US3189780A - Cathode ray tube controlled relay means - Google Patents

Description

June 151, 1965 MAsAo TAKAG: ETAL 3,189,780

GTHODE RAY TUBE CONTROLLED RELAY MEANS Filed April 1, 1963 2 Sheets-Sheet l MASAO TAKAGI ETAL CATHODE RAY TUBE CONTROLLED RELAY MEANS Jupe 15, 1965 2 Sheets-Sheet 2 Filed April l, 1963 CIRCUIT 3,189,769 CATHDE RAY TUBE CNTRGLLED RELAY MEANS Masao Talragi, lUlyo-iru, Kyoto, and Keizo Yamaii, iviishima-gnn, @salta-Su, lapapa, assignors to Tateisi Denki Kabushilriiraisha, Kyoto, Saipan, a corporation of apan Filed Apr. 1, i963, Ser. N 269,475 Claims priority, application Japan, Apr. 7, i962 (utility models), 37/l7,755, 37/f17,756 7 Ciairns. (Cl. 315-10) This invention relates to relay means and, more particularly, to relay means in which a cathode-ray tube and photoelectric means are employed to open or close electric circuits when a predetermined value of voltage has been reached.

Meter relays are adapted for general use as means for opening or closing a circuit when voltage under supervision has arrived at a predetermined level. These relays are generally provided with a needle which is able to be preset to a certain position and another needle which is movable in response to a given signal. Each of the needles carries a contact which is to be brought in touch with a contact mounted on the other. In response to a given signal, the movable needle is moved until the signal attains a predetermined level, when the two contacts touch -to produce a control signal to `actuate a relay.

Because of their mechanical construction, such meter relays have serious shortcomings: for example, the mechanical movable needle cannot respond instantaneously to a given signal due to the relatively great moment of inertia acting thereupon, especially when the signal liuctuates into transient values; and the existence of contacts involves many problems such as overheating due to contact resistance.

Accordingly, the primary object of this invention is to provide means for controlling electric circuits by output control signals produced when input signals arrive at predetermined levels, wherein no mechanical movable members such as a needle are employed. Instead, the relay means of the invention employs: a cathode-ray tube so constructed that the electron beam is deected in accordance with a given electrical signal and strikes a fiuorescent screen at a position thereon corresponding to the beam detlection so as to cause a luminous area to appear at that position; and light-sensitive means for receiving light from lthe luminous area on the screen of the tube when the given signal has attained a predetermined level, and producing a signal to open or close the electric circuit under control. Thus, in accordance with the invention no mechanical movable members such as needles are used, so that the above defects encountered in the meter relay can be eliminated.

Other objects, features and advantages of the invention will be more apparent from the following detailed explanation of preferred embodiments of the invention with reference to the accompanying drawings, wherein:

FIGURE l is a schematic showing of an embodiment of the invention, with the cathode-ray tube and photoelectric means having their Walls cut-away to show the insides thereof;

FlGURE 2 is a side sectional view of the cathode-ray tube and photoelectric means of FIGURE 1;

FIGURE 3 is a schematic, partially cut-away view of the cathode-ray tube and photoelectric means of another embodiment of the invention;

FIGURE 4 is a side sectional view of FIGURE 3;

FIGURE 5 is a schematic side view of another embodiment of the invention;

FlGURE 6 is a schematic bottom plan view of FIG- URE 5;

ld@ Patented ,inne l5, i965 FIGURE 7 is a schematic block diagram of another embodiment of the invention;

FIGURE 8 is a schematic diagram ofthe limiter circuit of FIGURE 7;

FIGURE 9 is a schematic block diagram of still another embodiment of the invention;

FIGURE 10 is a schematic diagram of the Schmitt circuit of FIGURE 9; and

FIGURE 11 is a schematic diagram of the NOT circuit of FIGURE 9.

The invention comprises: a cathode-ray tube including a iluorescent screen, electron gun means for emitting electrons, forming said electrons into a beam and focusing said beam onto said screen in a luminous area, and means for deiiecting said beam; means for applying an electrical signal to said deilecting means; photoelectric means including a photoelectric element for receiving light from said luminous area, said photoelectric means being so positioned relative to said screen that said photoelectric element receives light from said luminous area only when said electrical signal has reached a predetermined level; and means for opening or closing electric circuits in response to a signal produced when said photoelectric element is excited by light from said luminous area on said screen.

