US3735378A

US3735378A - Lamp failure indicating apparatus

Info

Publication number: US3735378A
Application number: US00138602A
Authority: US
Inventors: Namee J Mc
Original assignee: General Motors Corp
Current assignee: Motors Liquidation Co
Priority date: 1971-04-29
Filing date: 1971-04-29
Publication date: 1973-05-22
Anticipated expiration: 1990-05-22
Also published as: FR2135216A1; DE2220019A1; AU3956072A; GB1335803A; FR2135216B1; AU464229B2; CA955651A

Abstract

Apparatus for indicating inoperativeness of a lamp in any of several lamp arrays. Each lamp array includes one or more parallel connected lamps having predetermined electrical characteristics such that each operative lamp effects a certain voltage drop across the lamp array. A detector network monitors the voltage across each lamp array, which voltage is almost the same as the supply voltage, and generates a certain control signal when one of the lamps has become inoperative. An output amplifier that is responsive to the control signal effects energization of an indicator lamp to indicate that a lamp is inoperative when the certain control signal is generated. Several detector networks may be cascaded in parallel to monitor several lamp arrays and each detector network may be employed to monitor either one or two lamp arrays.

Description

H11 .Wm

[ LAMP FAILURE INDICATING APP ATlUS Primary Examiner-John W. Caldwell Assistant Examiner-William M. Wannisky [75] Inventor: James W. McNamee, Warren, Ohio Attorney-Jean L. Carpenter and Paul Fitzpatrick ABSCT [73] Assignee: General Motors Corporation,

Detroit, Mich.

22 Filed: Apr. 29, 1971 [21] Appl.No.: 138,602

Apparatus for indicating inoperativeness of a lamp in any of several lamp arrays. Each lamp array includes one or more parallel connected lamps having predetermined electrical characteristics such that each 340/248 operative lamp effects a certain voltage drop across the lamp array A detector network monitors the vow [58] Field of Search...................3l g(;gi2,sg5 1132948121511, g across each p array, which vohage is almost the same as the supply voltage, and generates a certain control signal when one of the lamps has become in- [56] References Cited UNITED STATES PATENTS 1/1969 Atkins...

operative. An output amplifier that is responsive to the control signal effects energization of an indicator lamp to indicate that a lamp is inoperative when the 3,421,157 3,457,561 7,1969 Zeislerm" certain control signal is generated. Several detector 3,629,799 12/1971 networks may be cascaded in parallel to monitor 3,631,441 12/1971 Murphy... several lamp arrays and each detector network may be 3,593,264 7/1971 Atkins..... employed to monitor either one or two lamp arrays. 3,171,096 2/1965 Murray et al 3,535,585 10/1970 4 Claims, 1 Drawing Figure LAMP FAILURE INDICATING APPARATUS This invention relates to light systems and, more particularly, to light systems in which failure of a lamp in any of several lamp arrays is indicated by an indicator device.

While the subject apparatus may be employed in conjunction with many different types of equipment, it is particularly advantageous when employed in a motor vehicle which may have several lamp arrays of similar electrical characteristics. For example, a motor vehicle may employ two headlamps in each of its front fenders for illuminating the roadway. The two headlamps referred to are often called high beams" and are connected in parallel to form a lamp array that is energized from a common power supply through a single conductor. While there have been numerous designs proposed for simultaneously monitoring two lamp arrays such as these to indicate when a lamp has failed in either of the lamp arrays, these previous proposals have not met with much success for a variety of reasons.

Among these reasons are the fact that the prior art proposals generally require that the apparatus monitoring the operativeness of the lamps consume a considerable amount of standby power even while all of the lamps are operative. Another reason is the fact that many proposals have required that resistors be inserted in series with the lamp arrays being monitored to provide a significant, and measurable, voltage drop in series with each of the lamp arrays and the power source. This proposal has similarly not received widespread acceptance as it both consumes a considerable amount of power in the added resistors and reduces efficient operation of the lamps.

Another disadvantage common to many prior proposals for indicating the presence of an inoperative lamp is the fact that each lamp array has often been monitored by a unique detector circuit which has not been readily adaptable for use in a system requiring additional detector circuits, or channels, to monitor the operativeness of other lamp arrays so that a common indicator device for indicating a failure inany of the lamp arrays being monitored could be used. Similarly, where a detector has been proposed as suitable for monitoring two or more lamp arrays, it has often been unsatisfactory in monitoring the operativeness of a single lamp. Both of the last mentioned disadvantages of the designs prevalent in'the prior art naturally result in considerable economic disadvantage in applications where it is most advantageous to employ a single detector unit that is readily adaptable in conjunction with similar detector units to monitor the operativeness of any number of lamp arrays.

