CA2257000C - Traffic control system using light emitting diodes - Google Patents

Abstract

A traffic control system (10) including a road (12), a vehicle (14) traveling on the road (12) and a traffic light assembly (16) operatively associated with the road (12) is disclosed. The traffic light assembly (16) comprises at least one traffic light or lamp means (18). The traffic light (18) includes a plurality of light emitting diodes (24) for projecting light rays to the vehicle (14) to provide the driver of the vehicle (14) with a visual signal. The system (10) further comprises a transmitter (20) including a power supply (30) and a modulator (72) for turning the diodes (24) on and off at a rate that is imperceivable to humans while optically transmitting information to a receiver (22) disposed on the vehicle (14).

Description

CA 022~7000 1999-01-11 TECHNICAL FIELD
This is a divisional of Canadian Patent application 2,169,037 filed February 7, 1996 entitled TRAFFIC INFORMATION SYSTEM USING LIGHT
EMITTING DIODES.
The subject invention relates to a trafffic control system of the type including a traffic light that provides an illuminated signal for controlling the movement of a vehicle.
BACKGROUND ART
Trafffic lights have been used since the early 1900's to control 10 vehicular traffic flow by advising drivers when and where to stop, proceed, turn, etc. The growing use of traffic lights attests to their effectiveness in directing traffic flow, reducing the incidence of accidents, and most recently to their utility in controlling the flow of trafffic through large metropolitan areas when used in conjunction with computer driven systems.
Interestingly, the modern trafffic light is no different today than it was decades ago. The basic light still employs red, yellow and green filters over incandescent lamps that are sequentially turned on and off by an electromechanical timing switch or by more CA 02257000 1999-01-11 ~' modern solid state traffic controllers. Recent developments in solid state lamps, i.e. light emitting diodes tL.E.D.) have permitted the replacement of the conventional incandescent-based traffic lights with S L.E.D.-based traffic lights. Examples of L.E.D.-based traffic lights can be found in United States Patent numbers 4,729,076 and 5,136,287. The main advantages of L.E.D.-based traffic lights over an incandescent-based traffic lights are much lower power usage, i.e. higher -10 luminous efCiciency~ and much longer lamp life. Other than luminous efficiency and lamp life, othe- important properties of light emitting diodes have nct yet been utilized in traffic control systems. The present invention incorporates improvements in traffic light technology made possible by the use of light emitting diodes.

S~RY OF ~ L~VS~'~lO~ ~ND ADVaNTAG~S

The present invention is a traffic control system for controllins the movement of a vehicle by providing a visual signal in the form of light rays and for using the light rays to transmit digital information. The sys~em comprises a traffic light including a plurality of light emitting diodes for ~. .. .

CA 022~7000 1999-01-11 projecting light rays and a power supply for supplying power to illuminate the diodes for a predetermined duty cycle consisting of an ON period followed by an OFF
period short enough to be humanly imperceiva~le. The present invention is characterized by a modulator for varying the length of time of the duty cycle to trans~lt the digital information.

The primary advantage of the present invention is the provision of a traffic c~ntrol system that utilizes the light e~itted from a traffic light to optically transmit digital information to the driver of a vehicle.

1~ Another advantage of the present invention is the provision of a traffic light retrofit comprising a plurality of light emit.ing diodes which can be used to replace incandescent-based t_afric lights to thereby cut energy and maintenance costs.

Another advantage of the present in~ention is the provision of a L. E. D .-Dased traffic light having a battery bac~-up for provldins emergency power.

.

