US3066223A

US3066223A - Bistable electro-optical network

Info

Publication number: US3066223A
Application number: US83730A
Authority: US
Inventors: James F Vize
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1957-12-27
Filing date: 1961-01-19
Publication date: 1962-11-27
Anticipated expiration: 1979-11-27
Also published as: US3042807A

Description

United States Patent Ofiice 3,066,223 Patented Nov. 27, 1962 3,055,223 BISTABLE ELECTROOPTICAL NETWORK James F. Vize, Rhinebeck, N.Y., assignor to General Electric Company, a corporation of New York Original application Dec. 27, 1957, Ser. No. 705,680, now Patent No. 2,997,596, dated Aug. 22, 1961. Divided and this application Jan. 19, 1961, Ser. No. 83,730

6 Cla ms. (Cl. 250-208) This invention-relates to bistable electro-optical networks, and more particularly to electrical networks including electroluminescent phosphors and photo-conductors as elements thereof and adapted to operate in either one of two stable states.

The phenomenon of electroluminescence upon which the operation of the networks of the present invention in part depends is the process by which certain semiconducting materials, known as phosphors, emit radiation under the primary stimulus of an applied electrical field or po tential. For a survey and bibliography on the subject of electroluminescence, reference is made to an article by G. Destriau and H. F. Ivey, Electroluminescence and Related Topics, Proceedings of the Institute of Radio Engineers, vol. 43 (1955), pp. 1911-4940.

As noted in the above article electroluminescent phosphors have in the past been used as light sources in devices frequently called electroluminescent capacitors or electroluminescent cells. Such devices often resemble a flat plate capacitor and may comprise two parallel planar electrodes which have sandwiched between them, in one form or another, an electroluminescent phosphor. The phosphor may be in the form of microcrystals suspended in a transparent plastic or dielectric binder. Alternatively, the phosphor may be in the form of a continuous, transparent crystalline layer such as that disclosed in US. Patent No. 2,709,765 to L. R. Koller, or in the form of single crystals as disclosed in US. Patent No. 2,721,950 to Piper and Johnson. In general the microcrystal-inplastic type of phosphor dielectric exhibits electroluminescence only under excitation by alternating electric fields, whereas in the two patents referred to above, the phosphors exhibit electroluminescence when excited by either alternating or unidirectional electric fields. The carrierinjection electroluminescence described in the aforementioned article is a type of electroluminescence excited by unidirectional electric fields.

Prior known electro-optical networks employing both electroluminescent phosphors and photoconductors dis posed and interconnected for mutual cooperation have been used as amplifiers or oscillators, etc. Networks of this type are shown in US. patent application Serial No. 585,027 now Patent No. 2,904,696 by R. E. Halsted and J. F. Elliott and US. patent application Serial No.

585,052 by C. F. Spitzer now Patent No. 2,975,290, both applications being assigned to the assignee of the instant application.

The term photoconductor as used herein is intended to apply to any material the impedance or conductivity of which varies as a function of the radiation emitted by a particular associated electroluminescent phosphor. A photoconductor is said to be in radiation-coupled relationship with an electroluminescent phosphor when they are so related that the impedance or conductivity of the nectedin electrical series relation and positioned in radiation-coupled relationship may be termed an electrooptical pair, for purposes of this specification. Such an electro-optical pair, when connected across a predetermined value of voltage is adapted to be bistable; that is, the pair will draw only one of two possible values of current, one high and one low. correspondingly, the intensity of radiation emitted from the electroluminescent cell Will be high or low, depending on whether the value of current is high or low. When the electro-optical pair is in its dark state (emission from the electroluminescent cell is low), application of an external radiation signal to the photoconductor will switch the pair to its other state. If two such independently stable electro-optical pairs are so mutually interrelated that proper signal feedback is transmitted from one pair to the other, a novel apparatus is obtained that has two stable states and is adapted to be shifted from either one of its stable states to the other by application of a radiation trigger signal to the respective photoconductor coupled to the dark electroluminescent cell. Such a bistable apparatus is extremely useful, especially in digital computers for register and counter elements. When used as a register element, the two stable states would be designated respectively as the binary digits 1 and 0. Input radiation triggering means is provided to independently switch the deviceto either of its stable states. When used as. a counter element, the device is switched from the stable state in which it is operating to its other stable state upon application of a common input trigger signal; that is, the device would return to a particular stable state after each two input trigger signals.

A bistable device employing these electroluminescent phosphors has the additional desirable feature of providing a visible indication of the state of the device. Thus, one electroluminescent phosphor glows only when the network is in one of its two stable states and the other electroluminescent phosphor glows only when the network is in the other stable state.

