Display Device And Method For Scanning Said Device

Abstract

A display device comprising a number of light emitting diodes disposed in a matrix array, a switching element being connected in series with each of the horizontal (row) and the vertical (column) scanning lines; said array being scanned in such a manner that the column lines are scanned with a pulse consisting of a negative part and a subsequent positive part while the row lines which are always biased with a negative voltage of a value smaller than that of the said negative part of the column scanning pulse are scanned with a negative pulse of a value larger than that of the said negative part of the column scanning pulse, a trigger pulse being applied to the gate electrode of the switching elements in synchronization with the column scanning pulse for a column which includes a light emitting diode to be energized, thus a light emitting diode which has been transferred to a conductive state being returned to the non-conductive state by the negative part of the column scanning pulse just before the relevant column is scanned for the second time, a bright display being thereby ensured.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a novel display device having a light emitter array and a method for scanning said array.

2. Description of the Prior Art

The conventional light emitter array which consists of a number of light emitting elements disposed in a matrix array, utilizes light emitting diodes (hereinafter, abbreviated as LED) as the light emitting elements; and the conventional method of scanning such an array for a lighted display is based on the principle that an LED connected at the cross point of the horizontal and vertical scanning lines that are being scanned emits light. With such a scanning method, an LED emits light only for a short instant during which the associated cross lines are being scanned. Therefore, in a large array consisting of a large number of rows and columns, the lighting period of an LED is rendered to be a small fraction of the whole scanning period of the array, resulting in a display of insufficient brightness. In an LED array having 50 rows and 50 columns, for example, the lighting period of a single diode is only 1/2,500 of a frame time, as LED has no persistency. In order to obtain, with the 50.times.50 LED array, an apparent brightness the same as that of the ordinary cathode ray tube which is 50 foot Lamberts (hereinafter, abbreviated as f.L) or so, the brightness of each LED must be as high as 50.times.2,500=125,000 f.L. Such a high brightness cannot be expected from any known LED, much less the operation with low voltage and small current.

As a measure to overcome the problem of the insufficient brightness resulting from the above-described point-at-a-time scanning, a method of line-at-a-time scanning has been invented, according to which lighting information for a whole row or column is stored in a delay line to subsequently energize the whole diodes in the row or column at a time. Such a method, however, requires considerably large delay lines which hamper design of a compact device.

SUMMARY OF THE INVENTION

An object of this invention is to provide a light emitter array which is capable to exhibit the same overall brightness as that obtained by the above-mentioned line-at-a-time method without using any delay line, this being achieved by connecting an element having the switching characteristics in series with each of the row and column scanning lines of the array.

Another object of this invention is to provide a light emitter array having the same capability as mentioned in the preceding paragraph, by constituting the array with light emitting elements which per se have the switching characteristics.

A further object of this invention is to provide a method for scanning the above-mentioned arrays for the satisfactory operation, according to which a specially designed combination of scanning pulses are applied respectively to row and column scanning lines.

A still further object of this invention is to provide a method for scanning the array consisting of light emitting elements having the switching characteristics, which enables the light emitting element to be energized during most part of a scanning period and therefore to present a display of outstandingly high brightness.

The features and merits of this invention will be clarified in the following descriptions given in connection with embodiments of the invention and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fundamental connection diagram of a light emitter array of the display device of this invention.

FIG. 2 is a diagram showing waveforms of the pulses applied to row and column terminals of the array shown in FIG. 1.

FIG. 3 is a diagram showing an alternative form of the pulse applied to the column terminals of the array.

FIG. 4 is a fundamental connection diagram of another embodiment of the light emitter array according to this invention.

FIG. 5 is a diagram showing waveforms of the pulses applied to row and column terminals of the array shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Example 1

Referring to FIG. 1 which shows the fundamental arrangement of this invention, indexes X.sub.1, X.sub.2, . . . X.sub.m, . . . X.sub.M designate terminals of the row scanning lines of the number of M, indexes Y.sub.1, Y.sub.2, . . . Y.sub.n, . . . Y.sub.N terminals of the column scanning lines of the number of N, index G a gate terminal of the array, indexes SX.sub.1, SX.sub.2, . . . SX.sub.m, . . . SX.sub.M and indexes SY.sub.1, SY.sub.2, . . . SY.sub.n . . . SY.sub.N elements having the switching characteristics such as silicon controlled rectifier elements (hereinafter, abbreviated as SCR) connected respectively to the row and column terminals X.sub.1, X.sub.2, . . . X.sub.m, . . . X.sub.M and Y.sub.1, Y.sub.2, . . . Y.sub.n, . . . Y.sub.N.