Another advantage of the present invention is a traffic light system that utilizes the light emitted from a traffic light to remotely detect and distinguish a "red", "yellow", or "green" light.

s ~IG~JRES IN T~ D~WIN~S

Figure 1 is a perspective view of the traffic control system of the present invention;

Figure 2 is a cross sectional view of the traffic light retrofit of the present invention;

Figure 2A depicts another aspect of the present invention;

Figure 3 is a cross sectional view of the traffic light assembly taken about line 3 - 3 of Figure l;

2~
Figure 4 is a schematic bloc~ diag am of the electronic circuitry utilized in the present invention;

Figure 5 is a sc~ematic bloc.~ diagram of additional circuit~y utilized in the present invention;

- CA 02257000 1999-01-11 - ' --Figure 6 is a signal diagram of amplitude versus time depicting one modulation scheme utilized in the present invention;

5Figure 7 is a schematic block diagram of the receiver of the present invention; and Figure 8 is a schematic bloc'; diagram of the simpli~led receiver of the present invention.

DET~ILED DESC~IPTION OF T~F~ PREEE~ED E.~BODI~NT

Referring to Figure 1, the traffic control system 10 of the present invention is shown. The t~affic control system lO includes a road 12, a vehicle 14 traveling on the road 12 and a traffic light assembly 16 ope-atiYely associated with the road lZ. The traffic light assembly 16 comprises at least one traffic light ar lamp means 18 for providing an illuminated signal.
Typically, the traffic light assembly 16 includes red, yellow and green traffic lights 17, 19, 21 for projecting red, yellow and green light rays to signal the driver of the vehlcle 14 to "stop", "yield", or "go", respectively. ~owever, it is to be understocd that while an intersection control light is depicted in CA 02257000 1999-01-11 ~--Figure 1, the benefits of this invention may be applied to a variety of other applications such as pedestrian c~ossing signals, e.g. WALR, DON'T WALX signals or any other illuminated "traffic related" message for controlling t_affic or simply providing information.

As shown in Figures 2 and 3, the preferred embodiment of each traffic lih' o~ lamp means 18 includes a plurality of light e~it~ing diodes 24 for trans~itting light rays from the t~affic light assembly 16. In other words, the red traf~ic light 17 comprises a first set of light emitting diodes 17A for projecting red light rays, the yellow traffic light 19 comprises a second set of light emitting diodes 19~ for pro~ecting yellow light rays, and the green traffic light 21 comprises a third set of light emitting diodes 21~ for pro~ecting green ligh~ rays. Each traffic light 18 fur her includes d.c. power connection means 26 secured to the traffic light 18 and elec'~ ically coupled to the diodes 24 for receiving d.c. power. The traffic light asse~bly 16 also includes a.c. power connection means Z8 seclred to the t-affic light 18 for receivin~ a.c.
power. The traffic light assembly 16 further includes d.c. power supply means 30 securec to each t_affic light Z5 18.

Referring to Figure 4, d.c. p~wer supply means 30 is electrically coupled to and between the a.c. p~wer connection means 28 and the d.c. pcwer connection means 26 for con~erting a.c. power received by the a.c. power connection me~ 28 to d.c. power so that d.c. pawer is supplied to said d.c. pawer connecti~n means 26. D.c p~wer supply means 30 may be ~f con~entional linear or switchmode design. D.c. power supply means 30 includes a step-down transformer to bring the a.c. line vol~ages of 120 - 140 volts down to approximately 3.5 volts and a standard full or half wave rectifier to convert the a.c power to d.c. power.

The traffic light assembly 16 is characterized by a battery 32 re~ovably connected to the traffic light 18 for supplying d.c. pcwer to said d.c. p~wer co~necLion ~c 26 to illu~ te the dicdes 24.
Preferably, the battery 32 is rec~argeable. For example, battery 32 cculd include any sealed lead -acid, nic~el - metal hydride, or lithium rec~argeable batteries. The traffic light ~CcPmhly 16 further c~y~ises battery charger means 33 secured to the tr~ffic light 18 and elec~rically coupled to and between the a.c. pawer connection means 28 and the d.c. power connection m~nC 26 for recharging the battery 32.

