|Publication number||US3828334 A|
|Publication date||Aug 6, 1974|
|Filing date||Apr 13, 1973|
|Priority date||Apr 13, 1973|
|Publication number||US 3828334 A, US 3828334A, US-A-3828334, US3828334 A, US3828334A|
|Original Assignee||Univ Iowa Res Found|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (8), Referenced by (33), Classifications (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
United States Patent 1 91 1111 3,828,334 Wallace Aug. 6, 1974  SYSTEM FOR REMOTE MONITORING OF 3,370,251 2/1968 Overstreet, Jr. 315/87 x Y T 3,587,082 6/1971 Muehter et a]. 340/409 x TOWER LIGHTING s S EM 3,626,401 12/1971 Flieder et al,.... 340/253 R  Inventor: Leonard M. Wallace, Ames, o a 3,660,674 5/1972 Bolinger 340/331 x Assignee: Iowa State University Research 3,715,741 2/1973 McWade et al. 340/223 X Foundation, Inc., Ames, Iowa Primary Examiner-John W. Caldwell  Filed: 1973 Assistant Examiner-Daniel Myer  App1 351 097 Attorney, Agent, or FirmDawson, Tilton, Fallon &
I Lungmus  US. Cl. 340/25l, 315/132, 340/248 R  Int. Cl. G08b 21/00 TRA T  Field of Search 340/251, 248, 253, 331, A tall tower equipped with a plurality of flashing 340/223, 409; 315/ 129-132, 88-93, 14 87 on lights and continuous obstruction lights is sensed at a remote location for indicating whether all lights References Clted are in proper working order, and if they are not, the
UNITED STATES PATENTS system identifies the fault to the remotely located op- 2,998,545 8/1961 Smyth 315/89 erator. I I 3,287,719 11/1966 Thornberg et a1... 340/248 R 3,289,097 11/1966 Martin 315/87 x 9 Clam, 3 D'awmg F'gures FIE-L 5 28 I CHANNEL 3 B I 7" I i AND 118 309 NOR INVERTER F" GATE CIRCUIT D/A CONVERTER SCHANNEL I REMOTE D A CONTROL 2 K I 52 STATION D/A CONVERTER A J D A C No.2
MONOSTABLE 55 af AND CIRCUIT 29 2 ICHANNELZ C 20 MONOSTABLE 25 7 B6 B 2 AND CIRCUIT '26 4/ MONOSTABLE a? AND CIRCUIT INVERTER d Z CIRCUIT 40 22 7 526 $2 s3 s4 AND s5 S6 s7 SYSTEM FOR REMOTE MONITORING OF TOWER LIGHTING SYSTEM BACKGROUND AND SUMMARY The present invention relates to improvements in monitoring a system of tower lights to make sure they are lighted; more particularly, the invention provides a system for monitoring tower lights at a remote location.
. The system has been found to be of particular use in monitoring the lighting system for a 2,000 foot television transmission tower. There are 20 separate lights on the tower. These are subdivided into obstruction lights and beacon'lights; There are seven obstruction lights, each located at a different elevation or level on the tower. The obstruction lights burn steadily. There are also seven beacon light levels, and these alternate with the obstruction light levels, the uppermost light on the transmitter tower being a single flashing beacon and probably the most important light from a safety standpoint. Each of the remaining beacon levels includes two separate lights to avoid shadow-that is, at least one of the flashing beacon lights will be visible no matter from which direction the tower is approached. All of the beacon lights flash synchronously, and the flashing is caused by a conventional flasher unit.
Safety requirements as well as governmental agencies dictate that the tower lights be checked every night for operability. The transmitter, however, may be remotelycontrolled from a location miles away from the tower; and this would require that an engineer drive back and forth to the tower merely to check the lights.
The present invention, then, is directed to a system The present invention senses signals representative of whether or not each set of beacon lights or each individual obstruction light is operative. The signals from the beacon lights are grouped in pairs except for the top one, and each pair is fed through an AND gate to a monostable circuit having an ON time longer than the cycle of the beacon lights. Hence, the output of the monostable circuits is ON as long as the flasher unit is working and all of the beacon lights are operative. The outputs of this first set of monostable circuits are fed to an AND gate, the output of which is fed to one input of a digital to analog converter. The other terminal of this first digital to analog converter is fed by a second AND gate directly sensing signals representative of the operativeness of each of the individual obstruction lights.