It is therefore a principal object of this invention to provide a bistable device comprising two mutually interrelated electro-optical pairs.

Another object of this .invention is to provide a novel bistable electro-optical network.

Another object of this invention is to provide an elect-rical network including electroluminescent phosphors and photoconductors as elements thereof and adapted to operate in either one of two stable states.

Another object of this invention is to produce an output signal from an electro-optical network in response to every two input signals.

Another object of this invention is to provide an electrical network including electroluminescent phosphors and photoconductors as elements thereof and adapted to operate in either one of two stable states, wherein the particular phosphors luminescing are indicative of the stable state in which the network is operating.

The foregoing objects are achieved by providing networks having first and second electro-optical pairs connected in parallel. A source of electrical energy is coupled across the parallel-connected electric-optical pairs. A point of the first electro-optical pair between the electroluminescent cell and the photoconductor thereof is coupled to a point of the second electro-optical pair between the electroluminescent cell and the photoconductor thereof. This coupling between the first and second electro-optical pairs is one means whereby feedback is provided so that the network can operate in only one of two stable states; that is, wherein the on electrooptical pair draws relatively large current and its electroluminescent phosphor emits a relatively intense radiant energy signal, and the off electro-optical pair draws relatively little current and its electroluminescent phosphor emits relatively little, if any, radiant energy. Upon application of a radiant energy trigger signal to the photoconductor of the olf" electro-optical pair the network shifts to its other stable state. In this other stable state the formerly off electro-optical pair is on and vice versa. a

The invention will be described with reference to th accompanying drawings, wherein:

FIGURE 1 is a circuit diagram of the bistable network of this invention, including one form of triggering means;

FIGURE 2 is a perspective view, partly in section, of an electro-optical pair useful in the circuit of FIG. 1.

In the bistable network of FIG. 1 an electro-optical pair 62 is connected in parallel with an electro-optical pair 63. Pair 62 comprises a series-connected electroluminescent cell 66 and photoconductor 65 positioned in radiation-coupled relationship indicated by the arrow and broken line between them. The arrows and broken lines elsewhere in FIG. 1 indicate the same radiationcoupled relationship and this convention will be employed throughout this application. Electro-optical pair 63 comprises'a series-connected electroluminescent cell 68 and photoconductor 67 positioned in radiation-coupled relationship. Appropriate feedback, to be described hereinafter, causes this network to operate in only one of of two stable states. 1

Examples of electro-optical pairs useful in the circuit of FIG. 1 are shown and described in the aforementioned U.S. patent application Serial No. 585,052 by C. P. Spitzer. A device 31 including such an electro-optical pair is shown in FIG. 2. Device 31 is adapted to receive either or both electrical and light input-signals and to produce either or both electrical and light output signals. In device 31 an electrode 32 consists of a rigid opaque metallic member serving as both an electrode and a supporting member and which is preferably polished for maximum light reflection. Deposited on one side of electrode 32 are an electroluminescent layer 33, a light-transmitting electrode 34, a photoconductive layer 35, and a light-transmitting electrode 36. Deposited on the other side of electrode 32 is an electroluminescent layer 43, a light-transmitting electrode 44, a photoconductive layer 46 and a light-transmitting electrode 47. Lead wires are soldered or otherwise electrically connected to each electrode.

Electroluminescent layer 33 and adjacent electrodes 32 and 34 comprise an electroluminescent cell EL-l, and electroluminescent layer 43 and

adjacent electrodes

32 and 44 comprise another'electroluminescent cell EL-2. The above two electroluminescent cells are connected in series by the common electrode-32 so as to make the cells EL-l and EL-2 effectively one electroluminescent cell which is in radiation-coupled relation with two photoconductors for reasons that will appear later. Similarly, photoconductive layer 35 and its electrodes 34 and 36 comprise one photoconductor PC-l and the photoconductive layer 46 and its electrodes 44 and 47 comprise a second photoconductor PC-2. A casing 48 which consists of the light-opaque, electrically-insulating material is used to, support the cells, photoconductors and a lens 49 adjacent electrode 36.

It should be understood that the word light, as used in this application, includesany radiation emitted by an electroluminescent phosphor to which a photoconductor is responsive and may for example, include ultraviolet or infrared radiation. 7

Light-transmitting, electrical conducting electrodes 34 and 36 may be layers of titanium dioxide or tin oxide, commonly referred to as conducting glass. Alternatively, a very thin light-transmitting layer of evaported metal, such as aluminum or silver, may be used. If the lighttransmitting, electrical conducting electrodes are titanium dioxide they may be prepared and rendered conductive in accordance with the teachings of U.S. Patent No. 2,717,844 to L. R. Koller.