It is therefore an object of this invention to provide a light system which includes two lamp arrays and a detector network for indicating when alamp has failed in either of the lamp arrays, the detector network drawing a minimum of power by monitoring a voltage substantially the same as the voltage applied by the power source.

It is also an object of this invention to provide lamp failure indicating apparatus for indicating failure of a lamp, the apparatus being operable only when the lamps are energized and designed in modular form whereby several modular channels may be cascaded in parallel to monitor a considerable number of lamp arrays while employing but a single indicator device for 2 indicating failure of a lamp in any of the lamp arrays being monitored.

It'is an additional object of this invention to provide lamp failure indicating apparatus for indicating failure of a lamp in either of two lamp arrays across which there appears a voltage drop substantially equal to the voltage of an applied power source and for monitoring but a single lamp, rather than all of the lamps in two lamp arrays, with a minimum of adaptation.

The foregoing and other objects and advantages of the subject invention will become apparent from the following description and accompanying drawing, in which is presented a schematic diagram of a light system embodying the principles of the subject invention.

As mentioned previously, the subject apparatus is particularly useful when employed in a vehicle. Accordingly, in the accompanying drawing a light system 10 is illustrated in which a suitable power source, such as a vehicle battery 12, energizes several lamps 14 through 17 through a normally open light switch 18 and first and

second resistors

20 and 22 that are each connected in series with two of the lamps 14 through 17. The lamps 14 through 17 are thus connected to form first and

second lamp arrays

24 and 26 that may each be mounted on one of the front fenders of the vehicle.

The voltages across each of the

lamp arrays

24 and 26, which voltages appear at first and second junctions 28 and 30, are monitored by a first detector network 32 that includes a differential comparator 34, a current source network 36, and a full wave detector 38 that generates a certain control signal when one of the lamps 14 through 17 is inoperative. An output amplifier 40 monitors the control signal and effects energization of a lamp 42 when the certain control signal is generated.

The light system 10 also includes a second detector network 44 that is similar to the first detector network 32 but which monitors the operativeness of yet another lamp 46. By way of example, and without limitation, the lamp 46 may be what is commonly termed a running lamp on the vehicle and is energized through an ignition switch 48 and a resistor 50 whenever the vehicle is in operation. Should the running lamp 46 become inoperative the second detector network 44 generates the certain control signal and effects energization of the indicator lamp 42 through the output amplifier 40.

The light system 10 will now be explained in detail.

The

resistors

20 and 22 are selected to be of substantially the same resistance. Similarly, the lamps 14 through 17 are each selected to have a certain resistance that is very much larger than the resistance of the

resistors

20 and 22 so that the current through each of the

resistors

20 and 22 is directly proportional to the number of lamps drawing the current. For example, if the battery 12 is selected to supply power at a continuous potential of 12 volts and the resistance of each of the lamps 14 through 17 is 599 times greater than the resistance of each of the

resistors

20 and 22, each of the lamps 14 through 17 will draw a certain current through the

respective resistors

20 and 22. In actual practice the

resistors

20 and 22 have been determined by selecting the wires connecting the

lamp arrays

24 and 26 to the battery 12 to have a predetermined low resistance rather than by selection of discrete resistor elements.

Accordingly, if it is assumed that current drawn by the first detector network 32 is negligible when compared with the current drawn by the lamps 14 through 17, which assumption proves valid in practice, it thus follows that a certain voltage drop is created across each of the resistors and 22 by the current drawn by each of the lamps 14 through 17. In the example just described, the current through each of the lamps 14 through 17 effects a 20 millivolt drop across the resistor in series with the lamp. It thus follows that in the illustrated embodiment, the voltage at each of the junctions 28 and is 11.96 volts when all of the lamps 14 through 17 are operative. Should one of the lamps in either of the

lamp arrays

24 or 26 become inoperative it will cease to draw current and the voltage at the corresponding junction 28 or 30 will be increased to l 1.98 volts. Accordingly, the first detector network 32 is designed to monitor the voltage at the junctions 28 and 30 and to generate the certain control signal when the monitored voltages are unequal by 20 millivolts.

