Patent Publication Number: US-6657381-B1

Title: Display device having a multi-layered structure with light-emitting devices mounted thereon

Description:
This invention relates to a display device with a plurality of light-emitting devices mounted at desired locations on a display surface of a multi-layered structure of the display device, and to a multi-layered structure and light-emitting devices for such display device. 
     BACKGROUND OF THE INVENTION 
     In his Japanese Unexamined UM Publication No. SHO 60-189084 U published on Dec. 14, 1985, Y. Nagai discloses an electric light board including light-emitting diodes each having a shorter electrode plug and a longer electrode plug of which a portion is insulated. The light-emitting diodes are pushed into a board including a plurality of soft, electrically conductive sheets and a plurality of soft, electrically insulating sheets. Y. Nagai does not show materials of the soft, electrically conductive sheets. 
     K. Kato et al., in their Japanese Unexamined UM Publication No. SHO 49-5374 U published on Jan. 17, 1974, disclose a position indicating device including light-emitting diodes each having a shorter electrode and a longer electrode with an insulating layer thereon, and a display plate including two conductive thin sheets with an insulating thin sheet interposed between the two conductive sheets. The conductive sheets are formed of metal, or rubber or synthetic resin with minute particles of conductive material mixed therein. 
     In Japanese Unexamined Patent Publication No. SHO 47-22093 A published on Oct. 6, 1972, S. Wada shows a display device including a laminate sheet of two or three conductive layers of metal or conductive resin and two or three insulating layers of soft synthetic resin, and light-emitting diodes each having longer and shorter contact needles coated with insulating material except the respective end portions thereof. The contact needles of the light-emitting diodes are pushed into the laminate sheet so as to contact the associated conductive layers. 
     With the above-described arrangements of the prior art display devices, electrical contact between the light-emitting diodes and the conductive layers is insufficient and unstable. In some cases, some diodes may lose sufficient electrical contact, which makes the diodes unable to emit light, or contact portions may be oxidized as time passes, so that power cannot be supplied to the diodes. 
     An object of the present invention is to provide a display device having a display surface, and a light-emitting device which can be mounted at substantially any desired position on the display surface. Another object of the present invention is to provide a conductive layer for a multi-layered structure for such display device, which can provide stable conductive contact with leads of a light-emitting device to be pushed into the multi-layered structure. A still further object of the present invention is to provide a display device in which a light-emitting device to be energized can be selected. 
     SUMMARY OF THE INVENTION 
     A display device according to the present invention includes a multi-layered structure and a plurality of light-emitting devices mounted on a display surface of the multi-layered structure. The multi-layered structure includes successively stacked first, second and third insulating layers with the first layer on the display surface side, a first conductive layer sandwiched between the first and second insulating layers, and a second conductive layer sandwiched between the second and third insulating layers. Each of the first, second and third insulating layers is formed of such a material that a needle or the like can be stuck into it. Also, each of the first and second conductive layers is a layer into which a needle or the like can be stuck and which contains fibers. Each of the light-emitting devices has a longer lead and a shorter lead, which are stuck into the multi-layered structure through the display surface at substantially any desired location. When stuck, the longer lead extends through the first and second insulating layers and the first conductive layer at least into the second conductive layer and contacts the second conductive layer. An insulating coating or covering is provided on the longer lead at a portion thereof which contacts the first conductive layer in the multi-layered structure when the longer lead is stuck into the multi-layered structure. This insulating coating insulates the longer lead from the first conductive layer. The shorter lead extends through the first insulating layer at least into the first conductive layer and contacts the first conductive layer when it is stuck into the multi-layered structure. 
     The first, second and third insulating layers may be of insulating foamed plastic. 
     At least one of the first and second conductive layers may be separated into plural sections of desired shapes. 
     The longer lead of each light-emitting device may be provided with an insulating coating on at least a part of a proximal portion thereof having a length equal to that of the shorter lead. The insulating coating has its distal end narrowed down. 
     The first and second conductive layers include woven or non-woven fabric. The fabric can establish stable electric contact between the leads of the light-emitting devices and the conductor layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a display device according to the present invention. 