Electroluminescent layers 33 and 43 may be phosphors such as zinc sulfide activated by three-tenths percent by weight of copper and written as ZnS:Cu, prepared as a continuous crystalline layer, as disclosed in the abovementioned patent 2,709,765 to L. R. Koller, or single crystalline phosphors of the type disclosed in the abovementioned Patent 2,721,950 to Piper and Johnson. These types of electroluminescent layers are responsive to both direct or alternating electric fields. The average brightness B of the light output of an electroluminescent phosphor as a function of the voltage V applied to it may be closely approximated by the expression,

troluminescent phosphor used and k is a constant of proportionality. Values of n, in Equation 1, range approximately from 1 to 7 for known phosphors.

Photoconductive layers 35 and 46 are thin light permeable layers of photoconductive material. The material may, for example, comprise cadmium sulfide or lead sulfide, which may besprayed, sputtered, or evaporated on one of the light-transniitting electrodes 34 or 36 and 44 or 47. More generally,

photoconductive layers

35 and 46 may, for example, consist of any of the sulfides, selenides, or tellurides of cadmium, lead, or zinc, or may be any other known photoconductor.

The physical arrangement of the device 31 is such that light emitted by electroluminescent cell EL-l falls on photoconductor PC-l. This cell and photoconductor are connected in electrical series relation between terminals 40 and 42, and therefore constitute an electro-optical pair such as pair 62 or 63 (FIG. 1). The current drawn by this pair depends on the voltage applied to the terminals light transmitting electrodes 34 and 44. Each of electroluminescent cells EL-l and EL2, is radiation-coupled to its respective photoconductor indicated generally by PC1 and F02. The relationship between the diagrammatic illustration of FIG. 1, for example, and the physical embodiment of FIG. 2 may be understood, by noting that there is one device 31 for each of the electro-optical pairs 62 and 63 (FIG. 1) and their radiation-coupled photo conductors 72 and 71, respectively.

Referring once more to FIG. 1, electro-

optical pairs

62 and 63 are connected in parallel across a source of electrical energy, such as constant current source 64. Electro-optical pair 62 comprises a photoconductor 65 and an electroluminescent cell 66 connected in series. Electro-optical pair 63 comprises a photoconductor 67 and an electroluminescent cell.68 connected in series. A connection point 69 is provided between photoconductor 65 and electroluminescent cell 66 and a connection point 70 between photoconductor 67 and electroluminescent cell 68. A pair of series-connected photoconductors'71 and 72 is connected between points 69 and 7t Photoconductors 71 and 72 are positioned in radiation-coupled relationship with respective electroluminescent cells 68 and 66. The network is switched from one stable state to another upon application of a radiation input trigger signal from atrigger source 74 to a photoconductor 73 connected between the photoconductor ends of electro-

optical pairs

62 and 63 and the common connection point of the series-connected photoconductors 71 and 72.

In the operation of the circuit of FIG. 1, current I from source 64 substantially divides between electro-

optical pairs

62 and 63. The network is in its designated first stable state when electro-optical pair 62 is on and in its designated second stable state when electro-optical pair 63 is on.

Assume now that the circuit is in its first stable state. Electroluminescent cell 66 is lighted and illuminates

photoconductors

65 and 72. Electroluminescent cell 68 is dark and consequently hotoconductors 67 and 71 are not illuminated and have a high resistance. The greater portion of current I therefore flows through electro-optical pair 62. Upon application of a rediation input trigger signal from trigger source 74 to photoconductor 73, a relatively low resistance is provided for current flow through

photoconductors

73 and 72 to electroluminescent cell 68. The impedance of

photoconductors

72 and 73 in parallel with the decreasing impedance of photoconductor 67 provide an impedance which is low as compared with that of photoconductor 65. The light output of elec troluminescent cell 68 thereupon tends to increase and the resistance of radiation-coupled photoconductor 67 tends to decrease so that a greater portion of current I is drawn through electro-optical pair 63. The increase of current through electro-optical pair 63 is accompanied by a decrease in current through electro-optical pair 62, a consequent decrease in the light output of electroluminescent cell 66, and a consequent increase in the resistance of photoconductor 65. The current in electro-optical pair 63 continues to increase and that in electro-optical pair 62 continues to decrease while the radiation input trigger signal is applied to photoconductor 73, until electro-optical pair 63 has reached its stable on condition and electrooptical pair 62 has reached its stable off condition. At this time the circuit is in its second stable state. Upon application of the next radiation input trigger signal to photoconductor 73, passage of current through photoconductor 71, which has been illuminated by electroluminescent cell 68, serves to change the state of the network once again. Thus, the network of FIG. 1 is changed from one stable state to the other by application of an electrical signal to the off electro-optical pair.