Referring in detail to FIGURES 1 and 2, there is shown a cathode-ray tube l including an electron gun generally designated at Z, a deflection system 3 consisting of a single pair of plates spaced apart symmetrically about the axis of the gun and having flared ends, and a liuorescent screen 4 deposited on the inside of the end `face of the tube. The tube is shown flat and the screen is of an elongated rectangular shape. Although the illustrated shapes are preferred, they may be of any other suitable shape. The electron gun consists of a cathode 5, a control grid 5', a first accelerating electrode 5, a second accelerating electrode 6, a beam-forming electrode 7 having an aperture formed therein, a focusing system 8 consisting of a pair of spaced plates, and a third accelerating electrode 9.

Electrons emitted from cathode 5 are accelerated as they pass through accelerating electrodes S, 6, formed by the aperture of electrode 7 into a beam having an elongated, narrow rectangular section, focused by means of focusing system 8, further accelerated by electrode 9, and then pass between deliecting plates 3. Within the space between the plates there is created an electrostatic field whose strength is determined by the strength of an electrical signal applied thereto. The electric signal may be `derived from any electrical phenomenon to be supervised, or any quantity to be regulated. For example, in the measurement of voltage, the signal may be derived from the voltage under measurement or a voltage proportional thereto; or if a quantity is to be automatically controlled, the signal may be derived from a voltage corresponding to the quantity.

To .the deilecting plates are connected terminals 11), l, between which is impressed `a signal voltage such :as described just above. This creates an electrostatic field proportional in magnitude .to the applied voltage. As the elect-ron beam passes through the electrostatic field, it is deflected an amount proportional to the magnitude of the held and, consequently, to the voltage under measurement or the quantity under :automatic control. The beam deflected in this manner finally strikes 4the uorescent lscreen at 1a position corresponding to the beam deliection so as to produce there a defined, luminous area. Preferably, the luminous area .on the screen takes the form lof a line, preferably extending along the short axis of the screen from the upper to the lower edge thereof. When the `applied voltage changes, the deflection angle of the beam also changes, causing the luminous area on the screen to move along the long axis of the screen a distance corresponding to the beam deilection.

rThe `structure of the cathode-ray tube of the invention `and its operation in connection with the electron beam deflection differ from .those of ordinary cathode-ray tubes, as follows: While ordinary cathode-ray tubes are provided with two sets of deiiecting plates, that is, horizontal `and Vertical deflection plates, the cathode-ray tube of the invention is provided with a single set of deecting plates so that the electron beam is deflected in .such a manner that displacement of the luminous `area on the screen takes place .along only one predetermined. axis of the screen. `If desired, there may be provided two or more sets of deecting plates of the same function, that is, detlecting their respective beams individually in the :same direction. In ordinary cathode-ray tubes, lthe luminous Iareajon the screen must take no other form than a tiny spot, whereas in the tube of the invention it may be a spot or a line or of any other suitable shape. Finally, the cathode-ray tube of the invention is prefer-ably hat, compared with ordinary `funnel-shaped tubes.

IIf a calibrated plate Iis provided for indicating the position of the luminous area on the screen, it will be possible to read the magnitude of the electrostatic field between the -deliecting plates and, consequent-ly, that of the voltage under measurement or the quantity under supervision, since any slight change in the Voltage or the quantity results in a corresponding lchange in the 4magnitude of the electrostatic Ifield and, consequently, in the position of the luminous area on the screen.

In front of the screen 4 there is placed a photoelectric means generally indicated 12 at such a position that it may receive light from the luminous area on the screen when the :area has moved to that position on the screen. The photoelect-ric means comprises a casing 13 with an open end facing the screen of the cathode-ray tube, a convex lens 14 iitted into the open end `of the casing, and a photoeiect-ric element -15 enclosed in the casing. The lens serves to collect light from the luminous .area on the screen and focus it onto the photoelectric element. The element may be composed of any `photoelectric material. Most widely used in cadmium sulfide which Vis highly sensitive to light in the visible region of the spectrum, and the most suitable is a germanium phototransistor.