The interconnection ~f the backup battery 32 with the traffic light 18 is shown in Figure 4, and essentially provides for a parallel power source with the d.c. power supply means 30. Ordinarily, the output voltage of d.c. power supply means 30 would be the same as the rated operating voltage of the traffic light 18 and the nominal voltage of the battery 32. That is, if the traffic light 18 were deslgned ~o operate at 12 Volts, then the battery 32 would also have a 12 Volt output and would be charged at a comparable (slightly higher~ voltage from bat'ery charser 33. The battery 32 does not supply power to the traffic light 18 until a power failure, i.e. until a.c. pcwer from utility lines is no longer being supplied to a.c. power connection means 28.

Whenever a.c. utility power fails, the entire function of the st~n~rd traffic controller is compromised in addition to failure of the traffic lishts th~celves. ~nless a contral uninte~L~ible power supply is used to power both the traffic lights and the controller, the lights may not be operated because the crit~cal t~;ng function and sequencing normally provided by the controller is not available. Therefore, a specific aspect of this invention is to provide a safety default condition whereby all the red stop lights at an intersection are flashed whenever a.c. utility power fails. The default condition effectively establishes a four-way stop under battery power.

To implement the four-way stop default condition, traffic lisht assembly 16 further comprises a.c. power monitoring circuit means 3~ secured to each traffic light 18 for monitoring a.c. power supplied to the a.c. power connection means 28. A.c. power monitoring circult means 34 is preferably included as part of a pcwer pack 3S which also includes battery 32.
The power pac~ 35 may be attached to each traffic light 18 by various means. Monitoring means 34 monitors the lS lamp c~rrent at all the traffic lishts (red 17, green 19 and yellow 21~ of t~e traffic light assembly 16. The traffic lisht assembly 16 further comprises switch means 36 secured to each traffic light 18 for electrically coupling the batt Q 32 to the d.c. power connection means 26 in respanse to a trigger signal from the a.c.
power monitor~ng means 34 ~t u~p~ed by a predeter~ined decrease in a.c. power supplied to the a.c. power connection means 28. The traffic light ~Cc~mhly 16 further ~ L ises pulsating circuit means 38 secl~red to 2S each traffic light 18 and ele~L-ic211y coupled to the switch means 36 for causing the switch means 36 to electrically couple the battery 32 to the d.c. pawer connection means 26 at a predetermined frequency to illuminate the diodes 24 at the predeter~ined frequency.
S Preferably, the pulsating means 38 is included as an integral part of the pawer pac~ 35. If monitoring means 34 detects a loss of power to all traffic lights 18 of traffic light assembly 16, switch mean~ 36 connects the battery 32 to traffic light 18 and pulsating means 38 causes trafric light 18 to flash at the predeter~ined frequency. Preferably, only the red traffic light 17 is connected to the battery 32 upon a power loss so as to create the default flashing red condition. A typical flash duty cycle of 30~ will extend the battery 32 life cycle to approximately 12 hours for a 24 Watt/Hr ra.ed battery 32. Thus, the present invention allows low cost, autonamous, rechargeable battery pac~s to be adapted to L.~.D. - based traffic lanps in order to pro~ide low cost emergency operation when line power outages oczur, and to recharge automatic~lly upon resumption of utility power.

A smzll, low pawer ra~io transmitter 40 may be secured to the t-afflc light 18 to advise a cent~al traffic engineerins office of power outases or other CA 02257000 1999-01-11 ~ -~

problems. The radio transmitter 40 is electrically coupled to the switch means 36 for transmitting a radio wave in response to the trigger signal sent by monitoring means 34. ~s shown in Figure 3, a digitally s encoded radio transmitt_r 40 can ~e used to identify the location of the malfunctioning traffic light assembly 16. Alternati~ely, a cable modem could be used to relay this information. The traffic light-assembly 16 may include delay means 42 secured to the traf.ic light 18 and electrically coupled to ~he radio transmitter 40 for delaying the tr~n-cm;~csion of the emitted radio wave a predetermined t~e after the switch m~c 3 6 recPives the trigger signal. This delay is needed so that one radio frequency may be utiliz~d by a number of t.affic light assemblies without radio frequency collision by any t-~o traffic light ~c-cPmhlies.