     FIG. 2A is a perspective view of a signboard for use in explaining the principle of the present invention, and FIG. 2B is a light-emitting diode (LED), showing how it is stuck into the signboard. 
     FIG. 3 is a cross-sectional view of the signboard along a line  3 — 3  in FIG.  2 A. 
     FIGS. 4A,  4 B,  4 C and  4 D show examples of conductive layers of the display device of the present invention. 
     FIGS. 5A,  5 B,  5 C,  5 D, and  5 E show various examples of LEDs useable in the present invention. 
     FIG. 6 is a cross-sectional view similar to FIG. 3, showing the LED of FIG. 5A stuck into a multi-layered board. 
     FIGS. 7A and 7B show how a lead is shaped, and FIG. 7C shows how the shaped lead can well contact the surrounding conductive layer. 
     FIGS. 8A and 8B show how the conductive layers are connected to power supply cables, and FIG. 8C shows another example of a power supply copper plate. 
     FIG. 9 shows a relay switch disposed between the display device and a power supply. 
     FIG. 10 shows a signboard with two display surfaces, and two LEDs which are respectively mounted on the two surfaces and can be turned on simultaneously. 
     FIG. 11 shows a signboard with two display surfaces, and LEDs which are mounted on the two surfaces and can be turned on selectively. 
     FIG. 12 shows a signboard with two different type LEDs mounted thereon, which are operated from different power supply voltages. 
     FIGS. 13A,  13 B and  13 C show LEDs having leads of various lengths for used with a signboard with three conductive layers. 
     FIGS. 14A,  14 B and  14 C show LEDs mounted on a signboard with three conductive layers and two display surfaces. 
     FIGS. 15A and 15B are perspective views showing top and bottom sides of an insulating strip having a predetermined shape, on which a plurality of LEDs are mounted. 
     FIG. 16A is a perspective view of a display device having a multi-layered structure shaped in a truncated cone, and FIG. 16B is a vertical cross-sectional view of the display device shown in FIG.  16 A. 
     FIG. 17A is a front view of a display device according to the present invention having a multi-layered structure shaped in a ball, and FIG. 17B is a vertical cross-sectional view of the display device of FIG.  17 A. 
     FIG. 18 is a schematic perspective view of a display device having a display surface divided into a plurality of display regions. 
     FIG. 19 is an exploded perspective view of the multi-layered structure of FIG.  18 . 
     FIG. 20 is an exploded perspective view of a multi-layered structure different from the one shown in FIG.  19 . 
     FIG. 21 is an exploded perspective view of another multi-layered structure. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Now, embodiments of the present invention are described with reference to the accompany drawings. Throughout the drawings, the same reference numerals are attached to the same or similar components. 
     FIG. 1 shows a perspective view of a multi-layered signboard  1  according to one embodiment of the present invention. The signboard  1  includes a multi-layered board  10  including three stacked insulating layers or boards  11 ,  12  and  13 , and two soft conductive layers  21  and  22 . The conductive layer  21  is interposed between the insulating layers  11  and  12 , and the conductive layer  22  is interposed between the insulating layers  12  and  13 . The signboard  1  further includes a plurality of LEDs  31  or light-emitting devices mounted on one display surface  15  of the board  1 , being stuck into the multi-layered structure. The shapes of the multi-layered structure and the display surface shown in FIG. 1 are flat and rectangular, but they may be circular, elliptic, stellar or of any other shape. The multi-layered structure may be shaped into a globe, a truncated cone, or any other curved shape, so the display surface is curved. 
     The conductive layers  21  and  22  are connected to power supply lines connected to the plus (+) and minus (−) terminals of a power supply  16 . In the arrangement shown in FIG. 1, the conductive layer  21  is connected to the minus (−) terminal of the power supply  16 , while the conductive layer  22  is connected to, the plus (+) terminal of the power supply  16 . 
     Each of the LEDs  31  includes a light-emitting portion  41  and a pair of cathode and anode leads  41  and  42 . The leads  41  and  42  have respective lengths corresponding to the depths of the conductive layers  21  and  22  from the display surface, respectively, which are poled similarly to the leads  41  and  42 , respectively. In FIGS. 2B and 3, the structure of each of the LEDs  31  is shown in greater detail. The longer lead  42  is provided with an insulating coating  44  over at least a portion which would contact the conductive layer  21  when the diode  31  is stuck into the multi-layered structure. Preferably, the insulating coating  44  extends from the proximal end  46  to a mid portion  47  of the lead  42 . The length of the insulating coating  44 , i.e. the distance between the proximal end  46  and the mid portion  47 , is slightly larger than the sum of the insulating layer  11  and the conductive layer  21 . 