The photoelectric element is connected to the input side of an amplifier 16, at the output side of which appears yan ampliiied voltage when the photoelectric element is ,appropriately illuminated. The loutput voltage controls an electric circuit, as through an auxiliary relay y17. The relay may be of an electromagnetic type having .an energizing coi-l 1S, which is excited by the output of the amplifie-r to open or close contacts 19, 20,

Mention has .already been made that a change in the voltage applied t-o the deflecting plates causes a corresponding change in the `deiiect-ion angle of the electron beam and, consequently, :a corresponding displacement or K the luminous area on the screen of the cathode-ray tube. If the displacement is such that light from the luminous area enters photoelectric means 12 preset to a certain position and excites photoe-lectric element 115, the output of amplifier 16 actuates relay 17. By selectively varying the position of the photoelectric means along the length of the screen, it is possible to regulate the value of the voltage applied to the deflecting plates when the relay is caused to operate. In other words, it is possible to detect that the voltage under measurement or the quantity under supervision has passed a predetermined value, and to take a proper measure such as eliminating the cause of the event through operation of relay 17.

With a single photoelectric means, it is possible to de- .rect that the voltage applied to the deflecting plates and, consequently, the quantity being supervised have passed through, that is, either increased from below to above or decreased from above to below, a predetermined level.

With .a Vpai-r of photoelectric means spaced a desired distance .apart from each other as shown in FIGURE `l or 3, it is possible to place both the upper and lower limits on the voltage under super-vision and detect that the voltage has gone out of the range. More than tworphotoelectric means may of course be provided, =In this case, the photoelectric element in each of the plurality of photoelectric means is excited when the luminous area moving across the screen has passed a position corresponding .to the position of each of the plurality of photoelectric means. Therefore, it is possible to detect the voltage under supervision having passed each of a plural-ity of predetermined values.

In accordance with the invention, the voltage to be applied .to the deiiecting plates of the cathode-ray tube may be either direct or alternating. When it is alternating, one -cycle of the voltage causes one reciprocating Imovement of the luminous area across the screen. The distance to be traversed by the 'luminous area varies with the amplitude rof the applied voltage. When the amplitude of the voltage and, consequently, that of movement or" the luminous area has exceeded a predetermined vaiue, the photoelectric element in a photoelectric means corresponding to the value is excited by the light from the luminous area to actuate relay 17.

For-the purpose of .the invention, lens 14 is not `always required. When there is only a slight amount of light 4falling on the photoelectric element, however, the convex lens helps to collect yand focus the light onto the element.

In the embodiment of FIGURES l and 2, the convex lens 14 is fitted into the open end of casing 13. In the embodiment of FIGURES 3 and 4, however, the end face of the cathode-ray tube 1, on the inside of which is deposited a iluorescent screen 4, isV formed into a .convex lens 21 extending over the length of the screen. Vlfhe len-s may occupy the whole end face of t-he tube, or either of .the upper and lower sections extending lengthwise of the tube face. The -lens may also occupy the middle section extending lengthwise of the tube. The function of the lens is quite the same as that of FIGURES l and 2.

As previously mention-ed in connection with explanation of the embodiment of FIGURES 1 and 2, it is preferable that the photoelectric means be `so arranged as to be selective-ly movable to any desired position along the length of screen 4. FIGURES 5 and 6 shows another embodiment of the invention provided with means suitable for such selective movement of the photoelectric means. To explain in further detail, the end face of the Ifiat cathode-ray tube 1 and, consequently, the screen 4 4is for-med into a `circular arc. A iixed shaft 31 is disposed either above or below the 4tube and concentric with the cent-er of the .arc of the Screen. ARotatable about the shaft is mounted an L-shaped lever 32 with its bent, free end carrying a photoelectric means 12 so that whereever along the arc of the screen the photoelectric means may have been rotated about the shaft to be positioned, it always yfaces the screen equidistantly spaced therefrom. Thus it becomes quite easy to move the photoelectric means to any desired position along the length 4of the screen, and to keep constant the amount of light falling upon the photoelectric element from the luminous area on the screen, regardless of the relative position of the photoelectric means. When a plurality of phot-oelectric means are provided, they are mounted on their respective L-shaped levers in the above-mentioned manner. In FIGURE 6 there are shown two L-shaped levers 32 carrying their respective photoelectric means 12.

Mention has already been made that as the voltage applied to the .deflecting plates of t-he cathode-ray tube varies, the luminous area on the screen moves across it. If the movement is too fast, it often happens that the luminous area runs past a position corresponding to the preset position of a photoelectric means too rapidly for the photoelectric element enclosed therein to 4receive a suicient amount of light to cause relay 17 to be actuated.

aisa'reo 5 FIGURES 7 and 8 show another embodiment of the invention provided with circuit means especially directed toward solving this problem. The circuit is arrange-d so that when the voltage applied to the deflecting plates of the cathode-ray tube varies enough to cause the luminous area on the screen to move beyond ythe preset position of the photoelectric means, the voltage is limited to such a value as to make .the luminous area stay vat the required position so that -t-he photoelectric element may receive a suiicient amount of light from the luminous area for the relay to be actuated.