Next, referring to FIG. 2 which shows waveforms of the row and column scanning pulses, it should be understood that the waveforms are presented on orthogonal coordinates, the abscissa standing for the time and the ordinate for voltage. In FIG. 2, indexes FY.sub.1, FY.sub.2, . . . FY.sub.n . . . FY.sub.N denote the waveforms of pulses to be applied to the terminals Y.sub.1, Y.sub.2, . . . Y.sub.n, . . . Y.sub.N, and FX.sub.m (M.gtoreq.m.gtoreq.1) denote the waveform of a pulse to be applied to a terminal X.sub.m. Reference numerals 1, 2, 3 respectively designate voltage levels 0, -V.sub.Y ' and +V.sub.Y in the waveforms FY.sub.1, FY.sub.2, . . . FY.sub.n . . . FY.sub.N and numerals 4, 5, 6 respectively denote voltage levels 0, -V.sub.L and -V.sub.H in the waveform FX.sub.m. Thus, it will be seen that a DC voltage of -V.sub.L is applied to the terminals X.sub.1, X.sub.2, . . . X.sub.m, . . . X.sub.M all the time, except that the voltage level on X.sub.m is lowered to -V.sub.H (V.sub.H >V.sub.L) at t.sub.m.sub.-1 and restored to -V.sub.L at t.sub.m. Voltage on each of Y.sub.1, Y.sub.2, . . . Y.sub.n, . . . Y.sub.N is normally at zero level, except that the voltage on Y.sub.n (N.gtoreq.n.gtoreq.1) is lowered to -V.sub.Y ' at a time

and finally restored to zero level at t.sub.n. The time t.sub.n ' may be an arbitrary time between t.sub.n.sub.-1 and t.sub.n. However, it will be understood that the time interval (t.sub.n '-t.sub.n.sub.-1) or (t.sub.n -t.sub.n') must not be shorter than the time required for the switching element to reverse the state, which varies depending on the characteristics of the element.

In this example, an SCR is used as each of the switching elements such as SX.sub.1, SX.sub.2, . . . SX.sub.m, . . . SX.sub.M and SY.sub.1, SY.sub.2, . . . SY.sub.n, . . . SY.sub.N. It is preferable in this case that the SCR operates with a comparatively low voltage and small current. That is, it preferably be turned to conductive with a gate current at most 100.mu.A., the operating voltage in the conductive state being 1 V or so. The gate terminal G is connected with the control terminals of all SCR. The cathodes of all row SCR and all column SCR are connected with the common line of the trigger circuit (not shown) respectively through a diode and a capacitor as shown in FIG. 1. The SCR are controlled by a trigger signal applied to the gate terminal G. Assuming that the voltage applied to an LED is V.sub.D and the hold voltage of the SCR is V.sub.S, the bias voltage level V.sub.L is set so as to be equal to 2V.sub.S +V.sub.D. Therefore, SCR elements which were once turned to the conductive state, maintain such a state; and the LED at the cross point of row and column scanning lines which are connected to such SCR that are in the conductive state keeps emitting light. Assuming that the mth row and the nth column are now being scanned, a negative voltage -V.sub.H is applied to the mth row as indicated with waveform FX.sub.m in FIG. 2, while a signal consisting of a negative-going pulse having the height of -V.sub.Y ' (V.sub.H > V.sub.Y ' .gtoreq. V.sub.L) and a subsequent positive-going pulse having an amplitude of +V.sub.Y is applied to the nth column as indicated with waveform FY.sub.n in FIG. 2. Now, considering an electric route starting at the terminal X.sub.m and ending at the terminal Y.sub.n through the mth row line and the nth column line, this circuit includes two SCR and one LED. At a certain time t between t.sub.n ' and t.sub.n.sub.-1, a voltage corresponding to V.sub.H -V.sub.Y ' is first impressed across the terminals X.sub.m and Y.sub.n. Then, at a time between t.sub.n and t.sub.n ', a voltage V.sub.H +V.sub.Y is applied. If this voltage V.sub.H +V.sub.Y is applied, the SCR elements which have been unconductive remain unconductive unless a trigger pulse is applied to the terminal G in synchronization with the voltage V.sub.H +V.sub.Y. Accordingly, no current flows through the LED. In order to cause a current to flow through the LED, a trigger pulse must be applied to the gate terminal G with an appropriate timing. Because of the switching characteristics of the SCR, the once triggered SCR remains conductive even after the time t.sub.m. In order to prevent any current from flowing through an LED which is connected with a row or column line that is not being scanned, the levels V.sub.L and V.sub.Y are set so that when a voltage V.sub.Y +V.sub.L is applied across the terminals X.sub.m and Y.sub.n, the SCR is not turned to conductive even if the trigger pulse appears at the gate terminal G. Further, the voltage levels V.sub.H and V.sub.Y and the height of the trigger pulse are chosen so that when a voltage V.sub.H +V.sub.Y is applied across the terminals X.sub.m and Y.sub.n, the SCR is turned to conductive by the trigger pulse applied to the gate terminal G. For the prevention of erroneous operation, a greater difference between V.sub.H and V.sub.L is more preferred. On the other hand, the same difference is desired to be as small as possible from the requirement that the amount of the light emission from an LED is not significantly affected by the application of V.sub.H.

