IMAGE READING DEVICE

Abstract: Improved driving circuitry for a light source used in a dot matrix display board, a copying machine or a facsimile machine etc. is disclosed. The light source includes a plurality of light emitting diodes (D11 to D44) each connected in parallel, and the quantity of light from each of the light emitting diodes can be balanced by providing proper resistors (R1 to R4). Additionally, the control of the total quantity of light from the light emitting diodes can be readily performed, since the total driving current is changed by changing the value of a variable resistor (Rv1).

Description: The present invention relates to an image reading device for reading a picture depicted on an applied original, comprising: light emitting means for emitting light to be directed onto said original, and picture detecting means responsive to an optical signal reflected from said original for detecting the picture depicted on said original, said light emitting means comprising, a first plurality of light emitting diode units arranged at least in one direction, each of said light emitting diode units being connected in parallel between first and second supply potentials. Description of the Background Art. Among many kinds of light emitting elements which are conventionally known, particularly a light emitting diode (LED) has been utilized in various electric appliances, with the recent development of the technique of manufacturing semiconductor devices. The present invention can be generally applied to a light emitting device having a plurality of light emitting elements, and as an example, the present invention is applied to a device which needs a plurality of light emitting diodes in the following description. A light emitting diode is used, in one application, as a light source for discharging for a copying machine, a laser beam printer or the like. Fig. 1 is a view schematically showing a conventional copying machine. The copying machine shown in Fig. 1 is seen, for example, in Japanese Utility Model Publication, No. 2-10544. Referring to Fig. 1, light generated from a light source 22 illuminates an original 21, and the light reflected from the original 21 is applied through mirrors and a lens to a photosensitive drum 23. Toner responds to the applied photo signal and is deposited onto the surface of the photosensitive drum 23 with an aid of a magnetic brush 4. The toner deposited on the drum 23 is transferred onto a sheet of transfer paper 27, and a picture depicted on the original 21 is therefore defected onto the transfer paper 27. After the transfer, the photosensitive drum 23 receives light generated from a light source for discharging 28, and is thereby made free from a charge state caused by the reflected light. The toner deposited on the photosensitive drum 23 is removed by a rotary brush 29. It is pointed out that in the copying machine shown in Fig. 1, a light emitting diode is often utilized as the light source for discharging 28. Fig. 2 is a view schematically showing an original reader in a conventional facsimile machine. The original reader shown in Fig. 2 is seen in "Nikkei Electronics" (No. 470, April 3, 1989), p.159. Referring to Fig. 2, an original 30 is inserted into the reading device through the drum 31 or 32. Light is illuminated onto the original 30 from a light source 33 for reading, and the reflected light is applied to a CCD 38 through mirrors 34 to 36 and a lens 37. A video signal based on a picture depicted on the original 38 is therefore produced in the CCD 38. The video signal is transmitted to other facsimile machines through a transmitting apparatus (not shown). It is pointed out that in a facsimile machine as shown in Fig. 2, a light emitting diode is frequently used as the light source 33 for reading. Additionally, a dot matrix display board, in which a plurality of light emitting diodes are utilized, is known. In the dot matrix display board, a plurality of light emitting diodes are arranged in a matrix, and information represented by characters or symbols is displayed by emission of light from selected light emitting diodes. In the above electrical apparatus, a plurality of light emitting diodes are employed to form a necessary light source, and the following problems arise unless the quantity of light generated from each of the light emitting diodes is constant. If, for example, the light quantity of each of a plurality of light emitting diodes used as a light source for discharge in a copying machine is not constant, the discharge from the photosensitive drum 23 shown in Fig. 1 cannot be performed in a uniform manner. In addition, unless the light quantity of each of the light emitting diodes used in the light source 33 of the original reader shown in Fig. 2 is constant, an accurate video signal cannot be obtained in the CCD 38. Thus, the original cannot be read accurately in a copying machine or a facsimile machine. Additionally, if the light quantity of each of the light emitting diodes arranged in a dot matrix display board is not even, a display displayed on the dot matrix display board is difficult to read and an error can be caused in reading in an extreme case. It is pointed out that a driving