The junctions 28 and 30 are connected to the respective anodes of first and second diodes 52 and 54 in the differential comparator 34. The cathodes of the diodes S2 and 54 are connected to the base terminals of first and

second PNP transistors

56 and 58 having their respective emitter terminals connected to the anodes of the respective second and first diodes 54 and 52. The collector terminals of the first and

second transistors

56 and 58 are connected to a point of reference potential, which is ground, through first and

second bias resistors

60 and 62 and a diode 64 that protects the first and second detector networks 32 and 44 from negligent reversal of the battery 12 polarity.

The current through the first and second diodes 52 and 54 is held at a substantially constant level by the current source network 36. The current source network 36 includes a pair of resistors 66 and 68 and a pair of diodes 70 and 72 connected between the second junction 30 and the anode of the polarity reversal diode 64. A Zener diode 74 is connected between the anode of the polarity reversal diode 64 and the junction 76 between the resistors 66 and 68 so as to maintain the junction 76 at a constant voltage.

The current source network 36 also includes a pair of

NPN transistors

78 and 80 and resistors 82 and 84 that are connected between the emitters of the

transistors

78 and 80 and the anode of the polarity reversal diode 64. The collectors of the

transistors

78 and 80 are connected to the cathodes of the respective first and second diodes 52 and 54 for drawing a predetermined current through the first and second diodes 52 and 54. The predetermined current level in a preferred design is on the order of 200 microamps and hence is so minute as not to affect the voltage at the junctions 28 and 30. The conductivity of the

transistors

78 and 80 is established by the voltage across the diodes 70 and 72, which voltage appears at a junction 86 that is connected to the bases of the

transistors

78 and 80. Since the voltage across the diodes 70 and 72 will vary with temperature changes the diodes 70 and 72 provide temperature compensation for the

transistors

56 and 58 by controlling the conductivity of the

transistors

78 and 80, as will subsequently become apparent.

When the first detector 32 is in operation the current source network 36 establishes the predetermined current through the first and second diodes 52 and 54. The first and second diodes 52 and 54 are thus held in a conductive condition in which the voltage across each of the diodes 52 and 54 remains substantially constant.

By selecting the diodes 52 and 54 to be matched so as to have substantially the same electrical characteristics the voltages which are applied to the emitter, or input, terminals of the

transistors

56 and 58 are equal and the voltages applied to the bases of the

transistors

56 and 58 are also equal. The voltage drop across the first and second diodes 52 and 54, which may be on the order of 0.7 volts, thus maintains the first and

second transistors

56 and 58 in a state of continuous conduction as class A operated amplifiers and their collector, or output, terminals conduct current through the bias resistors and 62. By further matching the diodes 52 and 54 to have the same characteristics as the emitter-base diodes of the

transistors

56 and 58 the current through the

transistors

56 and 58 may be made substantially the same as current through the diodes 52 and 54. This establishes the operating points of the

transistors

56 and 58 at the same level of conductivity.

The currents through the

bias resistors

60 and 62 generate first and second voltage signals which are monitored by the full wave detector 38, which includes third and fourth PNP transistors 88 and 90. The bases of the respective transistors 88 and 90 are connected to the collector terminals of the

respective transistors

56 and 58 and the emitter terminals of the transistors 88 and 90 are connected to the collector terminals of the

respective transistors

58 and 56. The same potential is thus applied to the base and emitter terminals of the third and fourth transistors 88 and 90 so long as the voltages on the collector terminals of the first and

second transistors

56 and 58 are equal. The third and fourth transistors 88 and 90 are thus held nonconductive and their collector terminals, which are connected together, provide a low voltage control signal so long as the lamps 14 through 17 are operative. The control signal is coupled by an isolation diode 92 to the output amplifier 40 to control the energization of the indicator lamp 42, which is connected to the battery 12 through a lead 94.

While the output amplifier 40 in the illustrated embodiment is a lamp driver circuit for controlling the energization of the indicator lamp 42, persons versed in the art will appreciate that the output amplifier 40 may instead be employed to control the energization of various other indicator devices. In the illustrated embodiment, the output amplifier 40 includes several cascaded NPN transistors 96 through 98 in what is known as a Darlington configuration, a capacitor 99 for protecting the transistors 96 through 98 from transients, an input resistor 100 through which the control signal is transmitted from the isolation diode 92 to the transistors 96 through 98, and a gain control network 102 which consists of a variable resistor 104 and several series connected diodes 106 through 108. The diodes 106 through 108 are selected to match the emitter-base diodes of the transistors 96 through 98 so as to provide temperature compensation within the output amplifier 40.