The digital-to-analog converter generates an output signal which assumes one of four possible levels and it is transmitted over an existing channel to the remote control station. One signal level represents that all of the obstruction lights and beacon lights are operative and that the beacon lights are flashing. A second signal level indicates that at least one of the beacon lights is not flashing (but does not indicate whether the light is burned out or the flasher unit is not operating). A third signal level indicates that at least one of the obstruction lights is not operating properly. A fourth signal level indicates that at least one beacon light is not flashing and at least one obstruction light is not operating properly.
In addition, predetermined ones of the obstruction light signals are fed to an OR gate. This OR gate also senses a predetermined number of beacon lights, but less than all. The output of this first OR gate generates a signal if any one of the obstruction lights or beacon lights that are sensed by it are operative, and it thus notifiesthe operator that the tower is lit even though one beacon light and one obstruction light are not operating as indicated by the first digital-to-analog converter output. The output of this first OR gate is fed to one input of a second digital-to-analog converter, the output of which is fed to the remote control station over a second channel.
The signal representative of the flashing beacon lights from a number of the beacon lights is differentiated and fed to a second OR gate, the output of which feeds a monostable circuit having an 0N time longer than the flashing period. Thus, the output of the monostable is ON as long as the flasher unit is operating correctly; and the output of this monostable circuit is fed to the other-input of the second digital-to-analog con-' verter.
In a preferred embodiment, the uppermost beacon light is sensed by circuitry which provides an alarm sig nal transmitted over a third channel to the remote monitoring location when some of the tower lights are'on, but the top beacon is not lighted. This alarm signal will not be generated when the lights are 011' during normal daylight hours.
In checking the system, the operator at the remote control station can select any of the first two incoming channels and read the corresponding voltage. The outputs of the first digital-to-analog converter have already been explained. The output of the second digitalto-analog converter generates a first signal level when the tower is both lit and flashing (although one or more lights may be out), a second signal level when a flasher unit has failed, and a third signal when the towers are not lit at all.
The present invention thus provides a remote monitoring system for tower lights for sensing the status of all lights on the tower, including both beacon lights and obstruction lights, and condenses this information into two channels (or three, if a separate alarm is required for the uppermost beacon light) of a transmitter remote control unit. The invention eliminates the need to have an engineer travel each night to the tower location to inspect the tower lights, and it facilitates checking of the status of the lights more often and more reliably under those conditions which hinder visual observation of the highest lights; for example, fog.
Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in their various views.
THE DRAWING FIG. 1 is a functional block diagram of the sensing and logic circuitry located at the tower;
FIG. 2 is a circuit schematic diagram for the monostable circuits of FIG. l; and
FIG. 3 is a circuit schematic diagram for the digitalto-analog converters of FIG. 1.
DETAILED DESCRIPTION Associated with each light on the tower is a sense wire that is fed to an indicator panel (not shown) at the base of the tower. The present invention makes use of these sense wires by feeding them to detection circuitry, identified generally by block in FIG. 1.
A separate sense wire is associated with each beacon light (Bl-B7), and it carries a signal when its associated beacon'light is operating. Similarly, there are seven obstruction lights, and the associated sense lines are designated 81-87.
The voltage on each of the sense lines fed into the detection circuitry 10 is 120 volts AC; hence, each line is coupled to a voltage divider such as a potentiometer 1 l to reduce it to a lower voltage level normally employed in commercially available logic circuits. A transformer could equally well be used in place of the potentiometer 11. The output of the potentiometer 11 is fed through a rectifying diode 12 and coupled to a smoothing capacitor 13. The output of the rectifier 12 is fed to both inputs of an AND gate 14. The output of the AND gate 14 is fed to a monostable circuit 15 which will be disclosed in greater detail in connection with the description of FIG. 2. It will be recalled that at the very top of the tower there is only a single beacon light, but for the remaining levels there are two separate beacon lights. Hence, for the two lights at level B2, the individual leads are designated respectively A and B. A network similar to that already described is used in connection with each of the leads B2A and B2B to reduce the voltage to logic level and to rectify it. Each of the separate output lines, however, is connected to a different input of an AND gate 16, and the output of the AND gate 16 feeds a monostable circuit 17. Identical networks are used at the indicator panel 10 to reduce and rectify the voltage of an associated sense line, and the resulting voltages are fed to separate inputs of AND gates designated 18-22 respectively. The output of the AND gates 18-22 feed respectively the inputs of monostable circuits 23-27. Monostable circuit 15 feeds one input of a two-input NOR gate 28. The outputs of the monostable circuits 17 and 23-27 feed a six-input AND gate designated by reference numeral 29.
