U.S. patent number 3,828,334 [Application Number 05/351,097] was granted by the patent office on 1974-08-06 for system for remote monitoring of tower lighting system.
This patent grant is currently assigned to Iowa State University Research Foundation, Inc.. Invention is credited to Leonard M. Wallace.
United States Patent |
3,828,334 |
Wallace |
August 6, 1974 |
SYSTEM FOR REMOTE MONITORING OF TOWER LIGHTING SYSTEM
Abstract
A tall tower equipped with a plurality of flashing beacon lights
and continuous obstruction lights is sensed at a remote location
for indicating whether all lights are in proper working order, and
if they are not, the system identifies the fault to the remotely
located operator.
Inventors: |
Wallace; Leonard M. (Ames,
IA) |
Assignee: |
Iowa State University Research
Foundation, Inc. (Ames, IA)
|
Family
ID: |
23379556 |
Appl.
No.: |
05/351,097 |
Filed: |
April 13, 1973 |
Current U.S.
Class: |
315/130; 315/132;
340/642 |
Current CPC
Class: |
G08B
26/00 (20130101) |
Current International
Class: |
G08B
26/00 (20060101); G08b 021/00 () |
Field of
Search: |
;340/251,248,253,331,223,409 ;315/129-132,88-93,149,87 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Caldwell; John W.
Assistant Examiner: Myer; Daniel
Attorney, Agent or Firm: Dawson, Tilton, Fallon &
Lungmus
Claims
I claim:
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 a plurality 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.
Description
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 remotely controlled 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 which permits
checking the operability of the tower lights at the remote control
station without having to visually observe the lights. The checking
of the lights may thus be accomplished more frequently and with
much greater facility than has been the case in the past.
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 notifies the 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
ON 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 converter.
In a preferred embodiment, the uppermost beacon light is sensed by
circuitry which provides an alarm signal 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 off 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
digital-to-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. 1; and
FIG. 3 is a circuit schematic diagram for the digital-to-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 10 in FIG.
1.
A separate sense wire is associated with each beacon light (B1-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 S1-S7.
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 11 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 C1--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 NOR 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-2) for instantaneous indications of out-of-tolerance
operation. All of the above units are known, and their outputs are
multiplexed together by a multi-system combiner (Mosley MSC-1) to
feed a single telephone line to the studio's 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 S1-S7, 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 S1-S7 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 .pi. 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 off transistor 57. Transistor 66
remains active, and it supplies enough current to produce a 1-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 resistive latter
network producing an output voltage of 2 volts. This particular
digital-to-analog 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 0
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 46-49 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 information 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.
* * * * *