The once triggered LED is not returned to the non-conductive state until the associated column is scanned for the next time, when a negative voltage -V.sub.Y ' appears at the terminal Y.sub.n. Thus, the bias voltage -V.sub.L existing at the terminal X.sub.m, a negative voltage V.sub.L -V.sub.Y ' is applied across the terminals X.sub.m and Y.sub.n. Therefore, the SCR elements SX.sub.m and SY.sub.n are turned to non-conductive, and the LED at the cross point of both relevant lines is no more fed with the current. As is clear from the above explanation, the duration of the light emission of the LED corresponds to the time required for scanning a row, that is, t.sub.m -t.sub.m.sub.-1.

In the above description, it has been assumed for the convenience of the explanation that SCR is used as the switching element. However, it will be understood that other elements having similar switching characteristics can be used. Further, in the above description, the waveform of the column scanning pulses has been shown as rectangular waves. However, the waveform can assume other shapes. An example of such waveforms is shown in FIG. 3, in which reference numeral 7 designates a waveform of the output pulse from a shift register, and 8 a waveform obtained by differentiating the waveform 7 and this is the waveform usable for the scanning pulse. Numerals 9, 10, 11 designate voltage levels O, -V.sub.Y ', +V.sub.Y respectively.

A sample of the above-described display device has been fabricated. In this sample, the light emitting diodes are Ga As.sub.0.6 P.sub.0.4 diodes disposed in a 6mm spaced matrix array of seven rows by five columns; and the switching elements are SCR of type 2SF656. Diodes of type 1S79H and capacitors of 10.mu.F are inserted in the gate circuit as shown in FIG. 2. This array is scanned by the method described above, the essential particulars for the operation being as follows: -V.sub.Y ' = -15V, t.sub.n ' - t.sub.n.sub.-1 = 20 .mu.s; +V.sub.Y = 5V, t.sub.n -t.sub.n ' = 5 =s; -V.sub.L = -14V, -V.sub.H = 31 .degree.V, t.sub.m -t.sub.m.sub.-1 = 140 .mu.s. The column scanning pulse is applied to the column terminals through a resistor of 2 k-ohm. A current of 10 mA. has been observed to flow through a light emitting diode in the conducting state. Under the above conditions, a brightness of 9 f.sup.. L has been obtained. On the other hand, the same array scanned by the conventional method has presented a brightness of only 2 f.sup.. L.

EXAMPLE 2

FIG. 4 shows another embodiment of this invention. In this embodiment, light emitting diodes having the switching characteristics are used as the light emitting elements in the array. In FIG. 4, terminals of similar functions to those shown in FIG. 1 are indicated with corresponding reference indexes. The waveforms of the scanning signals may be essentially the same as those shown in FIG. 2, though the peak values or levels of such signals may be different from those in the preceding embodiment depending on the characteristics of the elements. Further, the manner of the operation is also essentially the same as that described in connection with the first embodiment.