The operation of the apparatus just described may be summarized as follows. When all of the lamps 14 through 17 are energized, the voltages at the junctions 28 and 30 are equal. The first and

second transistors

56 and 58, which have their electrical characteristics matched, are thus held in a class A operation conductive condition and their output, or collector, terminals are at the same voltage. The third and fourth transistors 88 and 90 are held in a nonconductive condition by these voltage signals and the collectors of the third and fourth transistors 88 and 90 provide a low voltage control signal to the amplifier 40. The control signal thus provided is insufficient to forward bias the emitter-base junctions of the transistors 96 through 98 in the output amplifier so the transistors 96 through 98 are nonconductive and the indicator device 42 remains deenergized so long as all of the lamps 14 through 17 are operative.

Should one of the lamps 14 through 17 become inoperative, such as the lamp 14, the voltage across the lamp array containing the inoperative lamp is increased from approximately 11.96 volts to 11.98 volts. In this example, the voltage increase takes place at junction 28 and causes the second transistor 58 to become more conductive by increasing its emitter voltage while its base voltage is unchanged. The increase in voltage at the junction 28 effects a corresponding decrease in conduction of the first transistor 56 since the potential applied to its base by the first diode 52 increases from approximately 11.26 to 11.28 volts while the potential applied to its emitter by the second junction 30 remains at 11.96 volts. The resultant increase in current through the second transistor 58 and decrease in current through the first transistor 56 effect a corresponding voltage increase across the second bias resistor 62 and decrease in voltage across the first bias resistor 60.

Since the collector of the second transistor 58 is at a higher potential than the collector of the first transistor 56, the third transistor 88 is biased to a conductive condition while the fourth transistor 90 is maintained nonconductive. Once the transistor 88 has begun to conduct the voltage on its collector attains a certain predetermined level and this certain control signal is suffrcient when coupled through the isolation diode 92 and the input resistor 100 to bias the transistors 96 through 98 in the output amplifier 40 to a conductive condition, which effects energization of the indicator lamp 42 to indicate that one of the lamps 14 through 17 has failed.

While the indicator lamp 42 does not indicate which of the lamps 14 through 17 have failed, it serves to advise the vehicle operator that one of the lamps of the vehicle is inoperative so that he may visually ascertain which of the lamps has failed. Since the indicator lamp 42 may be energized whenever the certain control signal is applied to the input resistor 100 any number of detector networks similar to the first detector network 32 may be combined to provide the certain control signal when various other conditions have occurred within the vehicle.

This adaptability of the first detector network 32 has been illustrated in the drawing by provision of the previously recited second detector network 44, which monitors the operation of the running lamp 46. The second detector network 44 is identical in configuration to that of the first detector network 32 except for the deletion of the fourth transistor 90, which is unnecessary in the second detector network 44, and the employment of different resistance values for the first and

second bias resistors

60 and 62. Accordingly, circuit elements in the second detector network 44 are noted with the same identification numerals as employed for corresponding circuit elements in the first detector network 32 followed by a prime notation.

The second detector network 44 is connected to monitor the voltage at each side of the resistor 50. Accordingly, the voltage applied to the anode of the secllu and diode 54' is substantially 12 volts when the battery 12 is a 12 volt supply and the voltage at the anode of the first diode 52' is approximately 11.98 volts. This voltage ofiset tends to bias the first transistor 56' more conductive than the second transistor 58'. For this reason the biasing resistors 60' and 62' are adjusted so the collector voltages of the first and second transistors 56' and 58 are equal when the lamp is operative. Accordingly, the base and emitter voltages of the third transistor 88 are equal and the control signal on its collector is at a low potential so as to preclude the second detector network 44 from efiecting energization of the indicator lamp 42 when the lamp 46 is operative. It should be noted that the

bias resistors

60 and 62 in the first detector network 32 could similarly be adjusted to compensate for a voltage offset caused by one of the

lamp arrays

24 or 26 containing more lamps than the

other lamp array

24 or 26.

