bias resistor 64 which is connected in series with a voltage divider resistor 66 across the supply voltage conductor 36 to develop a cathode bias voltage. A condenser 68 is connectib le in parallel with the cathode resistor 64, through a conductor 7t and contacts 72 and 74 of the control relay 26, and with the cathode resistor 64 formsa time constant circuit for a purpose to be described hereinafter. The bias conditions, i.e., the quiescent value of electrode voltages developed by the aforementioned circuits, on the amplifier stage 2% are such that it is substantially non-conductive in the absence of a signal voltage across resistor 52.

The amplifier stage 22 is also an electron tube adapted for low voltage energization and the plate is connected through plate load resistor 76 to the supply voltage conductor 36. The output voltage from the first amplifier stage 20 is derived from its plate and applied directly to the grid of the amplifier stage 22. The cathode of amplifier stage 22 is maintained at a positive voltage with respect to ground by connection through a conductor 78 and a terminal 80 to the movable contact 82 of a voltage divider 84 which is energized from the supply voltage conductor 60. The grid of amplifier stage 22 is connected through resistor 86 and the energizing coil 88 of control relay 26 to ground. Thus, the plate load resistor 62, resistor 86 and the resistance of coil 88 form a voltage divider which maintains the plate of amplifier stage 20 at approximately half the supply voltage value. The bias conditions, i.e., the quiescent value of electrode voltages developed by the aforementioned circuits, on amplifier stage 22 are such that it is conductive due to the plate voltage of stage 20 when the latter is non-conductive in the absence of light on the cell 14.

The transistor stage 24 suitably comprises a transistor of the PNP type with the emitter electrode connected directly to the supply voltage conductor 36. The base electrode is connected through conductor 90 to the plate of amplifier stage 22 and the collector electrode is connected through the energizing coil 88 of relay 26 to ground. A condenser 94 between the base and collector electrodes provides degeneration for the alternating voltage components to prevent oscillation. The connection of the collector electrode through resistor 86 to the grid of amplifier stage 22 provides positive feedback which causes amplifier stages 22 and 24 to function as a bistable or flip-flop circuit. In the absence of light, the plate voltage of amplifier stage 20 on the grid of amplifier stage 22 permits current in the emitter to base circuit of the transistor which permits sufiicient current to flow in the emitter to collector circuit to energize or pull-in the control relay 26 and cause the movable contact 74 to engage the fixed contact 73. In condition, the control relay is effective to switch the headlamps to the upper beam.

The control relay 26 is adapted to control energization of the power relay 28 and is connected therewith through the manual control unit 18. The manual control unit includes a control switch which has a movable contact 96, and a fixed contact 98 corresponding to automatic control, and a fixed contact 100 which is an open circuit. The fixed contact 73 of control relay 26 is connected through a terminal 104 to the fixed contact 98 of the control switch. The movable contact 96 is connected through the energizing coil 106 of the power relay 28 to the battery 30. Thus when the control relay is energized,

in the absence of light on the photocell, the power relay- 28 is energized by the battery. The manual switch 18 also includes an override switch 102 which is normally open. The override switch 102 has its fixed contact connected through a terminal 188 and the conductor 78 to the cathode of amplifier stage 22 and its movable contact connected to ground. Thus when the override switch is closed, the amplifier stage 22 is caused to be conductive regardless of light on the cell 14 and the control relay is energized.

The power relay 28, having the energizing coil 106, includes a pair of fixed contacts 110 and 112 and a movable contact 114. The movable contact is spring biased into engagement with the fixed contact 118 and is connected with the battery 30. The headlamp circuit 10 includes a lower beam filament ,116 connected between the fixed contact and ground and an upper beam filament 118 connected between the fixed contact 112 and ground. When the power relay is deenergized, the lower beam filament 116 is energized and when the relay is energized, the upper beam filament 118 is energized.