It will be recalled that the beacon lights flash. The flashing is caused by a conventional flasher unit. There is thus associated with each flashing beacon level an AND gate which generates an output signal capable of triggering an associated monostable circuit only when all of the beacon lights on that particular level are working. For example, the output of AND gate 16 is a ONE only when both flashing beacon lights on level B2 are working. The time constants of the monostable circuits 15, 17 and 23-27 are set such that the resulting output voltage will be present for a period of time longer than the normal flashing period (a flashing period of the time required for a complete ON/OFF beacon flashing cycle). Preferably, the output of the monostable circuits is greater than approximately two flashing periods.
Turning now to FIG. 2, one embodiment of the monostable circuits 15, 17 and 23-27 is illustrated. The input lead is designated by reference numeral 30, and the input voltage is relatively high when the set of lights feeding the associated AND gate are all on. The input lead 30 is connected to a first NOR gate 31 and to the base of a transistor 32 which has its emitter grounded.
The output of the NOR gate 31 is coupled through a capacitor 33 to the input of a second NOR gate 34. Since there is only one signal input to the NOR gate 34, both input leads are connected in common, and the gate acts as an inverter. The output of the NOR gate 34 is coupled back as a second input to the NOR gate 31, and it is also the output of the monostable circuit. The junction between the collector of transistor 32, the capacitor 33 and the input to the NOR gate 34 is connected to a positive bias by means of a resistor 35. The combination of the capacitor 33 and resistor 35 form a time constant which determines the ON time of the monostable circuit when it is triggered.
In operation, when the signal on the input lead 30 is low (approximately ground level), transistor 32 is nonconducting and the output of NOR gate 31 is at its high voltage. Hence, the right-hand side of the timing capacitor 33 is charged to the positive voltage, and the output of the NOR gate 34 is at ground level.
When all of the beacon lights in a set associated with one of the AND gates 14, 16, 18-22 is lighted, a logic ONE input forces the output of NOR gate 31 to ground; and it also causes transistors 32 to conduct, thereby forcing the input of NOR gate 34 to a ZERO level. Since both inputs are at a logic ZERO, the output goes to a logic ONE.
When the beacon lights then turn off during a normal cycle, the signal on input 30 returns to the low voltage (logic ZERO), and transistor 32 is driven to cut-off, thereby permitting the right-hand side of capacitor 33 to charge towards the positive voltage by means of current flowing through resistor 35. The left-hand side of capacitor 33 is held at ground level by the output of the NOR gate 31 since at this time the output of NOR gate 34 is still a logic ONE, even though the other input of NOR gate 31 has gone to a ZERO. If the voltage at the input to NOR gate 34 should reach the minimum threshold voltage for turning it on, the output would go to a ZERO, thus indicating a failure. However, if the beacons are flashing under a normal cycle, the inputs to NOR gate 31 will again go positive before the capacitor 33 has had time to charge to a potential sufficient to change the state of NOR gate 34. When the input voltage does go positive, it will again cause transistor 32 to conduct, thereby discharging capacitor 33 to ground.
As long as the capacitor 33 remains discharged, the output of the monostable circuit will remain in a ONE condition. In other words, as long as the beacon lights in the set associated with a given monostable circuit are flashing, the monostable circuit will generate a ONE output signal. The feedback from the output of NOR gate 34 to the second input of NOR gate 31 insures an immediate discharge of capacitor Cl-that is, when the transistor 32 is caused to conduct, the output of NOR gate 34 goes to a ONE thereby forcing the output of the NORv gate 31 to ground, thereby providing a low resistance discharge path to the capacitor 33.
Returning now to FIG. 1, it will thus be appreciated that the output of the AND gate 29 is a ONE as long as all of the beacon lights are lighted and not off for a period longer than the output pulse of the monostable circuits just described. The output of the AND gate circuit 29 is fed to one input (indicated as input No. 2) of a first digital-to-analog converter (DAC) designated 37. The output of the DAC is fed directly to a first channel of a remote control transmitting unit which feeds information to the remote control station.