     The insulating coating  44  has preferably a larger thickness at the proximal end portion  46 , tapering toward the mid portion  47 , which gives a larger contact area to the lead  42  with respect to the inner wall off the hole formed by the lead  42  in the multi-layered board  10 . The larger contact area, in combination with the elasticity and static friction provided by the foamed plastic of the board, makes the LED  31  more firmly secured to the multi-layered board  10 . 
     FIGS. 2A,  2 B and  3  are for explaining the principle of the present invention. FIG. 2B shows how a LED  31  is stuck into the multi-layered board  10 , and FIG. 2A shows the LED  31  stuck into the board  10 . FIG. 3 is a cross-section along a line  3 — 3  in FIG.  2 . 
     The cathode and anode leads of a DC power supply (about 2 V)  16  are connected, optionally through a variable resistor  28 , to the conductive layers  21  and  22 , respectively, of the multi-layered board  10 . The leads  42  and  43  of the LED  31  are stuck into the board through the display surface  15 . The shorter lead  43  extends through the first insulating layer  11  and the first conductive layer  21  into the second insulating layer  12 , establishing an electrical contact between the lead  43  and the first conductive layer  21 . The longer lead  42  extends through the first insulating layer  11 , the first conductive layer  21 , the second insulating layer  12  and the second conductive layer  22  into the third insulating layer  13 , establishing electrical contact between the lead  42  and the second conductive layer  22  with the insulating coating  44  thereon isolating the lead  42  from the first conductive layer  21 . When forward current (of about 20 mA) is supplied to flow between the two leads, the LED  31  emits light. 
     Next, the insulating layers  11 ,  12  and  13  are described. The insulating layers  11 ,  12  and  13  shown in FIGS. 1-3 are preferably of a material which is relatively soft, exhibits some hardness when compressed, and is hard to deform, and into which a needle or the like can be stuck to form a hole therein. In the illustrated embodiments, foamed plastic layers having a thickness of, for example, 10 mm or 5 mm, are used. Typically, foamed polystyrene is used for the insulating layers, but other foamed plastics, such as foamed polyurethane, foamed polyethylene or the like can be used. 
     Alternatively, fiber layers having insulating property, such as insulating chemical fiber layers, may be used as the insulating layers of the present invention. 
     Multi-layered boards including three insulating layers having a thickness of 10 mm and 5 mm, and two conductive layers sandwiched between two adjacent insulating layers were experimentally prepared. The boards have various dimensions. The fabricated multi-layered boards had a thickness T (FIG. 1) of from about 15 mm to about 30 mm, a height H of from about 75 mm to about 900 mm, and a width W of from about 75 mm to about 600 mm. As shown in FIG. 3, a pointed needle like a paper fixing pin was stuck into the boards to thereby form guide holes  48 , and, then, electrodes of LEDs  31  could be inserted into the guide holes  48  and, hence, into the multi-layered boards  10  with ease. Since electrodes of ordinary LEDs are rigid, they can be stuck directly into the boards without forming the guide holes  48  beforehand. 
     The respective layers of the multi-layered board may be fixed together by means of insulating adhesive tapes  25  at four corners, as shown in FIG. 1, or they may be fixed together by bolts and nuts of insulating plastic at four corners. A pulp sheet or a plastic paper sheet which can be painted may be bonded by an adhesive to a display surface of the multi-layered board, so that any colors or drawings can be provided on the display surface of the display device. 
     Next, the conductive layers  21  and  22  are described in detail. Preferably, the material of the conductive layers  21  and  22  has high electrical conductivity and is such a material that guide holes for guiding the leads of the LEDs  31  as described above can be easily formed in them by sticking a needle into them and that sufficient electrical contact can be established between the conductive layers and the electrodes of the LEDs  31  inserted into the guide holes  48 . Also, it is important that the conductive layers  21  and  22  can hold the leads of the LEDs  31  firmly so that the LEDs  31  do not easily slip off the board. Each of the conductive layers may have a single-layered or multi-layered structure. 
     Such material may be a soft and electrically conductive material which can be formed into a sheet and into which a needle can be easily stuck. Such materials include conductive fabric coated with a conductive metal, a fine stainless-steel net, steel wool, and woven or unwoven fabric of carbon fibers. One of them may be used as a single layer, or two or more may be combined to form a multi-layered conductive sheet. The conductive sheet may be used singly or in combination with one or more of aluminum foil, copper foil and shield paper which provide high conductivity within the layer. 
     The metal-coated fabric may be woven or unwoven fabric of synthetic fibers, other chemical fibers or natural fibers coated with copper and/or nickel. The fibers include, for example, polyester, acrylic, and nylon fibers. 