To explain the circuit arrangement in further detall with refe-rence to FIGURES 7 and 8, a deiiecting circuit 41 is connected to the deecting plates 3 which are enveloped in a flat cathodearay tube 1 with a uorescent screen 4 deposited on the inside of the tube face. To the input side of the deecting circuit is connected a limiter circuit 42, to the terminal 43 of which is applied la voltage corresponding to a quantity under supervision. The limiter circuit applies its output to the deiiecting circuit, which in turn applies to the deflecting plates its output signal voltage proportional to its input, that is, the output from the limiter circuit. It will need hardly mentioned that because of the limiter circuit, no voltage higher than a predetermined level will be applied to lthe deflecting circuit 41.

As shown in FIGURE 8, the limiter circuit 42 has a tube 44 shown as a triode which functions as .a limit tube. This -tube has its anode 45 connected to an output terminal 46, to which is to be connected the deflecting circuit 41. The grid 47 of tube 44 is connected to the terminal 43. In order to vary the output of the limiter circuit, a variable resistor 49 is connected to the grid 47 of .tube 44:-, and the slider 50 of this resistor is connected to the photoelectric means 12 in such a manner that setting the slider to a point of the resistor results in a cor-responding setting of the photoelectric means to a position along the length f the screen. 'It will be easily seen that the value of grid bias applied to tube le and, consequently, the value of its output voltage is determined by the setting of variable resistor 49.

Suppose that the slider 5G is in Contact with a point T of resistor 49. The portion of the resistance existing between the point T and ground G will determine the value of grid bias, which will in tu-rn determine the v-alue of voltage appearing at terminal 46. Suppose that this output voltage is 80 v. 'Simultaneously with the setting of the slider 50, the photoelectric means i2 has been moved along the -length of the screen to a position corresponding to lthe position on the screen to which the luminous area on the screen is to be displaced when a voltage corresponding to the output, now supposed to 4be 80 v., of `the limiter circuit 42 is impressed on the deiiecting plates 3. With the photoelectric means thus positioned, so Along as the voltage under supervision is below 8O v., the luminous area on the screen will not be displaced enough for light therefrom to -fall on the photoelectric element in the photoelectric means. lEven if the Voltage happens to rise above 80 v., the circuit 42 l-imits it and holds it at 8O v., thereby preventing the luminous area on the screen from moving beyond the position corresponding to 8O v., and keeping the luminous area staying at that position so that light therefrom falls on the photoelectric element in an amount great enough for rel-ay 17 to be actuated without fail.

With the arrangement of FIGURES 7 and 8, in order for the photoelectric element to receive enough light from the luminous area on the screen of the cathode-ray tube for reliable operation of relay 17, the voltage to be applied to the deflecting plates is regulated so as to prevent the luminous area from moving beyond a position corresponding to the preset position of the photoelectric means. To accomplish the same purpose, the circuit arrangement of FGURES 9, l() and ll may be effectively employed.

With this arrangement, so long as the voltage under supervision is within a predetermined range, no signal is applied to the detlecting plates, with the luminous area staying motionless at a position on the screen, and it is only when the voltage under supervision iluctuates outside the predetermined range that the deflecting plates are impressed with a voltage suicient to move the luminous area on the screen to a position corresponding to the preset position of either of the two photoelectric means dening the range, so that light 'from the luminous area illuminates the photoelectric element suiciently for the auxiliary relay to be actuated.

In FIGURE 9 there is shown a cathode-ray tube 1 including a deflecting system 3 consisting of a pair of spaced plates 31 and 32. Plate 31 is connected to a deflection control circuit 51 consisting of a Schmitt circuit A, while plate 32 is connected to another deilection control circuit 52 consisting of a Schmitt circuit A and a NOT circuit B. The input sides of the two Schmitt circuits are connected to a terminal 53, t-o which a voltage to be supervised is applied.

The wiring diagrams of the Schmitt circuit and the NOT circuit are shown by way of example in FIGURES 10 and 11, respectively. As shown in FIGURE 10, the Schmitt circuit includes two triodes 54 and 55. With a voltage supplied from a B+ source, the grid 57 of triode 54 is biased negative due to a voltage drop across variable resistor 56 so that this triode becomes non-conducting. With triode 54 cut 0H, no anode current flows therethrough, causing no voltage drop across resistor 58. This makes the grid of triode 55 positive with respect to its cathode 60, so that this triode is kept conducting.