The transmitter is equipped with a remote control system which is a 30-channel remote control unit (Mosley PBR-30W), an automatic data printer (Mosley ADP-220) for F.C.C. logging, and a status control (Mosley SCS-Z) for instantaneous indications of outof-tolerance operation. All of the above units are known, and their outputs are multiplexed together by a multi-system combiner (Mosley MSC-l) to feed a single telephone line to the studios control point.
The control point is located at the studio where a multi-system combiner receiver splits the information to the data printer, status alarm, and the remote control unit.
The voltage output of the tower light monitor can be measured by the remote control meter and be logged by the data printer. The status control may also light a warning light if a beacon light fails.
Each sense line associated with an obstruction light S1137, is reduced in voltage down to logic level and rectified as indicated in connection with the beacon light sense lines; and they are all coupled to the inputs of an AND gate 39, the output of which feeds the number one input of the DAC 37. A local indicator light 40 is connected to the input number one of the DAC 37 and a second local indicator light 41 is connected to the number two input of the DAC 37. The output of the AND gate 39 is a ONE only as long as all of the obstruction lights S11S7 are continuously lighted. If any one of the obstruction lights goes out, the corresponding sense line input of the AND gate 39- goes to logic ZERO and the output also goes to ZERO.
Four of the seven inputs to AND gate 39, namely, sense lines associated with obstruction lights S1, S3, S5 and S7, are fed to inputs of an OR gate 45. Similarly, sense lines a, b, c and d (associated respectively with beacon light B1, B3B, B5B, and B7B) are fed to four separate inputs of the OR gate 45. These latter sense lines are also fed through capacitors 46-49 respectively into the inputs of an OR gate 50. The output of the OR gate 50 is fed into a monostable circuit 51 similar to that disclosed in connection with FIG. 2 and having an ON time again, approximately two flashing cycles. The output of the monostable circuit 51 feeds the number one input of a second DAC 52; and the output of the OR gate 45 is coupled to the number two input of the DAC 52. The output of the DAC 52 is connected to and feeds a second channel of the transmission unit feeding information to the remote control station. The inputs to the DAC 52 may be coupled to indicator lights 52a and 52b.
The output of OR gate 45 is'also fed through an inverter circuit 53 to the second input of the NOR gate 28. The output of the NOR gate 28 is then fed through an inverter circuit 54 to a third channel of the transmission unit.
Turning now to FIG. 3, the digital-to-analog converters 37 and 52 are similar, each having, as mentioned, a number one input and a number two input, as designated. The number one input is fed through an inverter circuit 55 to the base of a grounded emitter transistor 56. The collector of the transistor 56 is connected to the emitter of a transistor 57 and through a load resistor 58 to a positive voltage. The collector of the transistor 57 is connected to one terminal of a 11 resistive network 60, including resistors 61, 62 and 63.
The number two input of the DAC is connected by means of an inverter circuit 64 to the base of the grounded emitter transistor 65, the collector of which is connected to the emitter of a second transistor 66. This circuit is similar to that described in connection with the number one input lead, and the collector of transistor 66 is connected to the second terminal of the resistive network 60, which terminal is also the output of the digital-to-analog converter.
The digital-to-analog converter operates by the division of current through the resistive latter network including resistors 61, 62 and 63. When the inputs are both a logic ONE, transistors 57 and 66 are both conducting, and the combined current produces a 3-volt output signal across resistor 63. If input number one goes to a ZERO (also zero voltage), transistor 56 is driven to saturation through the inverter circuit 55, and it turns 0H transistor 57. Transistor 66 remains active, and it supplies enough current to produce a l-volt output signal.
If the signal on input No. 2 goes to a ZERO while the signal on input No. 1 remains a ONE, transistor 66 is turned off by the inverter 64 and transistor 65. In this case, transistor 57 remains conducting and it will supply current to the resistivelatter network producing an output voltage of 2 volts. This particular digital-toanalog converter circuit is not my invention. The values for the resistors in the latter network 60 for my preferred embodiment are as follows: resistors 61 and 63 are 8.2K ohms, and resistor 62 is 3.9K ohms.
The output voltage of the DAC may be measured at the receiver, after de-modulation, by an analog voltage meter 68.