     A fabric is preferred as the material for the conductive layers because it is easily handled, and easily cut, and hardly tears. A woven or unwoven fabric of conductive metal-coated synthetic fibers can be obtained by plating fibers, threads or fabric with copper or nickel, or plating with copper and then nickel. Metal-coated conductive woven fabrics of polyester fibers or acrylic fibers are commercially available from Daiwabo Co., Ltd., Osaka, Japan for a material for shielding electronic devices from electromagnetic waves, and may not have been used for any other purposes than electromagnetic shielding. 
     The thickness of a metal-coated conductive fabric used in the experiment was about 0.1 mm to 0.2 mm, but thicker conductive layers may be used in the present invention. The inventor recognized that even a single metal-coated fabric exhibited sufficiently high conductivity. 
     FIG. 4A shows an example of a conductive layer  21  or  22  which is formed of a laminate of a plurality of conductive fabrics  400 . FIG. 4B shows another example of a conductive layer  21  or  22 , formed of two aluminum thin sheets  401  with a sheet of steel wool  402  sandwiched between them. FIG. 4C shows a conductive layer formed of two aluminum thin sheets  401  with a fine steel net  403  sandwiched between them. FIG. 4D shows a still other example of a conductive layer including two aluminum thin sheets  401  sandwiching a woven or unwoven fabric  400  of carbon fibers. Although not shown, a single conductive cloth coated with copper and/or nickel may be used. When a conductive layer is formed of a metallic material, such as aluminum and steel wool, it is preferable to coat it with an anticorrosive paint so that an oxide film can be hardly formed, which can keep good electrical contact between the conductive layer and the diode leads. The conductive layer is preferably bonded to the entire surface of a foamed plastic layer or part thereof by an adhesive or an adhesive tape so that it may not slip off. When the conductive layer is formed of a laminate of conductive sheets, the respective sheets may be bonded together by an adhesive or an adhesive tape put on the entire surfaces or part of one or both conductive sheets. 
     Next, the structure of a light-emitting device used in the present invention is described. 
     A LED  31  used as a light-emitting device mounted to the multi-layered board  10  shown in FIGS. 1-3 can be fabricated by working a commercially available LED. Typically, the LED  31  includes substantially parallel, thin, long needle-like leads or conductive legs having some rigidity, and a light-emitting section  41  including an LED pellet  45  encapsulated in plastic, e.g. epoxy resin. There are commercially available color-LEDs which emit color light, e.g. red, yellow, green and blue, and operate from various power levels, e.g. 2 V and 4 V. LEDs are preferable light-emitting devices because they are easily procured, have a long life, are small in size, and can be rendered conductive by selecting the direction of a bias voltage applied to it. However, other light-emitting semiconductor devices, i.e. electro-luminescent cells or small lamps with two parallel needle-like lead soldered to the lamps, may be used instead. 
     FIGS.  5 A through FIGS. 5E show various examples of the LED  31  having different types of insulating coating or covering  44 . 
     FIG. 5A shows a LED having a tapered insulating coating on the proximal portion of its longer electrode lead. The coating covers the proximal 15 mm portion of the lead. The LED can be used with a multi-layered board including insulating layers having a thickness of 10 mm. The insulating coating is formed of the same plastic material as the one used for encapsulating the light-emitting portion of the LED, and is integral with the plastic encapsulation. 
     The LED shown in FIG. 5B has a tapered insulating coating similar to the one shown in FIG.  5 A. The insulating coating, however, of FIG. 5B is formed of a thermosetting plastic adhesive, which, when cured, is integral with the LED encapsulation and with the lead. 
     The insulating covering shown in FIG. 5C is provided with a separately prepared long sheathe or tube of hard, semihard or soft plastics, or rubber. Such plastic may be, for example, fluoroplastics or Teflon. The lead is inserted into the sheathe. If the sheathe is formed of hard plastics and has its distal end cut off obliquely, the lead with such plastic sheathe can be directly stuck into the board  10 . Alternatively, a needle may be stuck into the board to form a guide hole in the board  10 , the plastics tube is then placed in the guide hole, and after that, the lead of the LED is inserted into the plastics tube. If the tube is of hard plastics, the tube may be stuck into the board  10  without a guide hole pre-formed, and the lead of the LED can be inserted into the tube. The lead may be pulled out from the tube and, after that, a small amount of adhesive can be put into the tube before inserting the lead again, which makes the lead firmly secured to the plastics tube. 
     FIG. 5D shows the insulating covering provided by wrapping an insulating plastic adhesive tape around the lead portion. 
     FIG. 5E shows a LED having two, concentrically formed leads. The longer lead is needle-shaped and centrally extends, while the shorter lead is cylinder-shaped surrounding the need-like longer lead. Insulating plastics is placed into the space between the longer and shorter leads to provide insulation between them. Although not shown, insulating enamel or adhesive may be applied to the proximal portion of the longer lead in a substantially uniform thickness. 