In the most si~ple configuration, all the diodes 24 can be arranged on a disX, plate or printed circu~t substrate so that all of the di~des 24 are oriented in the same dm-ection. Preferably, the diodes 24 are mounted on circ~it board means 44. Circul' ~oard means 44 comprises c~ a planar printed circuit ~oard which includes c~nnection ~eans for elec~=ically coupling the diodes 24 in a plurality of series circuits 46 and for electrically coupling the series circuits 46 in parallel. Each series circuit 46 includes an e~ual num~er of light emit~ing di~des 24.

s Each traffic light 18 further comprises a traffic light housing 48. The light housing 48 comprises a hollow member 50 having a front open end 52 and a rear open end 54. The light housing 48 further comprises a front cover 56 for covering the front end 52 and a rear cover 58 for covering the rear end 5~. The diodes 24 are mounted within the hollow member 50 between the front and rear covers 56, 58. The front cover 56 comprises a transp~Tent glass or plastic material having a smooth auter surface 62. Transparent glass is necessary to allow light rays emitted by the diodes 24 to exit v~~tually un~mpe~ed and smooth surface 62 is preferrPd to avoid accumulation of dust and dirt.
m e front and rear covers 56, 58 for~ a hermetic seal wit~ the hollow ~ ~ 50. The t~affic cu..~s~l syst~m 10 further c~ ises an ~CcDmhly housing 64 for housing each traffic light 18. Each traCfic light 18 further includes mounting r~nC 66 for removably mounting each trafCic light 18 to t~e ~Cspmhly housing 64.

CA 02257000 1999-01-11 '~

The tra~fic light 18 is retrofitted to permit the use of the lisht in existing traffic li5ht assP~bly housings currently used to house incandescent traffic lights. Fcr ease of implementation, the traffic lights S 18 are preferably used with existing traff c lisht lens filters, which are nor~ally stippled or equipped with small lensatic ele ents to achieve correct beam dispersion. These lens elements are ordinarily designed to cperate in conjunction with a reflector and the quasi point source of a filament lamp. While a planar array of L.E.D.s w~ll operate satisfzctorily with existing filters and lenses, a mcra effective lens is needed to "fill-in" or illu~inate the spaces between diodes 24 and to steer the light rays emitted from the diodes 24 to the intended t~rget, such as the vehicla 14, to avoid wasting an~ l sh~ ene-sy. To this end, each traffic light 13 may include one or more ref-active elements 68 positioned bet-~een the diodes 24 and the front cover 56.
Refracti~e elements 68 act as a light ray steering means to steer the light rays emittad by the diodes 24 to the intended tarset. ~ltarnati~ely or in addition to ref~active elements 68, a spacer member 70 positioned betwean said circuit board means 44 and the d~cdes 24 can be used to p~int t~e dicdes Z4 in z pa-~icular direction. The spacar m~mker 70 includes an an~led - CA 02257000 1999-01-11 --~

s~rfacP 72 against which the diodes 24 abut to physically angle the diodes 24 relative to the road surfaca 12.

Another aspect of this invention relates to the ability of L.E.D. lamps to be pulsed at high rates;
i.e. to be turned on (illuminated) and off (not illuminated) at hiSh rates. By pulsing L.E.D.'s at hi~h rates, data can ke optically trans~itted from the traffic light 18 to a remotely located photodetector.
Thus, the diodes 24 can be used to tr~nsmit information to vehicle 14 such as announcing the presence of a traffic lisht and transmitting map coordinates, street names, directions or even trzffic advisories. Existing incandescent t~afClc lights cannot be used to t~ansmit digital data because the ther~al inertia of the fllaments in these lamps precludes thP rapid modulation of t~e supply current to cbtain detectable changes in light ou~ . Ob~iausly, slow on - of~ modulation of these lamps would be impractical as it would be annoying and confusing to drivers. The perceived luminosity of L.E.D. scurcas is a function of the average current that powers the L.E.D. For ex~mple, a d.c. value of 20mA
might y~eld roughly the same percPived luminosity from a L.E.D. as a s~uare wave (50% duty cycle) of 40mA peak - CA 02257000 1999-01-11 ~