Photoconductors 71 and 72 must have response times relatively slow compared to the time necessary for the network to switch from one stable state to the other in order that their resistances remain either high or low sufiiciently long for the switching operation to reach completion.

A resistor 75 is included in series relationship with electro-optical pair 63 and serves to effectuate resetting or initial setting of the bistable circuit upon the respective removal and reapplication or initial application of the source of current. With the source of current 64 removed, both electro-

optical pairs

62 and 63 are off. When current source 64 is applied to the network, current increases more rapidly in electro-optical pair 62 than in electrooptical pair 62. Thus the network will assume its first stable state.

In the above description of the operation of the circuit of FIG. 1 it may be seen that electro-

optical pairs

62 and 63 connected in parallel to constant current source 64 constituted a bistable network. Feedback is provided by the high internal impedance of constant current source 64. It is within the scope of this invention to provide other means for electrically triggering this bistable network than that shown in FIG. 1. Furthermore, this triggering means may be applied so that the network operates as either a register element or as a counter element.

A theory of the operation of the bistable apparatus of this invention, as presently understood, and a method of determining certain circuit parameters are described in a copending application Serial No. 705,680, filed December 27, 1957 now Patent No. 2,997,596, August 22, 1961, from which the present application has been divided.

While the principles of the invention have now been made clear in illustrative embodiments, there will be immediately obvious to those skilled in the art many modifications in structure, arrangement, proportions, the elements, materials, and components, used in the practice of the invention, and otherwise, which are particularly adapted for specific environments and operating requirements, without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

What is claimed is:

1. A signal responsive network comprising a pair of electrical circuit branches, each of said branches comprising an electroluminescent cell and a photoconductor connected in series, one end of each of said branches being connected to a first common point, the other end of each of said branches being connected to a second common point, the electroluminescent cell and the photoconductor of one of said branches being positioned in radiationcoupled relationship, and the electroluminescent cell and the photoconductor of the other of said branches being positioned in radiation-coupled relationship, a resistor connected in series in one of said pair of branches, a feedback means coupled between the first and second electro-optical pairs for providing feedback which causes the network to operate in only one of two stable states when a constant current source is connected in parallel with said pair of electrical circuit branches, and means for connecting a constant current source in parallel with said pair of electrical circuit branches.

2. A counter network element comprising at least five electrically distinct connection points, a first photoconductor connected between the first and second of said points, a first electroluminescent cell connected between the second and third of said points, a second photoconductor connected between the first and fourth of said points, a second electroluminescent cell connected between the third and fourth of said points, the first electroluminescent cell and the first photoconductor being further positioned in radiation-coupled relationship, the second electroluminescent cell and the second photoconductor being further positioned in radiation-coupled relationship, a third photoconductor connected between the second and fifth of said points, a fourth photoconductor connected between the fourth and fifth of said points, the first electroluminescent cell being further positioned in radiation-coupled relationship with the fourth photocon ductor, the second electroluminescent cell being further positioned in radiation-coupled relationship with the third photoconductor, and a fifth photoconductor connected between the first and fifth of said points.

3. A network element as defined in claim 2 further including a constant current source, and means for connecting said source to said first and third points.

4. A network element as defined in claim 3 further including triggering means positioned in radiation-coupled relationship with said fifth photoconductor and adapted to apply a radiation input trigger signal thereto.

5. A binary counter network comprising a first electroluminescent cell and a first photoconductor electrically connected in series, said first electroluminescent cell being positioned in radiation-coupled relationship with said first photoconductor, a second electroluminescent cell and a second photoconductor electrically connected in series, said second electroluminescent cell being positioned in radiation-coupled relationship with said second photoconductor, a third photoconductor positioned in radiationcoupled relationship with said second electroluminescent cell and having one terminal connected to a junction between said first electroluminescent cell and said first photoconductor, a fourth photoconductor positioned in radiation-coupled relationship with said first electroluminescent cell and having one terminal connected to a junction between said second electroluminescent cell and said second photoconductor, means for connecting a constant current source in parallel with the two series circuits consisting of said first electroluminescent cell in series with said first photoconductorand said second electroluminesconductor and triggering means positioned in radiation cent cell in series with said second photoconductor, and coupled relationship with said fifth photoconductor for switching means connected to a second terminal of said applying a radiation input trigger signal thereto.

third photoconductor and to a second terminal of said fourth photoconductor for momentarily coupling said 5 B third and fourth photoconductors in parallel with said References Cited In the file of thls Damnt first and second photoconductors, respectively. UNITED STATES PATENTS 6. A binary counter network as defined in claim 5 wherein said switching means consists of a fifth photo- 2,895,054 Loebner July 14, 1959