However, the light emitter array shown in FIG. 4 can be operated as a display device having an extremely high brightness if it is scanned by the following method. FIG. 5 shows the waveforms of the row and column scanning signals used in such a method. Indexes GY, GY.sub.2, . . . GY.sub.n, . . . GY.sub.N denote the waveforms of the scanning pulses to be applied respectively to the column terminals Y.sub.1, Y.sub.2, . . . Y.sub.n, . . . Y.sub.N, while GX.sub.m (M.gtoreq.m.gtoreq.1) denotes the waveform of the row scanning signal to be applied to the terminal X.sub.m. Reference numerals 11, 12 respectively denote voltage levels zero and +V.sub.Y is the waveforms GY.sub.1, GY.sub.2, . . . GY.sub.n, . . . GY.sub.N, and numerals 13, 14, 15, 16 respectively denote, in the waveform GX.sub.m, levels zero, -V.sub.L, -V.sub.H and an arbitrary level which is not lower than V.sub.Y. It will be seen from the diagram that a voltage of constant level -V.sub.L is normally applied to the row terminals; except that the voltage is raised to the level not lower than V.sub.Y at a time t'.sub.m.sub.-1 which is earlier by at least the time required for reversing the state of the element than the time t.sub.m.sub.-1 when the scanning for the mth row begins, thereby rendering all the conducting elements in the mth row non-conductive, and then the voltage is lowered to the level -V.sub.H (V.sub.H >V.sub.L) at the time t.sub.m.sub.-1, it being restored to the normal level -V.sub.L at the time t.sub.m. The potential of the terminals Y.sub.1, Y.sub.2, . . . Y.sub.n, . . . Y.sub.N is normally at zero level. However, the potential of an optional terminal Y.sub.n (N.gtoreq.n.gtoreq.1) is raised to the level +V.sub.Y at a time

and then is returned to zero level at

As in the operation with the first embodiment, a trigger pulse is applied to the gate terminal G in synchronization with the column scanning pulse applied to the column associated with the element to be energized.

Now, considering a light emitting element at the cross point of the mth row and the nth column, a voltage not lower than +V.sub.Y is first applied to the terminal X.sub.m during a period between t'.sub.m.sub.-1 and t.sub.m.sub.-1 to thereby make all elements in the mth row non-conductive. Then the potential of the terminal X.sub.m is lowered to -V.sub.H, while the columns are scanned with a pulse having a hight of V.sub.Y. Therefore, a voltage of vV.sub.H + V.sub.Y is applied to the element during a period between t.sub.n.sub.-1 and t.sub.n. If the trigger pulse is not applied to the gate terminal G at this instant, the element remains non-conductive and does not emit light. On the other hand, if the trigger pulse is applied, the element is turned to conductive and keeps emitting light until the mth row is scanned for the next time. It may be noted that the duration of the light emission is slightly different for each column, the ratio of the period for the first column to that of the last or the Nth column being M:M-1. However, if the number M is sufficiently large, the uniformity of the brightness over the display field will be satisfactory for practical purposes. If necessary, it is possible to visually equalize the unevenness of the brightness by arranging the sequence of the column scanning in a different mode from the geometrical sequence of the columns.

A sample of such a display device has been fabricated, in which PNPN type light emitting elements are disposed in a 6 mm spaced matrix array having seven rows and five columns. This array is scanned by the method described above in connection with FIG. 5. The essential particulars for the operation are as follows:

+V.sub.Y = +5V, t.sub.n -t.sub.n.sub.-1 = 5 .mu.s; -V.sub.L = -14V, -V.sub.H = -20V, t.sub.m -t.sub.m.sub.-1 = 140 .mu.s; the height of the positive pulse preceding the negative pulse in the row scanning = +5V (+19V on the basis of -V.sub.L) and the width of the last said pulse = 20 .mu.s. The current flowing through a conducting element is 10 mA. Under the above conditions, a brightness as high as 50 f.sup.. L has been obtained.