Should the lamp 445 become inoperative it will cease to draw current and substantially the entire potential of the battery 12 will be applied to the anode of the first diode 52' as well as to the anode of the second diode 54'. This effects an increase in the voltage applied to the base of the first transistor 56' so as to decrease its conductivity and effect an increase in the conductivity of the second transistor 58'. Accordingly, the voltage on the base of the transistor is decreased while its emitter voltage is increased. This turns on the transistor 88' and causes the generation of the certain control signal on its collector. The certain control signal is coupled from the collector of the third transistor 88 through a second isolation diode to the output amplifier 40 where it effects energization of the indicator lamp 42 in the fashion previously explained. The single indicator lamp 42 may thus be employed to indicate that one of any number of lamps has failed, the number only being limited by the number of detector networks employed.

Persons versed in the art will appreciate that various modification of this invention may be made without departing from its spirit.

What is claimed is:

1. In a light system having a first lamp array connected in series with a power source and a first resistor and a second lamp array connected in series with the power source and a second resistor, each lamp array comprising a certain number of parallel connected lamps having similar impedances,.the ratio of the impedance of thefirst lamp array to the impedance of the first resistor being the same as the ratio of the impedance' of the second lamp array to the impedance of the second resistor, apparatus for indicating when a lamp in either of the lamp arrays has failed comprising, in combination, a differential comparator for comparing the voltage across each of the lamp arrays and generating first and second signals having relative magnitudes related to the difierence between the voltages compared, the first and second signals being equal when the monitored voltages are equal and unequal when the monitored voltages are unequal, a full wave detector that is responsive to the first and second signals for generating a certain control signal only when the first and second signals are unequal, the control signal being other than the certain control signal when the first and second signals are equal, an indicator device for indicating that one of the lamps has failed, and an output amplifier that is responsive to the control signal for effecting an indication by the indicator device that a lamp has failed whenever the full wave detector generates the certain control signal.

2. In a light system having a first lamp array connected inseries with a power source and a first resistor and a second lamp array connected in series with the power source and a second resistor, each lamp array comprising a certain number of parallel connected lamps having similar impedances, the ratio of the impedance of the first lamp array to the impedance of the first resistor being the same as the ratio of the impedance of the second lamp array to the impedance of the second resistor, apparatus for indicating when a lamp in either of the lamp arrays has failed comprising, in combination, a differential comparator comprising first and second matched diodes having their respective anodes connected to sample the voltage across the respective first and second lamp arrays, first and second matched amplifiers each having a control terminal and input and output terminals between which a controlled current path is defined, the control terminals of the respective first and second amplifiers being connected to the cathodes of the respective first and second diodes and the input terminals of the respective first and second amplifiers being connected to the anodes of the respective second and first diodes, and first and second biasing impedances connected in series with the output terminals of the respective amplifiers; means for providing a predetermined load in series with each of the diodes whereby a predetermined potential diflerence is established across each of the diodes, the biasing impedances being selected to effect operation of the amplifiers so as to establish equal potentials on their output terminals when all of the lamps are operative and unequal potentials on their output terminals when a lamp has failed; detector means responsive to the potentials on each of the output terminals for generating a certain output signal when said potentials are unequal; an indicator device for indicating failure of a lamp in either of the lamp arrays; and means responsive to the detector means for effecting an indication by the indicator device that a lamp has failed when the detector means generates the certain output signal.

3. A light system comprising, in combination, first and second lamp arrays, each lamp array including at least one grounded lamp; a power source having a grounded terminal; first and second resistors connecting the respective first and second lamp arrays in series with the power source so as to connect each lamp in each lamp array in parallel with the other lamps in the same lamp array, the ratio of the impedance of the first lamp array to the impedance of the first resistor being the same as the ratio of the impedance of the second lamp array to the impedance of the second resistor, the resistances of the lamps in each lamp array being equal and very much greater than the resistor resistances whereby the current through each lamp effects a predetermined voltage drop across the resistor in series therewith; first and second diodes having their respective anodes connected to the respective junctions between the first resistor and the first lamp array and the second resistor and the second lamp array for partaking of the voltage across each of the lamp arrays; first and second controlled amplifiers each having control, input, and output terminals, the control terminals of the respective first and second amplifiers being connected to the cathodes of the respective first and second diodes and the input terminals of the respective first and second amplifiers being connected to the anodes of the respective second and first diodes; first and second biasing impedances connected in series with the output terminals of the respective first and second amplifiers whereby the voltages on the output terminals are responsive to the conductivity of the respective amplifiers; means for drawing a predetermined current through the diodes so as to effect a predetermined voltage drop across each of the diodes of sufficient magnitude to render the amplifiers partially conductive, the magnitude of the biasing impedances being selected when all of the lamps are operative so that each of the amplifier output terminals are at a certain potential whereby failure of one of the lamps effects a change in the conductivity of the amplifiers and a corresponding difference in the potential on the output terminals of the amplifiers; a detector for generating a certain control signal when the output terminals are at difierent potentials, the detector including third and fourth controlled amplifiers each having control, input, and output terminals, the control terminals of the third and fourth amplifiers partaking of the potentials on the output terminals of the respective first and second amplifiers and the input terminals of the third and fourth amplifiers partaking of the potentials on the output terminals of the respective second and first amplifiers, the output terminals of the third and fourth amplifiers being connected so as to provide a common terminal at which the control signal is generated, equal potentials on the output terminals of the first and second amplifiers biasing the third and fourth amplifiers to a first state of conduction in which they generate a first control signal on the common terminal and unequal potentials on the output terminals of the first and second amplifiers biasing the third and fourth amplifiers to a different conductive state in which they generate the certain control signal on the common terminal; an indicator device for indicating failure of a lamp; and means responsive to the certain control signal for effecting an indication by the indicator device that a lamp has failed when the detector generates the certain control signal.