current. The actual modulation of L.E.D. traffic lamps is rather simple, except for the fact that the percPi~ed lu~inosity of the lamps- needs to be essentially constant, as their main function is to visually alert S drivers. Message transmission is a secondary function that must not inter~ere with normal use. If the percaived luminosity of each traffic light is to remain c~nstant during operation, the average ~urrent delivered to the respective diodes must be essentially constant for periods greater than approximately 30 milliseconds.
This period corresponds to a flic~er frequency of a~out 33~z, which is perceptible by some humans.

Pulsing the dicdes 24 of the traffic light 18 allows the remote detection of the light rays illuminated there~y as well as the identification of the light rays, i.e., whether tie red, yellow, or green traffic light 17, l9, 21 is on. To distinguish whether eit~er the red, yellow, ar green traffic light 17, lg, 21 is ill~;nated, system 10 comprises a t~ansm~tt~r 20 including means for aptically transaitting data from one of the traffic lights 18 of assP~hly 16 by turning the first, second, and third set of diodes 17~, 19~, 21A on and off at three dist nc_ fre~uencies. For example, the fi_st set of light e~itting diodes 17A of the red , .. ~ .......................................... . .

CA 02257000 l999-Ol-ll ---traffic light 17 can be illuminated at 50 XHz, the second set of light emitting diodes l9A of the yellow traffic light 19 at 6~ X~z, the third set of light emitting diodes 21A of the green traffic light 21 at so ~Hz In this manner, the traffic lights 17, 19, 21, continue to serve their primary function of vlsually alerting drivers to stop, yield, or go while allowing the remote detecticn and identificatio~ of the lights The system 10 further comprises receiver means 22 located at a location remote from the traffic light assembly 16 For example, the recPiver means 22 can be disposed on the vehicle 14, as shown in Figure 1, for optically receiving the data transmitted by the t ansmitter 2~ The receiver means 22 further includes photo-detector mP~nc for converting the light rays emitted by the red, ~reen, and yellcw traffic lights into first, second and third ~LuuL signals of electric c~rrent haYing first, second and third output f-equencies, respecti~ely The rec~iver 22 further inc~udes di2ferential means for differentiating the first, second and thLrd output frequencies from each other to deter~ine whether the red, yellow ~r green t affic lisht is illu~inated The receiver 22 can be .

- . ~
~ CA 02257000 1999-01-11 --battery pawered, or pcwered by the electrical system of the vehicle 14.

Within the scope of the present invention and in addition to simple red, yellow and ~reen light detection, the system 10 may include digital data tr~n-cmission capability from the traffic light 18 to the vehicle 14. One or all colors (red, yellow, green and any turn signals, pedestrian ~essages, etc.) could be driven by a digital ccde generator to relay a message to oncoming vehicles, other tra~fic lights, or a central traffic center. Typically, each traffic light (red 17, yellow 19, and sreen 21) is "on" exclusively for a certain time, therefore all traffic lights 17, 19, 21 facing one direction could share the same message senerator which would then trans~it the same digital code for each c~lor change. Alte~natively, each individual traffic light 18 could be coded with a "color" precursor co~e to per~it mora sophisticated ~co~i ng schemes at the remote recPivers 22. The data rato ~ust ~e sufficiently high as to enable digital message t-~n~iC~ion within a single flash of a f~chi~g red or yellow traffic light 17, 19 in order to accommodate virtually all t~affic signal continsencies.
The most a~propriate use for s1lch traffic light message tr~n-cmi~sion capability w~uld be audible routing di~ections. For example, upon approaching a given intersection, the vehicle mounted receiver 22 might announce, ~Eastbound Metro Parkway at Hayes". N~
directional ambiguity exists because the message is only directed at oncoming traffic. Obviously, the transmitter 20 would have to be proçrammed to deliver the correct message, but this need be done only once, as the light remains at the same location for many years.