     If the insulating covering is firmly bonded to the lead, the lead can be directly stuck into the multi-layered board, as shown in FIG.  6 . On the other hand, if the lead with weak bonding between the lead and the insulating covering is stuck directly into the multi-layered board, the insulating covering may be removed or peeled off. In such case, a guide hole, like the hole  48  shown in FIG. 3, is preferably formed in the board beforehand, and the lead with such insulating covering is inserted into the guide hole. 
     The distal end of each lead of the LED can be tapered as shown in FIGS. 7A and 7B, so that the leads can be stuck into the board more easily, and firm electrical contact can be provided between the leads and the conductive layers as shown in FIG.  7 C. 
     Referring now to FIG. 8A, how to connect power supply lines  81  to the conductive layers  21  and  22  is described. In the example shown in FIG. 8A, each of the conductive layers  21  and  22  includes two conductive sheets, e.g. metal coated woven fabrics  83 . The other ends of the power supply lines  81  are connected to a DC power supply, such as the battery  16  shown, for example, in FIG.  1 . Each of the power supply lines  81  shown in FIG. 8A is a cable including a plurality of copper thin wires covered with an insulating vinyl covering. (Of course, a single wire line may be used instead.) The vinyl covering at the end of each cable to be connected to the conductive sheet  21  ( 22 ) is removed, and the copper thin wires are unbraided and spread into a fan-shape. Then, the spread copper wires are placed on one surface of one of the conductive sheets  83  and urged to contact that conductive sheet  83 . Spray starch is sprayed to secured the wires to the conductive layer. The wires may be soldered instead. Over the surface on which the copper wires are bonded, the other conductive sheet  83  is placed. If the conductive layer is single-layered, the copper wires are sandwiched between the conductive layer and one of the insulating layers. If the conductive layer includes more than two conductive sheets, the copper wires are placed between two of adjacent conductive sheets. 
     FIG. 8B shows another way of connecting the conductive layers  21  and  22  to power supply lines  82 , which may be a cable like the cable  81  or a single conductor. One end of each line  82  is secured to one surface of a small copper square sheet by solder  88  to thereby form a small power supply copper electrode plate  84 . The electrode plate  84  is placed between the conductive layer  21  ( 22 ) and one of the adjacent insulating layers  11  and  12  ( 12  and  13 ). Alternatively, the electrode plate  84  is placed between the two conductive sheets  83 . The copper plate  84  may be formed sawtooth as shown in FIG. 8C, with the teeth  86  extending in the direction transverse to the plane of the conductive layer. In the example shown in FIG. 8C, alternate teeth  86  extend in one direction, while the other teeth in the opposite direction. When sandwiched between the insulating layer and the conductive layer and between two conductive sheets of the conductive layer, the copper electrode plate  84  is firmly secured to the conductive layer to thereby provide firm electrical contact between them. 
     The DC power supply can be a battery like the battery  16  shown, for example, in FIG. 1, or a DC power supply including rectifiers and a smoothing capacitor, connected to an AC power source, which is adjusted to supply current of about 20 mA to each LED. The current supplied may be preferably adjusted by a variable resistor or a selected one of fixed resistors connected in circuit with the power supply. LEDs with built-in resistor, e.g. LEDs having a voltage rating of 5V manufactured by Hewlett-Packard Company may be advantageously used, which eliminates the need for using a separate resistor. 
     According to the present invention, the LEDs with the above-described structure can be mounted at any desired locations on the multi-layered signboard. Since they need not be soldered, they can be removed. Accordingly, one can adjust the positions of the LEDs, considering the overall artistic effect of his or her work on the signboard, to produce his or her work in a pointillistic manner. Once the positions on the display surface of the respective LEDs are determined, they may be fixed to the display surface with an adhesive applied over the lower surfaces of the respective light-emitting portions or over portions of the leads. 
     Referring to FIG. 9, the first conductive layer  21  is connected to the minus electrode of the battery  16 , while the second conductive layer  22  is to the plus electrode. A LED  91  has a longer anode lead connected to its anode, and a shorter cathode lead connected to the cathode. Hereinafter, this type of LED is referred to as “+L-S LED”. The signboard shown in FIG. 9 includes also another LED having a shorter anode lead and a longer cathode lead. This type of LED is referred to as “+S-L LED”. The structure of the LED  91  is the same as the previously described LED  31 , and the structure of the LED  92  is essentially the same as that of the LED  31  except that the longer lead is connected to the cathode of the LED  92 . The longer leads are in electrical contact with the conductive layer  22 , while the shorter lead is in electrical contact with the conductive layer  21 . A relay switch  90  is used to switch the connection of the electrodes of the battery  16  to the conductive layers  21  and  22 , which energizes the LEDs  91  and  92  to emit light alternately. A variable resistor  28  is connected between the battery  16  and the switch  90 . Accordingly, two display patterns, one formed of plural diodes  91  with the other formed of plural diodes  92 , can be selectively displayed by operating the relay switch  90 . 