current for each of the diodes should be controlled so that light generated from each of the diodes is even. Fig. 3 is a circuit diagram showing a conventional driving circuit of a light source employing light emitting diodes. Referring to Fig. 3, the driving circuit includes light emitting diodes D1 to Dn each connected in parallel, resistors R1 to Rn connected to the diodes D1 to Dn, respectively, and a constant voltage source E. The resistors R1 to Rn are used to avoid the above described problems by controlling a driving current for each of the diodes D1 to Dn. Since the driving current/quantity of light characteristic of each of the light emitting diodes D1 to Dn is not constant, the quantity of light is made uniform by setting the values of the resistors R1 to Rn properly. The resistors R1 to Rn can be replaced with resistors Rx1 to Rxk which are prepared in a factory and have various resistance values. A resistor having a proper value for each of the resistors R1 to Rn is connected whereby the quantity of light from each of the diodes D1 to Dn is made uniform. There exists a necessity for preparing many kinds of resistors having various resistance values in the factory. Additionally, even once proper resistors R1 to Rn are connected, and the quantity of light from each of the light emitting diodes D1 to Dn is made uniform, the total quantity of light frequently should be controlled. In other words, not only balancing the quantity of light from the individual light emitting diodes D1 to Dn, but also the quantity of light from all the other diodes D1 to Dn should be controlled in some cases. In such a case, conventionally, all the resistors R1 to Rn are replaced with others and time required for the replacement is increase due to the increase of the number of the light emitting diodes D1 to Dn to be used. Furthermore, besides the above described replacement in the factory, the control for the total quantity of light is often necessary in the maintenance of the electrical instruments. Like other electrical components, the quantity of light from a light emitting diode tends to decrease according to an increase in use. In such a case, the resistors R1 to Rn have to be replaced in maintenance, in order to insure a constant total quantity of light. The time required for the maintenance is increased accordingly. An image reading device according to the pre-characterizing part of claim 1 is known from WO-A-89/00299. The plurality of light emitting diode units is arranged in the direction of a scanning line of a document. DE-A-2925692 discloses display means using a matrix arrangement of light emitting diodes. Lines of serially connecting light emitting diodes are connected in parallel to power supply means. Each line of serially connected light emitting diodes includes a current limiting series resistor for absorbing over-voltage and to equalize the currents through each of the lines of light emitting diodes. A further resistor is connected in series to the power source for limiting the overall current flowing to all lines of light emitting diodes. This single resistor serves as a protecting unit for protecting the whole display device in case of short-circuits. To maintain its function on decreasing supply voltage or supply current, the display device according to DE-A-2925692 proposes to provide unequal numbers of serially connected light emitting diodes in one or more single lines. SUMMARY OF THE INVENTION One object of the present invention is to minimize the variation of absolute brightness among product units and thereby to simplify the control of the total quantity of light, in an image reading device having a light emitting device including a plurality of light emitting diode unit. Another object of the present invention is to reduce the number of components required for controlling the total quantity of light, in such a light emitting device. Yet another object of the present invention is to read accurately a picture depicted on an applied original, in an image reading apparatus. The above objects are solved by an image reading device constructed in accordance with the pre-characterizing part of claim 1 by the features specified in its characterizing part. Depending claims 2 to 5, each specify an advantageous development thereof. According to an essential aspect, the image reading device according to the present invention is characterized by a corresponding first plurality of individual driving current control means each connected in series to a corresponding one of said light emitting diode units for controlling individually a driving current flowing through each corresponding light emitting diode unit, and total driving current control means coupled to said first plurality of light emitting diode units for controlling the total driving current flowing through said first plurality of light emitting diode units and including controllable power supply voltage generation means connected to said first power supply potential for supplying each of a predetermined plurality of controllable power supply voltages to the parallel connection of said first plurality of light emitting diode units,and selecting means connected to said controllable power supply voltage generating means for selectively supplying one of the predetermined plurality of voltages to the parallel connection of said first plurality of light emitting diode units. In operation, the total driving current can be controlled without adjusting individual driving current controllers, since the total driving current controller controls the total driving current which flows throught the plurality of light emitting elements. The control of the total quantity of light generated from the plurality of light emitting diode untis therefore can be easily performed. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is schematic diagram of a conventional copying machine; Fig. 2 is a schematic diagram of an original reader in a conventional facsimile machine; Fig. 3 is a circuit diagram of a conventional driving circuit having a light source employing light emitting diodes; Fig. 4 is a circuit diagram of a light source driving circuit with an alternative diode array arrangement ; Fig. 5 is a characteristic view showing the relation between a total driving current and the quantity of light in the light source driving circuit shown in Fig. 4; and Fig. 6 is a circuit diagram of a light source driving circuit showing still another alternative arrangement of a diode array; and Figs. 7 and 8 are circuit diagrams of a light source driving circuit each showing an embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to Fig. 4, the light source driving circuit not belonging to the invention includes units of light emitting diodes connected in series each other D11 to D14, D21 to D24, D31 to D34 and D41 to D44; resistors R1 to R4 each connected in series to the units of light emitting diodes, respectively; a variable resistor Rv1 connected in series to the parallel connection of the units of light emitting diodes; and a constant voltage supply E. The light emitting diodes D11 to D44 have an alternative arrangement, namely a matrix array. Each of the resistors R1 to R4 controls a driving current which passes each unit of light emitting diodes. The quantity of light emitted from each unit of light emitting diodes can be uniformalized by setting the resistance value of each of the resistors R1 to R4 properly, i.e. by connecting resistors each having a proper resistance value to the resistors R1 to R4, respectively. The quantity of light emitted from each unit of the light emitting diodes are balanced and then the total quantity of light can be easily controlled by changing the resistance value of the variable resistors Rv1. Although the variable resistor Rv1 is required for balancing the quantity of light from the units of light emitting diodes, the adjustment for controlling the total quantity of light is remarkably simplified. Consequently, by controlling the resistance value of the variable resistor Rv1, the brightness of the diode array matrix can be changed in accordance with the ambient illuminance. A characteristic view showing the relation between the total driving current and the quantity of light in the light source driving circuit shown in Fig. 4 is shown in Fig. 5. In Fig. 5, the abscissa represents the length of a train of light emitting diodes, and the ordinate represents the quantity of light. By increasing the resistance value of the variable resistor Rv1, the total driving current is reduced thereby changing the quantity of light from line 61 toward line 63. It is pointed out that the light source driving circuit shown in Fig. 4 can be applied as a driving circuit for the above described light source for discharging in a copying machine or a light source for reading in a facsimile machine. Referring to Fig. 6, a light source driving circuit not belonging to the invention and showing still another alternative arrangement of a diode array includes light emitting diodes D1 to Dn connected in parallel to each other similar to Fig. 3, resistors R1 to Rn each connected in series to the respective diodes D1 to Dn, a parallel connection of resistors R A and R B connected in series to the parallel connection of light emitting diodes D1 to Dn, and a constant voltage supply E. Each of the resistors R1 to Rn are used for balancing the quantity of light of the respective light emitting diodes D1 to Dn. The total driving current of the light emitting diodes D1 to Dn can be controlled by selecting as resistors R A and R B a combination of two sets of various resistors Rx1 to Rxk, which are prepared in a factory. In other words, without using a variable resistor Rv1, a total driving current can be controlled in a wide range, since the total driving current can be controlled to various levels according to the combination of resistors R A and R B . In the above case, various resistance values for controlling the total driving current can be obtained by connecting further resistors in parallel in addition to the resistors R A and R B thus reducing the variety of resistors or the number of components to be prepared in the factory. Referring to Fig. 7, a light source driving circuit representing an embodiment of the present invention includes controllable power supply voltage generation means comprising a constant voltage source E, resistors R A1 to R Am which can