If an L.E.D. lamp is required to maintain an essentially constant, perceived output, the modulation imparted to the L.E.D. must be suf'~iciently ~ast and exhibit a constant a~erage duty cycle. Several digital madulation schemes are capable of this limitation.
Pulse position modulation (PPM), phase shift ~eyed (PS~) mcdulation and f-equency shl~t modulation (FS~) are typical for~s of an essentially constant duty cycle modulation. The F.S.~. modulation scheme is shown in Figure 6, as it is amcng the simplest to implement, and it is widely employed in wired and wireless communication links. ~o distinct fre~uencies F1 and F2 are used to signal the digital l's and O's or mar~ and space data that consti.ute the messase or data to ~e 2S transmitted. The ad~antage of using such a digital ~ . . ..

.. 19 mcdulation scheme is that the average L.E.D. lamp current (and power) r~m~ i n~ essentially constant irrespective of the data. The reason for this is readily apparent in the diagram depicted in Figure 6 S which indicates a constant 50~ duty cycle for any data stream. The result of such modulation i5 that the visible (perceived) brightness of the lamp is in~ariant, while high rat~ digital data is being trans~itted ~r even when no data is transmitted (all l's or o~s).
Ideally, the F.S.R. fre~uencies F1 and F2 representing the digital l's or 0's would be separated by several hundred Hertz or kilo Hertz to facilitate unambiguous detection. Having the fre~uencies within a 10~ band would allow a single tuned input amplifier to act as preselec'or filter to the frecuency detectors.
Operating the L.E.D. lamps in the 10 to 20 R~z range may be ad~antageous. This fre~uency band permits easy differ_ntiation f-om high intensity discharge (~.I.D.) lamps s~ch as street lights or neon lights. These light sources may ha~e s;m;lar specLral characteristics as the traffic lamps, and ther~y interfere with relia~le detection of the traffic lights 18.

Referring to Figure 5, the transmitter 20 that mcdulates the traffic light 18 is pcwered by the d.c.

- CA 02257000 1999-01-11 , ' .

power supply means 30. A.c. pcwer from a conventional traffic light controller in turn powers the d.c. power supply means 30. The transmitter 20 comprises a non-volatile local memory (E~P~OM~ or one time programmable tO.T.P.~ memory chip 78 for electronically storing the digital message to ~e transmitted. The message itself could be coded in virtually any for~ such as ASCII or a proprietary alphanumeric code. Additionally, digitally coded, compressed voice communication could be transmitted along wit~h the alphanumeric text or instead of it facilitating the regeneratian of high quality voice messages at the remote receiver 22. The transmitter 20 furt~er comprises a mic-oprocessor 72 for controlling the removal of the digital message stored in the ~emory chip 78. The microprocessor 72 also controls the tr~C~ission of the digital- message.

The tr~ncm;tter 20 further includes a solid state switch 80. The solid state switch 80 converts the coded digital ~~~ge developed by the ~i~o~Lo-essor 72 into F.S.~. for~at and thereafter transfers the F.S.g.
nodulated signal to the diodes 24 of the t_a~ric light 18. In other words, t~e switch 80 controls the power from the d.c. power supply means 30 to supply power tc illuminzte the dicdes 24 ~or a predeter~ined duty cycle CA 02257000 1999-01-11 .~

co~.sistins c~ an ~'i re~~od foilowed hy 2~ o~r pericd short enough to be humanly imperceivable. The microprocessor 72 further includes modulator means for varying the length of time of the duty cycle to t~ansmit the digital information. Microprocessar 72 also includes signal processor means for maintaining a constant ratio between the ON and OFF periods as the length of time of the duty cycle varies. Signal processor mezns includes means for establishing a first lens.h of time for the duty cycle to create a fi_st digital sisnal cor-esponding to a digital "l" and for estaklishing a secon~ length of time for the duty cycle to create a second digital signal corresponding to a digital "O". In other words, the first length of time correspon~s to frecuency F1 and the second length of time corresponds to freauenc~ F2;