In FIGURE 11, the NOT circuit has a triode 61 whose cathode 62 is biased positive so that when a relatively high voltage is applied to grid 63, a relatively low output appears at terminal 64, while when the voltage applied to the grid is relatively low, the output appearing at the terminal is relatively high.

As previously mentioned, a voltage to be regulated or a voltage proportional thereto is applied to terminal 53 and thence to the input sides of the two Schmitt circuits. Now suppose that it be intended to keep the voltage within the range of 60 to 70 v. Appropriate adjustment of the respective variable resistors 56 of the two Schmitt circuits makes it possible that the Schmitt circuit of circuit Sl operates when the voltage applied to terminal 53 exceeds 70 v. or a value proportional thereto, and that the Schmitt circuit of circuit 52 operates when the voltage applied to terminal 53 exceeds 60 v. or a value proportional thereto.

So long as the applied voltage is within the range of 60 to 70 v., the Schmitt circuit of circuit 51 does not operate, so that the triode S5 is conducting, applying a voltage V1 to the detlecting plate 31, this voltage V1 being equal to the voltage of the B source minus the drop across resistor 65. On the other hand, the Schmitt circuit of circuit S2 is in operation, with its triode 55 non-conducting, so that the triode 61 of the NOT circuit is conducting, applying to the dellecting plate 32 a voltage V2 which is equal to the voltage of the B source minus the drop across resistor 66. If the circuits are so arranged that the two voltages V1 and V2 become equal, the electron beam within the cathode-ray tube experiences no deflection and the luminous area produced on the screen of the tube is held at the center thereof.

When the input voltage at terminal S3 happens to rise above 70 v., the Schmitt circuit of circuit 51 operates so that the triode 55 -becomes non-conducting and the voltage applied to the detlecting plate 31 becomes equal t0 the voltage of the B source, This causes a deflection of the electron beam proportional to the difference between the two voltages V1 and V2, and a corresponding displacement of the luminous area on the screen. Then, with a photoelectric means 121 preset to a position correspending to the displaced position of the luminous area,

light therefrom falls upon the photoelectric element contained in the photoelectric means sufliciently to produce an output, which is amplified by an amplifier 161 to actuate an auxiliary relay 171. Upon actuation of the relay, an automatic control system operates to reduce the voltage under control to a required level. When the voltage has thus been decreased below 70 v., the Schmitt circuit of circuit 51 is rendered inoperative again, so that the voltage V1 is reduced to its initial level, i.e., equal to V2, and the luminous area on the screen is returned to the center thereof; consequently, no light therefrom enters the photoelectric means 121 and the relay 171 is de-energized.

If the voltage at terminal 53 decreases below 60 v., the triode 54 of the Schmitt circuit of circuit 52 becomes non-conducting, rendering the triode 55 conducting and the triode 61 non-conducting; consequently, the voltage V2 becomes equal to the voltage of the B source, This voltage is applied to the deflecting plate 32, whereby the luminous area on the screen is displaced in the direction opposite to that in the previous case where the voltage to be regulated exceeded 70 v. A photoelectric means 122 is preset to a position corresponding to the displaced position of the luminous area, so that light therefrom falls -on the photoelectric element to excite it. The output therefrom is amplified by an amplifier 162 to actuate an auxiliary relay 172. Upon actuation of the relay, the automatic control system operates to increase the voltage applied to terminal 53 until it exceeds 60 v., when the circuit 52 recovers its operation and the luminous area on the screen returns to the center thereof and the control system stops its operation. Y

It will be apparent from the foregoing that with these circuit arrangements, the luminous area on the screen of the cathode-ray tube is brought to a predetermined position thereon when the voltage under supervision has gone outside a prescribed range, so ythat however rapidly the voltage may iiuctuate outside the range, a sufficient amount of light from the luminous area enters the photoelectric means for the auxiliary relay to be actuated without fail.

.It should be recognized that the embodiments disclosed herein are merely representative and that further modifications and changes may be made without departing from the true scope and spirit of the invention.