OPERATION When the uppermost beacon light is flashing correctly, the output signal of monostable circuit 15 remains a ONE. Hence, the output of NOR gate 28 remains ZERO and the signal on channel three is a ONE. If this ONE signal goes to a ZERO, a detector which may be equipped with an audible alarm at the remote location may be used to indicate that the uppermost beacon light has gone out. In order to avoid the triggering of the alarm during daylight hours when the lights are off, the output of OR gate 45 is fed through inverter circuit 53 which will generate a ONE signal when all of the lights are out, to force the output of NOR gate 28 to a ZERO at the same time, thereby insuring a ONE signal on channel three.
When all of the obstruction lights on levels 2-7 are operating correctly, the AND gate 39 generates a ONE signal to the No. 1 input of the DAC 37. When the flasher unit is working correctly and all of the beacon light sets are operating, the monostable circuits 17 and 23-27 continuously generate ONE output signals, and the AND gate 29 generates a ONE. Hence, the No. 2 input to the DAC 37 is also a ONE.
With both inputs to the DAC 37 being ONEs, the output voltage is 3 volts. This signal is representative of the fact, therefore, that all beacon lights are flashing normally and all obstruction lights are on. If any one of the obstruction lights goes out, the output of the AND gate 39 will go to a ZERO, and the output of the DAC 37 drop to two volts, indicating that one or more obstruction lights has failed, but that the beacon lights are all operative.
If one or more beacon lights fails, then its associated monostable circuit will not energize the corresponding input lead of AND gate 29, and the No. 2 input of the DAC 37 will go to ZERO. This condition (as well as a flasher unit failure) will result in an output signal of 1 volt on the output of the DAC 37. If both one or more obstruction lights and one or more beacon lights fails, both inputs to the DAC 37 will be ZERO, and the output will also be volts. If the four obstruction lights having sense lines feeding the OR gate 45 are lit, or any one of the four beacon lights feeding that OR gate are lit, the No. 2 input of the DAC 52 will be-a ONE. Further, if any one of these four beacon lights are flashing correctly, the sense pulses will be differentiated by one of the capacitors 4649 to send a pulse by means of the OR gate 50 to the monostable circuit 51, and as long as the flashing continues to occur, the output of the monostable circuit 51 will remain a ONE. With both inputs to the DAC 52 being ONE, the output signal on channel 2 is 3 volts, indicating that power is on and the flasher unit is operative.
If the output of the monostable circuit 51 goes to ZERO, the output of the DAC 52 goes to two volts, indicating that although power is being supplied to some of the lights, the flasher unit is out. If power is lost, the output of both the OR gate 45 and the monostable circuit 51 will go to ZEROs and the output of the DAC 52 will be correspondingly 0 volts.
In summary, an operator at the remotely located control station using a conventional analog voltmeter (such as is schematically shown at 68 of FIG. 3) will switch first to channel No. 1 and read the voltage there. If it is 3- volts, all obstruction lights are ON and all beacon lights are operative and flashing. If he reads two volts on channel 1, he knows the one or more obstruction lights has failed. If he reads 1 volt, he knows that one or more beacon lights are out or the flasher unit base failed. If he reads 0 volts, he knows that at least one beacon and at least one obstruction light are out.
He then switches to channel 2, and if he reads 3 volts, he knows that the tower is lit by at least some lights, and that it is flashing. This would be helpful informa tion in the event that he had detected a 0-volt level from channel 1..If the second channel reads 2 volts, the operator knows that although some lights are operating but the flasher unit is not operating. If the voltage on the second channel is 0, the operator can surmise that power has been lost and that the tower is not lighted.
Finally, if the uppermost beacon B1 goes out at night and at least one of the other three beacons or one of the four obstruction lights sensed by OR gate 45 is lit, then channel 3 will generate an alarm signa (0 volts).
Having thus described in detail a preferred embodiment of the present invention, persons skilled in the art will be able to modify certain of the circuits which have been disclosed and to substitute equivalent elements for those illustrated while continuing to practice the principle of the invention; and it is, therefore, intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims.
1. Apparatus for remote monitoring of a lighting system for a tower including flashing beacon lights and continuously burning obstruction lights comprising:
first logic circuit means responsive to a plurality of said beacon lights for generating a first signal representative of said beacon lights being operative and flashing; second logic circuit means responsive to a plurality of said obstruction lights for generating a second signal representative of said obstruction lights being on; conversion circuit means receiving said first signal and said second signal for generating an output signal assuming one of aplurality of states including a first signal state when both said first and second signals are present, a second signal state when only said first signal is present, a third signal state when only said second signal is present, and a fourth signal state when neither said first nor second signal levels is present; and means for transmitting said output signal of said conversion circuit to a remote location.