     FIG. 10 shows another example of signboard which has two display surfaces on opposite sides of the board. The two conductive layers  21  and  22  are connected in the same manner as in FIG.  9 . FIG. 10 shows one +L-S LED  91  mounted on one display surface, the upper surface in FIG. 10, and one +S-L LED  92  mounted on the other, lower display surface. The variable resistor  28  is connected between the battery  16  and the conductive layer  22 . The LEDs  91  and  92  of the signboard shown in FIG. 10 are energized simultaneously. A larger number of LEDs  91  and  92  can be mounted on the signboard. 
     Another example is shown in FIG. 11, in which there are shown two +L-S LEDs  91 - 1  and  91 - 2  and two +S-L LEDs  92 - 1  and  92 - 2  mounted on the opposite display surfaces of the signboard. The LEDs  91 - 1  and  92 - 1  are mounted on one display surface, and the LEDs  91 - 2  and  92 - 2  are on the opposite display surface. The variable resistor  28  is connected between the battery  16  and the switch  90 . By switching the connections of the DC supply  16  between the conductive layers  21  and  22  with the relay switch  90 , the LEDs on each display surface are alternately energized to emit light. Specifically, when the conductive layer  21  is connected through the switch  90  to the minus electrode of the battery  16 , with the conductive layer  22  connected to the plus electrode of the battery  16 , the LED  91 - 1  on the upper display surface and the LED  92 - 2  on the lower display surface are energized. When the switch  90  is switched to connect the conductive layer  21  to the plus electrode of the battery  16 , the LEDs  92 - 1  and  91 - 2  emit light. A larger number of LEDs  91  ( 91 - 1 ,  91 - 2 , . . . ,  91 -n) and  92  ( 92 - 1 ,  92 - 2 , . . . ,  92 -n) can be mounted on the signboard to form two desired patterns on each of the display surfaces. By operating the relay switch  90 , either one of the patterns on each display surface can be selected for display. 
     FIG. 12 shows another embodiment of the present invention. In this embodiment, the multi-layered display device includes four insulating layers  124 ,  125 ,  126  and  127  and three conductive layers  121 ,  122  and  123 , and two types of LEDs  128  and  129 . The lengths of the leads of the LEDs  128  and  129  are different. In the example shown in FIG. 10, the LED  128  has a shorter cathode lead which is in electrical contact with the first conductive layer  121 , and a longer anode lead which is in electrical contact with the second conductive layer  122 . The LED  129  has a shorter cathode lead which is in electrical contact with the first conductive layer  121 , and a longer anode lead which is in electrical contact with the third conductive layer  123 . 
     A voltage source of about 2 volts is connected between the first and second conductive layers  121  and  122  through a switch, and a different voltage source of about 2 volts is connected between the first and third conductive layers  121  and  123  through a switch. By selectively turning on and off the respective switches, the LEDs  128  and  129  are selectively turned on to emit light. When both switches are turned on, both LEDs  128  and  129  are energized to emit light. If the two types of LEDs have different operating voltages, for example, if the LED  128  operates with 2 V, while the LED  129  operates with 4 V, a 4 volt voltage source is connected between the first and third conductive layers  121  and  123 . 
     A larger number of such LEDs  128  and  129  can be mounted on the signboard. 
     As is understood from this example, by using three or more conductive layers and different types of light-emitting devices having leads of different lengths, the different types of light-emitting devices can be mounted at desired locations on a display surface and can receive operating voltages necessary for them. 
     As shown in FIGS. 13A,  13 B and  13 C and in TABLE I, there are six combinations of lengths of anode and cathode leads of LEDs which can be mounted on a display surface of a signboard having three conductive layers. 
     In TABLE I, “+” denotes an anode lead, and “−” denotes a cathode lead. A letter “S” represents a shorter lead, “M” does a middle-length lead, and “L” represents a longer lead. The shorter lead S extends to be in electrical contact with the first conductive layer  121 , the middle-length lead M extends to be in electrical contact with the second conductive layer  122 , and the longer lead L extends to be in electrical contact with the third conductive layer  123 . 
     By using two different operating voltages of about 2 V and about 4 V with these LEDs, there are twelve combinations available. If LEDs emitting four colors of light are used in combination, there are forty-eight combinations. In FIGS. 13A-13C, various connections of a 2 V source and 4 V source are also shown. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE I 
               