To consider the operation of the inventive system, assume that there is no light on the photoconductive cell 14. Accordingly, the cell exhibits a high resistance and the signal voltage across the resistor 52 is substantially zero and the amplifier stage 20 is essentially non-conductive. Since the grid of amplifier stage 22 is held at the voltage of the plate of amplifier stage 2i and is approximately equal to its cathode voltage, the amplifier stage 22 is conductive. Accordingly, the plate to cathode circuit of amplifier 22 presents a low resistance in the base circuit of the transistor stage 24 and emitter to base current flows. This permits current flow from the emitter to collector through the energizing coil 88 of the control relay 26 and the relay is sufficiently energized to cause movable contact '74 to engage the fixed contact 73. Thus, the power relay 106 is energized from the battery 3t: and the movable contact 114 engages the fixed contact 112 to energize the upper beam filament 118 of the eadlamp circuit.

With the control relay 26 energized, the sensitivity switching circuit, including potentiometer resistor 40 and conductor 42, is connected to ground through fixed contact 73 and movable contact 74. The voltage applied across the cell 14 is thus determined by the position of the movable contact 44 on potentiometer resistor 49 and the value of signal voltage across resistor 52 for a given light intensity is thus established. The potentiometer resistor 40 is termed the dim sensitivity control and permits adjustment of the value of light intensity at which the control relay will drop out or become deenergized.

Consider now the dimming or lower beam switching cycle. As the light intensity on the photocell T4 is increased, as by the headlamps of an oncoming vehicle, the signal voltage developed across the resistor 52 becomes increasingly positive. Consequently, the amplifier stage 20 becomes increasingly conductive and the voltage on the plate thereof diminishes. As a result, the voltage on the grid of amplifier stage 22 becomes less positive and reduces the conduction of amplifier stage 22. This decreases the emitter to base current in the transistor stage 24 and hence the emitter to collector current through the energizing coil 88 of control relay 26 decreases. The decrease of current through the energizing coil 88 causes its magnetic field to collapse and produce a negative-going voltage which is applied through resistor .86 in a regenerative sense to the grid of amplifier stage 22 to further reduce the grid voltage and drive the amplifier stage 22 to the nonconductive state to deenergize the relay 26. This causes the movable contact "1'4 to engage the fixed contact 72 thereby interrupting the energizing circuit for the power relay 28. Accordingly, in the power relay, the movable contact 114 engages the fixed contact 110 and energizes the lower beam filament 116 in the headlamp circuit.

The deenergization of the control relay 26 also interrupts the sensitivity switching circuit extending from potentiometer resistor 40 through conductor 42 to ground. (Donsequently, the entire voltage on supply voltage condoctor 36 is applied from the movable contact 44 across the photoconductive cell circuit. This increased voltage across the photocell 14 increases the sensitivity of the photocell circuit by a factor of 10 to 12 and therefore the same signal voltage will be developed across load resistor 52 by a light intensity of much lower value.

The value of signal voltage applied to the grid of amplifier stage 20 when the control relay 26 is deenergized may 'be adjusted by the variable load resistor 52. Thus the value of light intensity at which the control relay 26 will be reenergized may be established by adjustment of the resistor 52 which is termed the hold sensitivity control.

With the control relay 26 deenergized and the lower beam filaments energized, the system is in readiness for the return or upper beam switching cycle. The time constant of the cell 14 is appreciably longer than the acceptable time constant for automatic headlamp control and to improve the system performance, the time constant circuit including condenser 68 and resistor 64 are provided. When the light intensity on the cell 14 decreases rapidly, as occurs when an oncoming vehicle passes, the

signal voltage across the load resistor 52 decreases at a rate determined by the time constant of the cell. The time required for the signal voltage to decrease to the hold value is of the order of one second whereas it is desired to return the headlamp circuit to the upper beam withina time interval of 0.2 to 0.3 second. The condenser 68 is connected between cathode and ground when the control relay 26 is in the lower beam position and therefore is charged to hold the cathode of amplifier stage 20 at a positive voltage relative to ground. Upon the sudden decrease of light intensity, the grid voltage "starts to decrease immediately but the cathode voltage decreases at a rate determined by the time constant of condenser 68 and resistor 64. Consequently, there is a time delay in the voltage change of the cathode preventing it from tracking the grid voltage and a sufiicient difference betwen the grid and cathode voltages is achieved within the required time interval for headlamp switching. Since the conduction of amplifier stage 20 is immediately decreased, the amplifier stage 22 becomes more conductive and its plate voltage decreases. Accordingly, transistor stage 24 becomes increasingly conductive and the increased current flow through the coil .88 causes a regenerative voltage to be fed back through resistor 86 to the grid of amplifier stage 22 to increase .theconduction therethrough sufiiciently to energize the control relay 26.-