What is claimed is:

1. An electrical system for generating an output signal upon occurrence of a predetermined input signal value, comprising a cathode ray tube including means for generating an electron beam, a screen which luminesces at points of impingement of said beam thereon, and beam deiiecting means coupled to the system input and operable to deiiect said beam to different positions on said screen in response to different input signal values; light-sensitive means for receiving light from such luminescence and positioned adjacent said screen at a location corresponding to a predetermined input deflection signal value; a deflection limiter circuit interposed in said system input and coupled to said detiecting means, said limiter circuit including circuit means operable to stop beam deiiection at the location of said light-sensitive means; and output means responsive to said light sensitive means including means for generating an output signal upon occurrence of an input signal value causing beam deflection to such location.

2. The system defined in claim 1 further including positioning means carrying said light-sensitive means and operable to adjust the position thereof to different deflection locations along said screen, and wherein said detiection limiter circuit includes adjustable circuit means operatively coupled to said positioning means whereby positioning of the latter adjusts maximum beam deection permitted by said limiter circuit to the location of said lightsensitive means.

3. The system defined in claim 1 wherein said deflection limiter circuit includes additional circuit means operable to preclude deflection of said beam when the input signal is less than a preselected value.

4. The system defined in claim 3 wherein said system includes first and second such light-sensitive means at iirst and second locations, respectively, and associated first and second such deflection limiter circuits, the preselected value for said first limiter circuit being greater than the preselected value for said second limiter circuit, and NOT circuit means coupled with the output of said second limiter circuit whereby the latter is rendered operative to deflect said beam to the location of the second lightsensitive means when said input signal value is less than its preselected value and to preclude such deection when said input signal value is greater than its preselected value, whereby deiiection to said first and second locations occurs when said input signal value is greater than and less than the range established by the preselected values for said first and second limiter circuits, respectively.

5. The system defined in claim 1 wherein said cathode ray tube includes a curved front face having said screen on the inside surface thereof, and wherein said system further includes positioning means carrying said lightsensitive means thereon and operable to adjust the position thereof to different deiiection locations along said screen, said positioning means comprising an arm pivotally mounted at substantially the center of curvature of said front face and carrying said light-sensitive means at the outer extremity thereof adjacent said front face.

6. The system defined in claim 1 wherein said beam deecting means is operable to deiiect said beam to impinge upon said screen at locations lying substantially along a linear path thereon, said system further including lan elongated optical lens extending along said path and having a substantially convex cross section throughout whereby to collect light from luminescence of said screen and focus the same on said light-sensitive means at any location along said path.

7. The system defined in claim 1 wherein said output means comprises a relay and actuating means operable to actuate said relay in response to occurrence of said input signal value causing detiection to such location.

References Cited by the Examiner FOREIGN PATENTS 453,248 9/36 GreatBritain.

DAVID G. REDINBAUGH, Primary Examiner.

Claims (1)

1. AN ELECTRICAL SYSTEM FOR GENERATING AN OUTPUT SIGNAL UPON OCCURRENCE OF A PREDETERMINED INPUT SIGNAL VALUE, COMPRISING A CATHODE RAY TUBE INCLUDING MEANS FOR GENERATING AN ELECTRON BEAM, A SCREEN WHICH LUMINESCES AT POINTS OF IMPINGEMENT OF SAID BEAM THEREON, AND BEAM DEFLECTING MEANS COUPLED TO THE SYSTEM INPUT AND OPERABLE TO DEFLECT SAID BEAM TO DIFFERENT POSITIONS ON SAID SCREEN IN RESPONSE TO DIFFERENT INPUT SIGNAL VALUES; LIGHT-SENSITIVE MEANS FOR RECEIVING LIGHT FROM SUCH LUMINESCENCE AND POSITION ADJACENT SAID SCREEN AT A LOCATION CORRESPONDING TO A PREDETERMINED INPUT DEFLECTION SIGNAL VALUE; A DEFLECTION LIMITER CIRCUIT INTERPOSED IN SAID SYSTEM INPUT AND COUPLED TO SAID DEFLECTING MEANS, SAID LIMITER CIRCUIT INCLUDING CIRCUIT MEANS OPERABLE TO STOP BEAM DEFLECTION AT THE LOCATION OF SAID LIGHT-SENSITIVE MEANS; AND OUTPUT MEANS RESPONSIVE TO SAID LIGHT SENSITIVE MEANS INCLUDING MEANS FOR GENERATING AN OUTPUT SIGNAL UPON OCCURRENCE OF AN INPUT SIGNAL VALUE CAUSING BEAM DEFLECTION TO SUCH LOCATION.

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