4. In a light system having a grounded lamp connected in series with a grounded power source and a resistor whereby current through the lamp efiects a predetermined voltage drop across the resistor, apparatus for indicating failure of the lamp comprising, in combination, a differential comparator for monitoring the voltage across the resistor and for generating first and second signals indicative of the operativeness of the lamp, the differential comparator including first and second matched diodes having their anodes connected to the respective terminals of the resistor, the anode of the first diode being connected to the resistor terminal that is connected to the lamp and the anode of the second diode being connected to the other resistor terminal, first and second controlled amplifiers each having control, input, and output terminals, the confiol terminals of the respective first and second amplifiers being connected to the cathodes of the respective first and second diodes and the input terminals of the respective first and second amplifiers being connected to the anodes of the respective second and first diodes, and first and second impedances connecting the respective output terminals of the respective first and second amplifiers to a point of reference potential whereby the first and second signals are generated on the output terminals of the respective first and second amplifiers and an output terminal on which is generated the control signal; an indicator device for indicating that the lamp has failed; and an output amplifier that is responsive to the control signal and effective when the certain control signal is generated to cause the indicator device to indicate that the lamp has failed. I I0! 4 t t

Claims

1. In a light system having a first lamp array connected in series with a power source and a first resistor and a second lamp array connected in series with the power source and a second resistor, each lamp array comprising a certain number of parallel connected lamps having similar impedances, the ratio of the impedance of the first lamp array to the impedance of the first resistor being the same as the ratio of the impedance of the second lamp array to the impedance of the second resistor, apparatus for indicating when a lamp in either of the lamp arrays has failed comprising, in combination, a differential comparator for comparing the voltage across each of the lamp arrays and generating first and second signals having relative magnitudes related to the difference between the voltages compared, the first and second signals being equal when the monitored voltages are equal and unequal when the monitored voltages are unequal, a full wave detector that is responsive to the first and second signals for generating a certain control signal only when the first and second signals are unequal, the control signal being other than the certain control signal when the first and second signals are equal, an indicator device for indicating that one of the lamps has failed, and an output amplifier that is responsive to the control signal for effecting an indication by the indicator device that a lamp has failed whenever the full wave detector generates the certain control signal.

2. In a light system having a first lamp array connected in series with a power source and a first resistor and a second lamp array connected in series with the power source and a second resistor, each lamp array comprising a certain number of parallel connected lamps having similar impedances, the ratio of the impedance of the first lamp array to the impedance of the first resistor being the same as the ratio of the impedance of the second lamp array to the impedance of the second resistor, apparatus for indicating when a lamp in either of the lamp arrays has failed comprising, in combination, a differential comparator comprising first and second matched diodes having their respective anodes connected to sample the voltage across the respective first and second lamp arrays, first and second matched amplifiers each having a control terminal and input and output terminals between which a controlled current path is defined, the control terminals of the respective first and second amplifiers being connected to the cathodes of the respective first and second diodes and the input terminals of the respective first anD second amplifiers being connected to the anodes of the respective second and first diodes, and first and second biasing impedances connected in series with the output terminals of the respective amplifiers; means for providing a predetermined load in series with each of the diodes whereby a predetermined potential difference is established across each of the diodes, the biasing impedances being selected to effect operation of the amplifiers so as to establish equal potentials on their output terminals when all of the lamps are operative and unequal potentials on their output terminals when a lamp has failed; detector means responsive to the potentials on each of the output terminals for generating a certain output signal when said potentials are unequal; an indicator device for indicating failure of a lamp in either of the lamp arrays; and means responsive to the detector means for effecting an indication by the indicator device that a lamp has failed when the detector means generates the certain output signal.