If the time constant circuit, including condenser 68, were connected during the dimming cycle, dimming action would result from flashes of light of less intensity than the dimming level. A sudden increase of light in- :.tensity, as by a momentary flash, would cause the grid voltage of amplifier stage 20 to increase rapidly and the condenser 68 would tend to prevent the cathode voltage .from increasing therewith and the amplifier stage 20 would become conductive rapidly. To prevent this undesirable operation, the time constant circuit, including condenser 68, is interrupted during the dimming cycle by .the control relay 26 when its movable contact 74 engages ;the fixed contact 73.

' System sensitivity, which may be considered in terms ;of distance from the oncoming vehicle at which dimming poems, is provided by the movable contact 82 on the potentiometer resistor 84. The position of the movable --contact 82, which is accessible to the vehicle driver, deytermines the cathode voltage in amplifier stage 22 and adjustment thereof changes the dim sensitivity and the .hold sensitivity proportionally. Additionally, it is noted .that the resistor 38 and the potentiometer resistor 40 are connected across the contacts of the control relay to minimize the destructive eliect of switching the power relay current. The resistor 50 in the photoconductive cell circuit limits the current flow therein to prevent the ,grid signal voltage across load resistor 52 from becoming .sufliciently positive to cut-off the plate current of the xamplifier stage 20.

The modification of the circuit as shown in Figure 2,

involves the hold and dim sensitivity controls. The load resistor 52 is a fixed resistor to prevent variation of the grid circuit resistance of amplifier stage 20 which tends '.'to shift the keying point of the amplifier. Additionally, a filter condenser 120 is connected across the fixed load resistor 52'. The hold sensitivity control is adjusted by changing the voltage across the photoconductive cell ch cuit and for this purpose, a potentiometer resistor 40' in series with the resistor 38 is connected across the supply voltage conductor 36 and ground. The movable contact 44' of potentiometer resistor 40' is positioned to establish the desired value of hold sensitivity. The dim sensitivity control is provided in the sensitivity switching circuit 42 by a variable resistor 122 which is adjusted to establish the desired dimming level. Otherwise, the circuit configuration and operation is the same as that of the circuit in Figure 1.

' Additional modifications of the inventive system are represented by the circuit of Figure 3. In Figure 3, the energizing circuit for the photoconductive cell circuit extends from the battery 30 through conductor 32, terminal 34 to terminal and thence through conductor 60' and the potentiometer resistor 130 to ground. The photoconductive cell circuit is connected between the movable contact 132 of the potentiometer resistor 130, through the photoconductive cell 14, terminal 48, current limiting resistor 50 and then in parallel through variable load resistor 52 and the sensitivity switching circuit 42' which includes variable resistor 124 and shunt condenser 126. The variable load resistor 52 serves as the hold sensitivity control and the variable resistor 124 serves as the dim sensitivity control. In this arrangement, the sensitivity switching circuit 42, which is interrupted by the control relay on the dimming cycle, modifies the signal voltage by a change of load resistance in the photoconductive cell circuit rather than by change of voltage across the photoconductive cell circuit as in the circuits of Figures 1 and 2. The system sensitivity in this arrangement is adjusted by positioning the movable contact 132. of potentiometer resistor 130 which alters the voltage across the photoconductive cell circuit and changes the dim and hold control levels proportionally.