The t_a-.ic light assem~ly 16 may ~nclude power sensins mear.s 74 for sensing when d.c. pcwer is delivered from the d.c. powe- supply means 30 and a reset generator 76 for reset'ing the microproc~ssor 72 each time d.c. power is delive-ed to the traf.lc light 18 to initlate a new dlgital message secuer.ce. The t_afCic light assembly 16 may also include ambie-.. light Z5 sensor means 60 for varying the luminosity of the diodes .

- CA 02257000 1999-01-11 t 24 in response to ambient light variations. Ambient light sensor means 60 includes a photodetector 60 coupled to a power control element 82. Power control element 82 controls the quantity of power delivered to the diodes 24 in response to the level of ambient light detected by the photodetector 60. For example, at night, the voltage of the pulses delivered to the tra fic light 18 may be reduced to dec-ease briahtness to fu__her reduce power consumption while in bright sunli5ht the sensor means 60 would cause the power control element 82 to increase the voltage of pulses delive_ed to the traffic lisht 18. The power control elemen. 82 could consist of a linear voltage resulator, o_ a low loss high frequency pulse width modulation (~.W.~.) cont_oller, both or which are comme~cially available devices. Note that if puIse control is utilized to control lamp brightness, the operating f-ecuency of such a device would have to be significantly greater than t.~e modulation frecuencv in order to nat af L ect the digital coding.

The companion receiver 22 receives the digital infor~ztion transmitted by the diodes 24. Refer-ing to Figure 7, receiver 22 consists of an o~tlcal wavelength selecl-ve filter 84, a collec_ion lens 86, and a :- CA 02257000 1999-01-11 .-receiver photodetector 8$ coupled to a tuned amplifier 90 and a limiter 92. The photodetector 88 converts the light rays emitted by the dicdes 24 into an output signal cf electric current. The cptical wavelength selective filter 84 allows only selective frequencies of light, such as F1 or F2, to reach the photo detector 88.
The collection lens 86 directs the llght rays emitted by the dlodes 24 to the photodetector 88. The ampll ie_ 90 ampliCies the output sicnzl. The amplifier 90 is typically peaked to respond preferentially to the F.S.K.
f-ecuencies employed by the transmitter 2 0. F.S.K.
detection may be advantageously provided by a receiver mlcroprocessor 94 whlch then also handles character decoding (using nonvolatile resident memory) and the lS messase dlsplay functlons. ~he receiver 22 further c~mprises visual messase disolav means 96 for visually displaying the digital infor~ation transmitted by the diodes 24. Typically, a licuid c~stal, vacuum fluorescent, L.E.D., or miniature C.R.T. display wauld be used to display the messase trans~itted by the traffic light 18. The receiver 22 may also be e~uipped with a voice generation means 98 comprisins a speaker means 99 for audibly conveyin5 the digital in~or~ation ._ans~it.ed by the dicdes 24. For voice message zoplications, de~icated voice synthesis intesrated CA 02257000 1999-01-11 '~

circuits would convert the digitized, transmitted voice announcement back into human speech for review by the driver. Modern linear predicti~e coding techniques and specialized voice I.C.1s (as used in many toys) could be used to simplify the delivery of audible messages of several seconds duration.

Referring to Figure 8, a simplified alternate receiver 22 1 is disciosed for use in detecting whether the red 17, yellow 19, or green 21 traffic light is illuminated. Rece1ver 2Z' conslsts of an optical wavelength selective filter 84', a collection lens 86', and receiver photodetector 88' coupled to a tuned amplifier 90' and a limiter 92'. The receiver 22' furthe- includes an alar~ generator 100 for driving a s~ea~~ 9~'. As an example, the receiver 22' could be set up to sound an alarm upon the detection of a red lisht as transmitted by the first set of diodes 17A.