3. A light system comprising, in combination, first and second lamp arrays, each lamp array including at least one grounded lamp; a power source having a grounded terminal; first and second resistors connecting the respective first and second lamp arrays in series with the power source so as to connect each lamp in each lamp array in parallel with the other lamps in the same lamp array, the ratio of the impedance of the first lamp array to the impedance of the first resistor being the same as the ratio of the impedance of the second lamp array to the impedance of the second resistor, the resistances of the lamps in each lamp array being equal and very much greater than the resistor resistances whereby the current through each lamp effects a predetermined voltage drop across the resistor in series therewith; first and second diodes having their respective anodes connected to the respective junctions between the first resistor and the first lamp array and the second resistor and the second lamp array for partaking of the voltage across each of the lamp arrays; first and second controlled amplifiers each having control, input, and output terminals, the control terminals of the respective first and second amplifiers being connected to the cathodes of the respective first and second diodes and the input terminals of the respective first and second amplifiers being connected to the anodes of the respective second and first diodes; first and second biasing impedances connected in series with the output terminals of the respective first and second amplifiers whereby the voltages on the output terminals are responsive to the conductivity of the respective amplifiers; means for drawing a predetermined current through the diodes so as to effect a predetermined voltage drop across each of the diodes of sufficient magnitude to render the amplifiers partially conductive, the magnitude of the biasing impedances being selected when all of the lamps are operative so that each of the amplifier output terminals are at a certain potential whereby failure of one of the lamps effects a change in the conductivity of the amplifiers and a corresponding difference in the potential on the output terminals of the amplifiers; a detector for generating a certain control signal when the output terminals are at different potentials, the detector including third and fourth controlled amplifiers each having control, input, and output terminals, the control terminals of the third and fourth amplifiers partaking of the potentials on the output terminals of the respective first and second amplifiers and the input terminals of the third and fourth amplifiers partaking of the potentials on the output terminals of the respective second and first amplifiers, the output terminals of the third and fourth amplifiers being connected so as to provide a common terminal at which the control signal is generated, equal potentials on the output terminals of the first and second amplifiers biasing the third and foUrth amplifiers to a first state of conduction in which they generate a first control signal on the common terminal and unequal potentials on the output terminals of the first and second amplifiers biasing the third and fourth amplifiers to a different conductive state in which they generate the certain control signal on the common terminal; an indicator device for indicating failure of a lamp; and means responsive to the certain control signal for effecting an indication by the indicator device that a lamp has failed when the detector generates the certain control signal.

4. In a light system having a grounded lamp connected in series with a grounded power source and a resistor whereby current through the lamp effects a predetermined voltage drop across the resistor, apparatus for indicating failure of the lamp comprising, in combination, a differential comparator for monitoring the voltage across the resistor and for generating first and second signals indicative of the operativeness of the lamp, the differential comparator including first and second matched diodes having their anodes connected to the respective terminals of the resistor, the anode of the first diode being connected to the resistor terminal that is connected to the lamp and the anode of the second diode being connected to the other resistor terminal, first and second controlled amplifiers each having control, input, and output terminals, the control terminals of the respective first and second amplifiers being connected to the cathodes of the respective first and second diodes and the input terminals of the respective first and second amplifiers being connected to the anodes of the respective second and first diodes, and first and second impedances connecting the respective output terminals of the respective first and second amplifiers to a point of reference potential whereby the first and second signals are generated on the output terminals of the respective first and second amplifiers, the first and second signals being equal when the lamp is operative and unequal when the lamp is inoperative; a detector that is responsive to the first and second signals and is effective to generate a certain control signal when the first and second signals are unequal, the detector including a third controlled amplifier having control and input terminals connected to the output terminals of the respective first and second amplifiers and an output terminal on which is generated the control signal; an indicator device for indicating that the lamp has failed; and an output amplifier that is responsive to the control signal and effective when the certain control signal is generated to cause the indicator device to indicate that the lamp has failed.