The speed up of switching on the return or upper beam switching cycle is provided by the condenser 68 connected across the cathode bias or self-bias resistor 64 and forming a time constant circuit therewith. In order to] counteract the effect of the condenser 68' on the dimming or lower beam switching cycle, the condenser 126, forming a time constant circuit with variable resistor 124, causes a time delay in the change of grid voltage in amplifier stage 20 corresponding to the time delay in the change of cathode voltage developed by the condenser 68f.

An additional modification, shown in the circuit of Figure 3, causes the upper beam headlamp circuit to be energized when the power relay is deenergized and the lower beam headlamp circuit to be energized when the power relay is energized. In this arrangement, the upper beam filament is connected between the fixed contact and ground and the lower beam filament 116 is connected bet-ween the fixed contact 112 and ground. Additionally, the connections to the control relay are modified so that the fixed contact 72 is connected to the power relay energizing coil 106 and battery 30 so that the power relay is deenergized when the control relay is energized. The fixed contact 73 is connected with the sensitivity switching circuit 42 to connect this circuit to ground when the control relay is energized on the upper beam switching cycle. A fixed resistor 128 is connected between the supply voltage conductor 36 and a fixed contact 72 of the control relay to minimize the destructive effect of current interruption on the contacts of the control relay. A condenser 94' is connected across the energizing coil 88 of the control relay to bypass the alternating voltage components to prevent oscillation of the system. Otherwise, the circuit configurations and operation is the same as described with reference to Figure 1.

Although the description of this invention has been given with respect to a particular embodiment, it is not to be construed in a limiting sense. Numerous variations and modifications within the spirit and scope of the invention will now occur to those skilled in the art. For

7 a definition of the invention, reference is made to the appended claims.

I claim:

1. An automatic control system for vehicle headlamps having upper and lower beam circuits, a light responsive device for developing a signal voltage and having a time constant longer than that desired for switching from one beam circuit to the other, an amplifier having an input circuit including said device and having an output circuit, a relay connected with said outptu circuit for energization thereby and including switching means operati 'ely connected with said upper and lower beam circuits for selective energization thereof, said amplifier including a biasing means developing a, bias voltage corresponding to the conduction in its output circuit, and timing means connected to said biasing means and to said switching means to delay change of said bias voltage when one of said beam circuits is-energized, said timing means being disconnected from said biasing means to permit instantaneous change of said bias voltage when the other of said beam circuits is energized.

2. An automatic control system for vehicle headlamps having upper and lower beam circuits, a light responsive device for developing a signal voltage and having a time constant longer than that desired for switching from one beam circuit to the other, an amplifier having an. input circuit including said device and an output circuit, a relay connected with said output circuit for energization thereby and including switching means operatively connected with said upper and lower beam circuits for selective energization thereof, said amplifier including a self-biasing resistor in its input circuit, and a timing condenser being connected across said resistor through said switching means to delay the chmge of self-bias when one of said beam circuits is energized and being disconnected from said resistor by said switching means to permit instantaneous change of said self-bias when the other of said beam circuits is energized.

I 3. A light responsive control system comprising a voltage source, a photo-conductive cell and a load resistor serially connected across the voltage source, said photo-1 conductive cell having a time constant longer than that desired for the system, an electron tube having plate, cathode, and grid electrodes, means applying a voltage between the plate and cathode electrodes, a self-biasing resistor and said load resistor being connected between the grid and cathode electrodes for supplying the signal voltage to the grid corresponding to the photoconductive cell current variations, relay means connected with the plate electrode and responsive to plate current variations, a condenser connected across said self-biasing resistor to prevent instantanenous change of voltage of said cathode electrode whereby the grid voltage changes at a greater rate than the cathode voltage to cause the plate current to change at a greater rate than the photoconductive cell current.

4. A light responsive control system comprising a voltage source, a photoconductive cell and a load resistor serially connected across the voltage source, said photoconductive cell having a time constant longer than that desired for the system, an electron tube having plate, cathode, and grid electrodes, a cathode resistor connected between the cathode electrode and ground, means applying a voltage between the plate electrode and ground, said load resistor being connected between the grid electrode and ground for supplying the signal voltage to the grid corresponding to the photoconductive cell current variations, relay means connected with the plate electrode and responsive to plate current variations, a condenser connected across said cathode resistor to prevent instantaneous change of voltage of said cathode electrode whereby the grid voltage changes at a greater rate than the cathode voltage to cause the plate current to change at a greater rate than the photoconductive cell current.