be selected for controlling a total driving current, and as selecting elements switches SW1 to SWm connected in series to the resistors R A1 to R Am , respectively. By turning on one of the switches SW1 to SWn selectively, one of the resistors for controlling the total driving current is determined. Referring to Fig. 8, a light source driving circuit showing another embodiment of the present invention includes a power supply voltage generation circuit 40 for receiving power supply voltage from the constant voltage supply E and for generating power supply voltages E1 to Em having different voltage levels, and a selecting element 41 for selecting one of the power supply voltages E1 to Em. The respective power supply voltage E₁, E₂, E m which determines the total driving current flowing through the parallel connection of light emitting diodes D1 to Dn is selected by the selecting element 41, and therefore, the total quantity of light from the light emitting diodes D1 to Dn can be controlled. As described above, the total quantity of light of a light emitting diode arrangement can be easily controlled by applying the total driving current control means as shown in Figs. 7 and 8 in order to control the total driving current which flows through a plurality of light emitting diodes. Additionally, the number of components i.e., the number of resistors to be prepared in a factory is reduced since the variety of resistors required for controlling the total quantity of light is reduced. The application of the above mentioned light driving circuit for driving the light source for discharging 28 in the copying machine shown in Fig. 1 or for driving the light source 33 for reading in the facsimile machine shown in Fig. 2 provides an image apparatus capable of reading a picture depicted on an applied original accurately.

Claims: An image reading device for reading a picture depicted on an applied original, comprising: light emitting means (33) for emitting light to be directed onto said original, and picture detecting means (38) responsive to an optical signal reflected from said original for detecting the picture depicted on said original, said light emitting means comprising, a first plurality of light emitting diode units (D11, D21, D31, D41) physically arranged at least in one direction, each of said light emitting diode units being connected in parallel between first and second supply potentials, characterized by a corresponding first plurality of individual driving current control means (R1, R2, R3, R4) each connected in series to a corresponding one of said light emitting diode units for controlling individually a driving current flowing through each corresponding light emitting diode units, and total driving current control means coupled to said first plurality of light emitting diode units (D11, D21, D31, D41) for controlling the total driving current flowing through said first plurality of light emitting diode units and including controllable power supply voltage generation means (40; 41) connected to said first power supply potential for supplying each of a predetermined plurality of controllable power supply voltages to the parallel connection of said first plurality of light emitting diode units (D11, D21, D31, D41), and selecting means (41; SW₁-SW m ) connected to said controllable power supply voltage generation means for selectively supplying one of the predetermined plurality of voltages to the parallel connection of said first plurality of light emitting diode units (D11, D21, D31, D41). The image reading device in accordance with claim 1, wherein said first plurality of individual driving current control means comprises a first plurality of resistance elements (R₁-R₂) each connected in series to a corresponding one of said light emitting diode units (D11, D21, D31, D41). The image reading device in accordance with claim 1, wherein said controllable power supply voltage generation means comprises, a parallel connection of a fourth plurality of resistance elements (R A1 to R Am ) each connected to receive the first power supply potential, and the same fourth plurality of switching means (SW1 to SWm) each connected to one of said fourth plurality of resistance elements and having one switching means selectively turned on. The image reading device in accordance with claim 1, wherein said first plurality of individual driving current control means comprises, a fifth plurality of prepared spare resistance elements (Rx1 to Rxk), and each resistance element of said first plurality of resistance elements (R1 to Rn) being replaceable with one resistance element of said fifth plurality of spare resistance elements. The image reading device in accordance with claim 1, wherein each light emitting diode unit of said first plurality of light emitting diode units (D11, D21, D31, D41) comprises a second plurality of light emitting diodes (D11-D14) connected in series and physically arranged in a second direction so as to form a dot matrix light emitting means (33).

Inventors: INOGUCHI TSUKASA, INOGUCHI, TSUKASA
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1991
Main classification: H05B37
IPC codes: G06K15, H01L33, H04N1, E02D3, G03G15, E21B7, H05B37, H05B33