             
            
               
                   
                   
               
               
                   
                   
                 Length of Anode Lead (+) 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 S 
                 M 
                 L 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Length of 
                 S 
                 None 
                 +M-S 
                 +L-S 
               
               
                   
                 Cathode 
                 M 
                 +S-M 
                 None 
                 +L-M 
               
               
                   
                 Lead (−) 
                 L 
                 +S-L 
                 +M-L 
                 None 
               
               
                   
                   
               
            
           
         
       
     
     FIGS. 14A,  14 B and  14 C show another examples of signboards, which include three conductive layers and use two opposing surfaces as display surfaces. LEDs having shorter, middle-length and longer leads like those shown in FIGS. 13A-13C are used. Each of the shorter leads is in electrical contact with the nearest one of the three conductive layers. Each of the middle-length leads is in electrical contact with the middle one of the three conductive layers, and each of the longer leads is in electrical contact with the farthest one of the three conductive layers. 
     It may sometimes be difficult to display exactly desired characters, curves or shapes by directly mounting LEDs on a display surface of a multi-layered display board. In such a case, as shown in FIGS. 15A and 15B, a plurality of LEDs can be stuck into and bonded to a thin insulating strip  151  formed in a desired shape, e.g. T-shape. After that, the strip  151  with LED leads extending out from the bottom surface thereof (FIG. 15B) is mounted on the display surface by sticking the leads into the multi-layered board. 
     Although not shown, a plurality of LEDs are arranged adjacent each other to form a desired pattern without using a strip, and the light-emitting portions of adjacent LEDs may be bonded together by an adhesive, for example. Alternatively, the LEDs arranged in a pattern may be integrated by applying and curing resin to the light-emitting portions. 
     Such integrated LEDs may be divided into one or more groups, with a plurality of leads to be brought into contact with the same conductive layer interconnected by a conductor  152 , for example, as shown in FIG.  15 B. At least one of the interconnected leads is configured as a corresponding lead of, for example, the LED  31  shown in FIG.  3 . The remaining leads, e.g. the leads  153  and  154 , are cut short or bent to extend along the bottom surface of the strip  151 . Then, the number of leads to be stuck into the multi-layered board is reduced, which facilitates the handling of LEDs. 
     In the above-described examples, the multi-layered board has flat surfaces. However, the multi-layered structure may be formed in a globe, a cylinder, a truncated-cone, or any other shape with a non-flat surface. 
     FIG. 16A shows a truncated-cone shaped display device  160 . FIG. 16B is a vertical cross-sectional view along a vertical plane containing the center axis C of FIG.  16 A. The truncated-cone shaped multi-layered structure  165  has a display surface  170 , four insulating, foamed plastics layers  161 ,  162 ,  163  and  164  successively disposed in the named order with the layer  161  disposed outmost. The structure further includes a first conductive layer  166  sandwiched between the insulating layers  161  and  162 , a second conductive layer  167  sandwiched between the insulating layers  162  and  163 , and a third conductive layer  168  sandwiched between the insulating layers  163  and  164 . A plurality of LEDs  169   a  and  169   b  are mounted on the display device, with the three conductive layers  166 ,  167  and  168  are connected to a switching controller and power supply arrangement  171  shown in block. 
     FIG. 17A is a front view of a ball-shaped display device  175 , and FIG. 17B is a vertical cross-sectional view along the line  17 B— 17 B in FIG.  17 A. The multi-layered structure used in this display device  175  includes successively stacked four insulating foamed plastics layers  181 ,  182 ,  183  and  184  with the layer  181  located outmost, conductive layers  185 ,  186  and  187  disposed between respective adjacent ones of the insulating layers  181 - 184 . The display device  175  further includes a base  178  and a multi-layered structure supporting rod  176  extending upward from the base  178 . A bundle of three power supply lines extends in the rod  176 . A switching controller and DC power supply  179  is disposed in the base  178 . The three power supply lines are connected between the DC power supply and the respective ones of the conductive layers. 
     FIG. 18 shows another example of a display device  200  having a display surface divided into plural display regions. The multi-layered structure of the display device  200  includes first, second and third insulating layers  201 ,  202  and  203 , and first conductive layer  206  disposed between the first and second insulating layers  201  and  202 , and a second conductive layer  207  disposed between the second and third insulating layers  202  and  203 . 
     FIG. 19 is an exploded view of the multi-layered structure of FIG. 18 useful in explaining the structure. The first conductive layer  206  is divided into three regions  311 ,  312  and  313  by insulating gaps  276  and  277  formed in the layer  206 . Electrodes  241 ,  242  and  243  are connected to the regions  311 ,  312  and  313 , respectively. The second conductive layer  207  is divided into three regions  351 ,  352  and  353  by insulating gaps  278  and  279  which extend in the direction generally perpendicular to the gaps  276  and  277  in the first conductive layer  206 . Electrodes  246 ,  247  and  248  are connected to the regions  351 ,  352  and  353 , respectively. 