2. The apparatus of claim 1 further comprising means at said remote location responsive to said received output signal states for detecting the same upon reception.
3. The system of claim 1 wherein said first logic circuit means comprises AND gate means having an input sense line for each of said plurality of beacon lights, the signal on said sense lines being ONE when its associated beacon light is lit and being ZERO when it is not lit; and monostable circuit means responsive to said AND gate means for generating an output signal for a predetermined time after said AND gate means generates a ONE output signal representative of the flashing of all beacon lights associated therewith, said output signal of said monostable circuit lasting for a time at least as great as the normal flashing period of said beacon lights, whereby the output signal of said monostable circuit will remain a ONE as long as all of said beacon lights are lit under normal operation.
4. The apparatus of claim 1 further comprising third logic circuit means including an OR gate receiving a plurality of sense lines from said obstruction lights and a plurality of separate sense lines from said beacon lights for generating a third logic signal representative of any one of said sense obstruction lights or beacon lights beint lit; fourth logic circuit means receiving sense lines from a plurality of said beacon lights for generating a fourth timed logic signal only when one of said sensed beacon lights is turned on, said fourth output signal lasting for a time at least as long as the normal flashing period of said beacon lights; and second signal conversion means receiving said third logic signal and said fourth logic signal for generating a second output signal and transmitting the same to a second channel to said remote location, said second output signal being in a first state when both said third and fourth logic circuit means are actuated, a second state when only said third logic circuit means and not said fourth logic circuit means is actuated, and a third state when neither said third nor fourth logic circuit means is actuated.
5. The apparatus of claim 1 wherein the uppermost light is a flashing beacon light, and further comprising fifth logic circuit means responsive to said uppermost light and to a plurality of other lights on said tower for generating an alarm signal only when said uppermost beacon light is not lit and at least one other light on said tower is lit; and means for transmitting said alarm signal to said remote location.
6. In a system for remote monitoring of warning lights on a tower including a plurality of continuously lighted obstruction lights spaced at different elevations and a plurality of beacon lights spaced at different elevations and periodically lit by a flasher means, each of said lights having an associated sense wire for indicating whether the light is on, the improvement comprising: first gate circuit means responsive to said sense lines associated with said beacon lights and including monostable circuit means for generating an output signal of a duration longer than a flashing cycle for generating a first signal representative of all of said beacon lights being lighted with normal frequency; second AND gate circuit means receiving the sense lines associated with said obstruction lights for generating a second signal only when all of said obstruction lights are lighted; and signal conversion circuit means responsive to said first signal and said second signal for generating an output signal, said output signal being a first level when said first and said second signals are both ONES, said output signal being a second level when said first signal is a ONE and said second signal is a ZERO, said output signal being a third level when said first signal is a ZERO and said second signal is a ONE, and said output signal being a fourth level when both said first and second signals are ZEROs.
7. The system of claim 6 further comprising OR gate means receiving a plurality of said sense lines of said obstruction lights and a plurality of said sense lines of said beacon lights for generating a third signal when any one of said sensed obstruction lights or any one of said sensed beacon lights is on; logic circuit means responsive to a plurality of said sense lines of said beacon lights for generating a third signal comprising a pulse of predetermined length greater than the normal flashing cycle of said beacon lights when any one of said sensed beacon lights is flashing; and second signal conversion means responsive to said third signal and said fourth signal for generating a second output signal, said second output signal being a first level when said third and said fourth signals are ONES, said second output signal being a second level when said third signal is a ONE and said fourth signal is a ZERO, said second output signal being a third level when both of said-third and fourth signals are ZEROs.
8. The system of claim 6 wherein each of said first and second output signals are transmittdd to a remote location over separate channels, said system further comprising an analog meter at the remote location for detecting the signal level of each of said channels.
9. The apparatus of claim 8 wherein the uppermost light is a flashing beacon light, and further comprising fifth logic circuit means responsive to said uppermost light and to a plurality of other lights on said tower for generating an alarm signal only when said uppermost beacon light is not lit and at least one other light on said tower is lit; and means for transmitting said alarm signal to said remote location.
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|U.S. Classification||315/130, 340/642, 315/132|