It will be understood that the waveforms of the scanning pulses shown in FIGS. 2 and 5 have been simplified for easy understanding of the fundamental concept of this invention, and that pulses of other various waveforms can be used. For example, it has been assumed in the previous description that the width of the column scanning pulse is 1/N of that of the row scanning pulse. However, since there exists no specific relation between the width of the column scanning pulse and the duration of the light emission of the element, the width of the column scanning pulse can be optionally set, so far as it is longer than the time required for reversing the state of the element or otherwise conflicting in the timing relationship. Further, it is not essential that the pulse for making a conducting element non-conductive (the negative pulses in FY.sub.1, FY.sub.2, . . . FY.sub.n, . . . FY.sub.N shown in FIG. 2, or the positive pulse of GX.sub.m in FIG. 5) continues to the pulse for making the element conductive (the positive pulses in FY.sub.1, FY.sub.2, . . . FY.sub.n, . . . FY.sub.N in FIG. 2, or the negative pulse of GX.sub.m in FIG. 5), but it is essential for the former to precede the latter. Also, it will be clear that the rows and the columns can be mutually replaced in the above description.

Claims

I claim:

1. A display device comprising a light emitter array having a plurality of light emitting diodes disposed in a matrix array, a plurality of terminals each assigned to each of the rows and columns of the array, a plurality of elements having switching characteristics each assigned to each row of said array with the anode thereof connected with the cathode of every light emitting diode in the row and the cathode thereof connected with the terminal of the row, a plurality of elements having switching characteristics each assigned to each column of said array with the cathode thereof connected with the anode of every light emitting diode in the column and the anode thereof connected with the terminal of the column, and a gate terminal connected with the control terminals of all of said elements having switching characteristics.

2. A display device comprising a light emitter array having a plurality of light emitting elements that have switching characteristics, said elements being disposed in a matrix array, a plurality of terminals each assigned to each row of said array and each connected with the cathode of every light emitting element in the row, a plurality of terminals each assigned to each column of said array and each connected with the anode of every light emitting element in the column, and a gate terminal connected with the control terminal of very light emitting element in the array.

3. A method for scanning the display device as defined in claim 1, characterized in that the column terminals are scanned with a pulse consisting of a starting negative part and a subsequent positive part, while the row terminals are normally provided with a negative voltage which is less negative than said negative part of the column scanning pulse, but a row terminal which is being scanned is provided with a negative voltage which is more negative than said negative part of the column scanning pulse if a light emitting element to be energized is connected with said row terminal, and a trigger pulse is applied to said gate terminal in synchronization with a column scanning pulse applied to the column terminal connected with the light emitting element to be energized, so that a light emitting element which has been turned to conductive is returned to the non-conductive state before the column terminal with which said conducting light emitting element is connected is scanned for the next time.

4. A method for scanning the display device as defined in claim 2, characterized in that the column terminals are scanned with a pulse consisting of a starting negative part and a subsequent positive part, while the row terminals are normally provided with a negative voltage which is less negative than said negative part of the column scanning pulse, but a row terminal which is being scanned is provided with a negative voltage which is more negative than said negative part of the column scanning pulse if a light emitting element to be energized is connected with said row terminal, and a trigger pulse is applied to said gate terminal in synchronization with a column scanning pulse applied to the column terminal connected with the light emitting element to be energized, so that a light emitting element which has been turned to conductive is returned to the non-conductive state before the column terminal with which said conducting light emitting element is connected is scanned for the next time.

5. A method for scanning the display device as defined in claim 2, characterized in that the potential of the column terminals is normally set at zero level, but a positive pulse is applied to a column terminal during the scanning period for the column if a light emitting element to be energized is connected with the column terminal, while the row terminals are normally provided with a constant negative voltage which is sufficient for maintaining a triggered light emitting element in the conductive state, the row terminals being scanned with a pulse which includes a positive part not lower than the column scanning pulse and a subsequent negative part, and a trigger pulse is applied to said gate terminal in synchronization with a column scanning pulse applied to the column terminal connected with the light emitting element which has been turned to conductive is returned to the non-conductive state before the column terminal with which said conducting light emitting element is connected is scanned for the next time.