     In the laminated structure, the regions  311 ,  312  and  313  in the first conductive layer  206  overlap the regions  351 ,  352  and  353  in the second conductive layer  207 . The electrodes  241 ,  242  and  243  are connected to the same polarity electrodes, the minus electrodes in the illustrated example, of three DC sources  261 ,  262  and  263  in a power supply apparatus  260 , respectively, while the electrodes  246 ,  247  and  248  are connected through a switch apparatus  250  to the other polarity electrodes, the plus electrodes in the illustrated example, of the DC sources  261 ,  262  and  263 , respectively. The switch apparatus  250  includes nine switches  251 - 259  arranged in a matrix. Broken lines  271 ,  272 ,  273  and  274  drawn on the upper, display surface  250  of the display device  200  shown in FIG. 18 are projections of the gaps  276  and  277  in the first conductive layer  206  and the gaps  278  and  279  in the second conductive layer  207 . Thus, the display surface  250  is divided into nine display regions  211 ,  212 ,  213 ,  221 ,  222 ,  223 ,  231 ,  232  and  233 . When LEDs are mounted on the display surface  250 , they are divided into nine groups. When, for example, the switch  251  is closed, the LEDs in the region  211  are selected. If the switch  255  is closed in addition to the switch  251 , the LEDs in the regions  211  and  222  are selected. 
     FIG. 20 is an exploded perspective view of another structure realizing the display device  200  shown in FIG. 18 having plural display regions. The display device  201  is essentially the same as the display device  200  shown in FIGS. 18 and 19, except that the insulating layers, too, are divided into plural portions. The first insulating layer  201  is divided into portions  301 ,  302  and  303  corresponding to the regions  311 ,  312  and  313  of the first conductive layer  206 , by gaps  281  and  282  corresponding to the gaps  276  and  277  in the first conductive layer  206 . The third insulating layer  203  is divided into three portions  361 ,  362  and  363  corresponding to the regions  351 ,  352  and  353  in the second conductive layers  207  by gaps  283  and  284  corresponding to the gaps  278  and  279  in the second conductive layer  207 . There grooves  285  and  286  extending midway into the second insulating layer  202  from its upper surface, which correspond to the gaps  276  and  277  in the first conductive layer  206 . There are two grooves  287  and  288  extending midway into the second insulating layer  202  from its bottom surface, which correspond to the gaps  278  and  279  in the second conductive layer  207 . The gaps and grooves in the insulating layers and the gaps in the conductive layers can be left open, but they may be filled with insulating rubber, vinyl sheets or the like. 
     The structure shown in FIG. 20 results from cutting the gaps  281  and  282  in the laminate board from the upper, display surface to extend into the second insulating layer  202 , which also results in forming the gaps  276  and  277  in the first conductive layer  206  and the grooves  285  and  286  in the second insulating layer  202 , and cutting the gaps  283  and  284  in the laminate board from the bottom surface to extend into the second insulating layer  202 , which also results in the formation of the gaps  278  and  279  in the second conductive layer  207  and the grooves  287  and  288  in the second insulating layer  202 . When this method is employed, a display device essentially the same as the display device  200  shown in FIGS. 18 and 19 can be fabricated in a simple manner. 
     FIG. 21 shows another structure for realizing the display device  200  shown in FIG. 18 having plural display regions. The first conductive layer  206  is divided into nine regions  371 ,  372 ,  373 ,  374 ,  375 ,  376 ,  377 ,  378  and  379  which correspond respectively to the display regions  211 ,  221 ,  231 ,  212 ,  222 ,  232 ,  213 ,  223  and  233 . Electrodes  381 - 389  are connected respectively to the regions  371 - 379  in the first conductive layer  206 . An electrode  391  is connected to the second conductive region  207 . The electrodes  381 - 389  are connected through associated switches (not shown) to one en of a DC power supply (not shown), and the electrode  391  is connected to the other end of the DC power supply. By selective closing one or more switches connected to the respective regions  381 - 389  of the first conductive layer  206 , the LEDs in the selected display regions are energized to emit light. 
     According to the present invention, a plurality of separate conducive layers and one or more light-emitting devices each having two leads connected to different ones of the conductive layers. Accordingly, it is not necessary to form two or more conductor strip patterns in one conductor position to apply a required voltage between the two leads of each light-emitting device. In contrast, when a printed circuit board is used to mount one or more light-emitting devices thereon, conductor patterns for applying at least two potentials must be formed on the board. 
     The display device of the present invention may be used as, for example, an artistic display, a signboard, a toy and a map board for selectively indicating the locations of famous places by light-emitting devices. The display device shown in FIG. 17 may be used as a sparkling ornament. The display device may be used on a wall, a ceiling or door. A sparkling fan may be obtained by forming a display device of the present invention in a thin, disc-like shape, and attaching a handle with a battery disposed therein, to the disc-shaped display device. The display device according to the present invention may be used in many other ways. 
     The examples shown and described heretofore are only typical ones, and any people skilled in the art can consider various modifications without departing the scope of the present invention.