Once the reouisite hardware is installed in a c-itical nl~m~er cf t~af,~ic lights 18 and ve~icles 14, the t_~ffic control system 10 may be used to transmit trafflc advis~ry messa~es. That is, the system 10 need nct be limited to repetitive prerecorded messages. A

t_a-Ci_ lisht 18 positioned at specific points alon~ a road 12, such as an intersection, may be directly accessed by cable or a wireless link from a central traffic authority to trans~it messages to passing motorists. One method to accomplish such a ~unction would be to download digital messa~e data to selected tra~ic lights 18 for subsequent relay to passing vehicles 12. Time-out or "sunset" provisions in the sof.wzre or traffic light t_ar.smitter 20 hardware could be i~?lemented to erase messages that were no longer per_inent.

Another use for the tra~fic control syste~ 10 described herein involves the use of the digitally coded tra~c lishts 18 as "signpcstsl' for vehlcular 1_ navicztion systems. Digitallv modulated tra~fic lights 18 could serve as convenient, low cost sign posts .or vehicular navigation systems, whe e unambiguous loc-~tion is essential. Traffic lights are ubi~uitous, and are easily adapted to transmit dlgital location infor~ation as well as ~ther data.

The invention has been described in an illus.rative manner, and it is to be understood that the ter~-nology which has been used is intended to be in the nature of words of description rather than of limitation.
Obviously, many variations and modifications are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims wherein reference numerals are merely for convenience and are not to be in any way limiting, the invention may be prac~iced otherwise than as specifically described.

_ .

Claims (6)

What is claimed:

1. A traffic control system (10) for controlling the movement of a vehicle (14), said system (10) comprising:
traffic light means (16) for projecting light rays to a vehicle (14), said traffic light means (16) including first light means (17) for projecting first light rays to the vehicle (14) and second light means (19) for projecting second light rays to the vehicle (14);
a transmitter (20) including means for illuminating said first light means (17) at a first frequency to transmit said first light rays from said first light means (17) at said first frequency and for illuminating said second light means (19) at a second frequency to transmit said second light rays from said second light means (19) at said second frequency;
a receiver (22,22') including photodetector means (88,88') for converting said first light rays into a first output signal of electric current having a first output frequency and for converting said second light rays into a second output signal of electric current having a second output frequency; and characterized by said receiver (22,22') further including differential means (92,92') for differentiating said first output frequency from said second output frequency to detect the presence of one of said first or second light rays.

2. The traffic control system of claim 1 wherein said first light means (17) comprises a first set of light emitting diodes (17A).

3. The traffic control system (10) of claim 2 wherein said second light means ( 19) comprises a second set of light emitting diodes (19A).

4. The traffic control system (10) of claim 3 wherein said receiver (22) further includes means (96) for visually indicating detection of illumination from one said first (17) or second (19) light means.

5. The traffic control system (10) of claim 1 wherein said receiver (22,22') further includes means (98,99,100,99') for audibly indicating detection of illumination from said first (17) or second (19) light means.

6. A method for optically transmitting data from a traffic light assembly (16) to a receiver (22,22,) disposed on a target (14) wherein the traffic light (18) is of the type for controlling the movement of the target (14) and is of the type including a first light (17) for projecting first light rays to the target (14) and a second light (19) for projecting second light rays to the target (14); said method comprising the steps of:
illuminating the first light (17) at a first frequency to transmit the first light rays from the first light (17) at the first frequency;
illuminating the second light (19) at a second frequency to transmit the second light rays from the second light (19) at the second frequency, converting the first light rays into a first output signal of electric current having a first output frequency;

converting the second light rays into a second output signal of electric current having a second output frequency; and differentiating the first output frequency from the second output frequency to detect the presence of either the first or second light rays.