5. A light responsive control system comprising a.

voltage source, a photoconductive cell and-a load liesistor serially connected across the voltage source, said photoconductive cell having a time constant longer than that desired for the system, an amplifier having input and output circuits andincluding an electron tube having plate, cathode, and grid electrodes, a self-biasing resistor connected between the cathode electrode and ground, means applying a voltage between the plate electrode and. ground, said load resistor being connected in said input circuit between the grid electrode and ground for supplying a signal voltage to the grid corresponding to the photo,- conductive cell current variations, a relay having an energizing coil, a movable contact and a fixed contact, one contact being connected to ground and said coil being connected in said output circuit for energization thereby, a condenser connected between the cathode electrode and the other contact whereby the condenser is, connected across the self biasing resistor when the contacts are closed to prevent instantaneous change of voltage of said cathode electrode and allow the grid voltage to change at a greater rate than the cathode voltage to speed up the change of energization of said relay when the light intensity changes, said condenser being disconnected from across the self-biasing resistor when the contacts are open whereby the grid voltage and the cathode voltage change at the same rate.

6. A light responsive control system comprising a voltage source, a photoconductive cell and a load resistor serially connected across the voltage source, said photoconductive cell having a time constant longer than that desired for the system, an amplifier having input and output circuits and including an electron tube having plate, cathode, and grid electrodes, a self-biasing resistor connected between the cathode electrode and ground,

means applying a voltage between the plate electrode and ground, said load resistor being connected in said input circuit between the grid electrode and ground for supplying a signal voltage to the grid corresponding to thereby, a first timing condenser connected across said self-biasing resistor to prevent instantaneous change of voltage of said cathode electrode and allow the grid voltage to change at a greater rate than the cathode voltage to speed up the change of energization of said relay when the contacts are open, a second timing condenser connected across said load resistor through said contacts whereby the grid voltage and the cathode voltage change at the same rate when the contacts are closed.

7. A light responsive control system comprising a voltage source, a photoconductive cell and a load resistor serially connected across the voltage source, said photconductive cell having a time constant longer than that desired for the system, an amplifier having input and output circuits and including an electron tube having plate, cathode, and grid electrodes, at self-biasing resistor connected between the cathode electrode and ground, means applying a voltage between the plate electrode and ground, said load resistor being connected in said input circuit between the grid electrode and ground for supplying a signal voltage to the grid corresponding to the photoconductive cell current variations, a relay having an energizing coil, a movable contact and a fixed contact, one contact being connected to ground and said coil being connected in said output circuit for energization thereby, a first timing condenser connected across said self biasing resistor to prevent instantaneous change of voltage of said cathode electrode and allow the grid voltage to change at a greater rate than the cathode voltage to speed up the change of energization of said relay when the contacts are open, a second timing condenser and an additional load resistor connected across the first-mentioned load resistor throug said contacts whereby the grid voltage and the cathode voltage change at the same rate when the contacts are closed.

8. An automatic control system for vehicle headlamps having upper beam and lower beam circuits, a voltage source, a photoconductive cell and a first resistor in series connection across the voltage source for developing a signal voltage across the first resistor corresponding to the light intensity on said cell, a relay including switching means operatively connected with said upper and lower beam circuits, an amplifier having an input circuit connected across the first resistor and an output circuit connected with said relay means for causing actuation of the switching means at a predetermined value of light intensity, a second resistor connected in parallel with said first resistor through the switching means for changing the value of the signal voltage corresponding to a given value of light intensity whereby the relay means actuates said switching means upon the occurrence of a different value of light intensity.

, 9. An automatic control system for Vehicle headlamps having upper and lower beam circuits, a light responsive device for developing a signal voltage corresponding to the intensity of incident light, a control relay having switching means operatively connected with said upper and lower beam circuits and having an energizing coil for actuating said switching means, a first amplifying device having its input circuit connected with the light responsive device and being non-conductive when there is no light on said light responsive device, a second amplifying device having its input circuit connected with the output circuit of the first amplifying device and being conductive when the first amplifying device is non-conductive, a third amplifying device having its input circuit connected to the output circuit of the second amplifying device and including said energizing coil, said third amplifying device being conductive when the output circuit of the second amplifying device is conductive, and positive feedback circuit from the output circuit of the third amplifying device to the input circuit of the second amplifying device whereby a change of signal voltage causes an inverse change of conduction of the first and third amplifying devices and the feedback to second amplifying device promotes rapid change of conduction therebetween to change the energization of said relay.

10. An automatic control system for vehicle headlamps having upper and lower beam circuits, a light responsive device for developing a signal voltage corre- 'sponding to the intensity of incident light, a control relay having switching means operatively connected with said upper and lower beam circuits and having an energizing coil for operating said switching means, a first electron tube having an input circuit extending between its grid and cathode and including said light responsive device, a second electron tube having an input circuit extending between its grid and cathode and having its grid connected with the plate of the first electron tube, a transistor having an input circuit extending between its base and emitter and connected across the output circuit of the second electron tube, said transistor having an output circuit extending between its emitter and collector electrodes and including said energizing coil whereby a change of signal voltage causes an inverse change of conduction in the output circuits of the first electron tube and said transistor.

11. An automatic control system for vehicle headlamps having upper and lower beam circuits, a light responsive device for developing a signal voltage corresponding to the intensity of incident light, a control relay having switching means operatively connected with said .upper and lower beam circuits and having an energizing coil for operating said switching means, a first electron tube having an input circuit extending between its grid and cathode and including said light responsive device,

a second electron tube having an input circuit extending between its grid and cathode and having its grid con- 10 ,nected with the plate of the first electron tube, a tr'ari sistor having an input circuit extending between its base and emitter and connected across the output circuit of the second electron tube, said transistor having an output circuit extending between its emitter and collector electrodes and including said energizing coil, and a positive feedback circuit from the collector of said transistor to the grid of the second electron tube whereby a change of signal voltage causes an inverse change of conduction in the output circuits of the first electron tube and the transistor, and the feedback to the second electron tube promotes a rapid change of conduction between the first electron tube and the transistor output circuits to change the energization of said relay.

12. An automatic control system for vehicle headresponsive device for developing a signal voltage corresponding to the intensity of incident light, a control relay having switching means operatively connected with said upper and lower beam circuits and having an energizing coil for actuating said switching means, a first electron tube having an input circuit extending between its grid and cathode and including said light responsive devices and being nonconductive when there is no light on said device, a second electron tube having its grid connectedi with the plate of the first electron tube and being con-- ductive when the first electron tube is nonconductive, a: transistor having its base connected with the plate of": the second electron tube, its emitter connected with said"-' voltage source, and its collector connected with said; energizing coil, and a resistive feedback path connected between said energizing coil and the grid of said second! electron tube whereby the positive feedback voltage to: the second electron tube promotes the change of current through said energizing coil in response to variations; of said signal voltage.

13. An automatic control system for vehicle head lamps having upper and lower beam circuits, a voltage; source, a photoconductive cell and a load resistor serially: connected with said voltage source for developing a: signal voltage corresponding to the intensity of light upon said cell, a control relay having switching means operatively connected with said upper and lower beam circuits and having an energizing coil for actuating said switching means, a first electron tube having an input circuit extending between its grid and cathode and including said load resistor and having an output circuit extending between its plate and cathode and including a plate resistor and said voltage source, said first electron tube being nonconductive in the absence of light on said cell, a second electron tube having an input circuit extending between its grid and cathode and connected across the output circuit of the first electron tube and having an output circuit including a plate resistor and said voltage source, said second electron tube being conductive when the first electron tube is nonconductive, a transistor hav-. ing an input circuit extending between its emitter and; base and connected across the plate resistor of the sec-, ond electron tube, and having an output circuit extending: between its emitter and collector circuits and connectedtl across said energizing coil, and a resistive feedback cir cuit connected between said collector and grid of the, second electron tube to provide positive feedback there-l between to rapidly cut off conduction of said second; electron tube and said transistor when said signal voltages, reach a predetermined value.

14. An automatic control system for vehicle headlamps having upper and lower beam circuits, a photoconductivet cell and a load resistor serially connected across a volt-. age source for developing a signal voltage corresponding; to the intensity of incident light, said photoconductive cell having a time constant longer than that desired forthe system, a first electron tube having its grid connected with said load resistor, its cathode connected to ground"; through a self-biasing resistor and its plate electrode cona endasnected to said voltage source through a plate resistor, a relay having an energizing coil, a movable contact and a fixed contact, one contact being connected to ground, a condenser connected between the cathode of the first electron tube and the other contact whereby the condenser is connected across the self-biasing resistor when the contacts are closed to allowlthe grid voltage to change at a greater rate than the cathode voltage, a second electron tube having its grid connected with the plate of the first electron tube, its cathode connected to ground and its plate connected with said voltage source through a plate load resistor, a transistor having its emitter connected with said voltage source and its base connected with the plate of the second electron tube whereby input current flows in the transistor when the second electron tube is conductive, said transistor having its collector connected through said energizing coil to ground whereby said relay is energized in the absence of light on the photoconductive cell and a positive feedback connection between said coil and the grid of the second electron tube to promote rapid deenergization of said relay when the signal voltage increases to a predetermined value.

l5. An automatic control circuit for vehicle headlamps having upper and lower beam circuits, a light responsive device for developing signal quantity corresponding to incident light intensity and having a time constant longer than that desired for switching from one beam circuit to the other, an amplifier having an input circuit including said device and having an output circuit, the current conduction in the output circuit being variable in accordance with the signal quantity, a relay connected with said output circuit for energization thereby and including switching means operatively connected with said upper and lower beam circuits for selective energization thereof, bias means connected with the am plifier and responsive to current conduction in the output circuit to develop a bias condition which decreases the effect of the signal quantity when the light intensity is increasing, and means connected with the bias means for maintaining said bias condition momentarily during a decrease of light intensity whereby the bias condition increases the effect of the signal quantity and causes immediate actuation of the switching means when the light intensity decreases.

16. An automatic control circuit for vehicle headlamps having upper and lower beam circuits, a light responsiveidevice for developing signal: quantity corresponding to light intensity and. having a time constant longer than that desired for' switching from one beam circuit to the other, an amplifier having an input circuit including: said device and having an output circuit, the current conduction in the output circuit being variable with the signal quantity, a relay connected with said output circuit for emergization thereby and including switching means operatively connected with said upper and lower beam circuits for selective emergization thereof, means connected with the amplifier for reducing the sensitivityof the amplifier to change the effect of the signal quantity when the light intensity changes in one sense, and means for maintaining the changed sensitivity momentarily duringa change of light intensity in the other sense whereby the last-mentioned means increases the effect ofthe signal quantity and causes immediate actuation of the switching means when the light intensity changes in said other sense. a

l7. Anrautomatic control circuit for vehicle headlamps having, upper and lower beam circuits, a light re sponsive device for developing signal quantity corre sponding to incident light intensity and having a time constant longer than that desired for switching from one beam circuit to the other, an amplifier having an input circuit including said device and having an output circuit, the current conduction in the output circuit being variable in accordance with the signal quantity, a relay connected with said output circuit for energization there by and including switching means operatively connected with said upper and lower beam circuits for selective energization thereof, a resistance-capacitance bias circuit connected with the amplifier and responsive to current conduction in the output circuit to develop a bias condition which decreases the effect of the signal quantity when the light intensity is increasing, and means connected with the resistance-capacitance bias circuit for maintaining said bias condition momentarily during a decrease of light intensity whereby the bias condition increases the effect of the signal quantity and causes immediate actuation of the switching means when the light intensity decreases.

References Cited in the file of this patent UNITED STATES PATENTS 2,829,307 Miller et a1 Apr. 1, 1958

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