Apparatus for selectively displaying analogue quantities of plural input data signals
The present invention relates to a display apparatus for displaying analogue quantities (input data) as the ratio of the analogue quantities to a predetermined value in the form of a graph. The display apparatus is adapted particularly to the instrument panel of vehicles such as automobiles for display of analogue values representing the number of revolutions per minute of an automobile&#39;s engine, the temperature at the various portions of the automobile, the residual fuel and the like. 
     For displaying the detected ratio of the analogue input value (input data) to a predetermined value, a conventional display apparatus uses a circuit for comparing input data and another circuit for displaying the comparison result of the input data. Since the conventional display apparatus is provided with the display circuit and the comparing circuit separately, it requires a number of circuit components which result in a high cost. 
     Accordingly, an object of the present invention is to provide a display apparatus with a simple circuit construction. 
     Another object of the present invention is to provide a display circuit with a comparing function. 
     Other object of the present invention is to provide a display device which can receive input data and select a desired one from the input data thereby to display it. 
     This invention provides a single-circuit arrangement capable of comparing the selected input data signal with the reference value predetermined and then displaying the result of the comparison, so that the circuit arrangement is simple in construction. 
     In more particular, a datum selected from the input data is applied to a first common terminal in the form of an analogue signal (voltage or current). In order to cause the corresponding number of the light emitting diodes to the analogue quantity of the selected input signal to light up, the light emitting diodes receive each reference step voltage, which stepwisely changes, at the one-side terminals in accordance with the order of the light emitting diodes set in array, while they receive at the other-side terminals the voltage which has been applied thereto and now appears at the first common terminal. Current flows through the light emitting diodes when they are forwardly biased by the potential difference appearing between the one-side terminals with the reference voltage applied thereto and the other-side terminals with the input data applied thereto, so that such biased light emitting diodes make a light-emission. 
     There are two ways for displaying the input data: One is that the number of light emitting diodes in light emitting state shows the analogue value of the input data; The other is that the number of light emitting diodes in light non-emitting state indicates the analogue value. The ways are selectable with which electrode of the light emitting diodes is connected between the terminals providing the reference step voltages and the first common terminal, i.e. which terminal the anode of the light emitting diode is connected to. The present invention permits either electrode of the light emitting diodes to be connectable to the common terminal. When the anode of the light emitting diode is connected to the common terminal, light is emitted from the light emitting diode of the number corresponding to the analogue value if the reference step voltage is positive. Conversely, in the case of the cathode of light emitting diode being connected to the common terminal, the light-emitting diodes of the number corresponding to the analogue value are turned off. 
     In this specification, the term &#34;light emitting diode&#34; is used to define a component which has at least two terminals and the impedance of which largely varies by the polarity of the voltage applied across the two terminals thereof and emits light when the current flowing therethrough reaches a predetermined value. 
     The components of the name other than the above which are now or will be commercially available, are also applicable to the display apparatus of the present invention if they have the above-mentioned characteristic. Thus, such components must also be involved in the above term of &#34;light emitting diode&#34;. 
    
    
     Other objects and features of the present invention will be apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 shows a block diagram of an embodiment of a display apparatus according to the present invention; and 
     FIG. 2 shows a circuit diagram of a preferred embodiment of the block diagram of FIG. 1. 
    
    
     FIG. 1 shows a block diagram of a display apparatus of a preferred embodiment according to the present invention. In the figure, a switching circuit 1 selects one of input data V 1  to V n . The selected input data is in turn transferred to a comparison and display circuit 5, while, at the same time, a signal for selecting the voltage determined by the selected input data is fed to a reference-voltage production circuit 3. 
     The comparison and display circuit 5 compares the input data voltage from the switching circuit 1 with the output voltage from the reference voltage production circuit 3. Through the comparing operation, when the input data voltage is larger than the output voltage of the reference voltage production circuit 3, the input data are displayed. When the above relationship in amplitude between the input data voltage and the reference output voltage is reversed, the input data are not displayed. 
     FIG. 2 shows a concrete construction for a preferred embodiment of the block diagram in FIG. 1. In FIG. 2, reference numeral 10 is a switching signal production circuit for producing switching signals S1 to Sn, 111 to 11n and 121 to 12n designate switches which are turned on and off in response to switching signals S1 to Sn, 31 to 3 (m + 1) and 511 to 51m are resistors, 521 to 52m designate light emitting diodes, and E 1  to E n  are voltages determined by the input voltages V 1  to V n . 
     The switches 111 to 11n receive at the input side the input voltages V 1  to V n , while they are at the output side connected to a common terminal A. The common terminal A is connected to the anodes of the light emitting diodes 521 to 52m, through resistors 511 to 51m. 
     The switches 121 to 12n receive at the input side the voltages E 1  to E n , and are at the output side connected in common to a series circuit consisting of resistors 31 to 3 (m + 1). The respective junction points of the resistors 31 to 3 (m + 1) are connected to the cathode sides of the light emitting diodes, respectively. It will be noted here that the voltages E 1  to E n  are produced by a combination of, for example, a DC power source 41, resistors R 4 , and diodes D 4 . If the variable range of each of the input voltages representing the input data is identical, a common voltage may be used for the voltages E 1  to E n . In this case, switches 121 to 12n become unnecessary. 
     In the circuit construction of FIG. 2, assume that the gth switches 11g and 12g are selected. Upon this switch selection, the switch 11g produces an input voltage V g . The switch 12g produces the voltage determined by the input voltage V g , which is in turn fed to the resistors 31 to 3 (m + 1). 
     At this time, the output voltage V s31  to V s3m  appearing at the junction points of the respective resistors 31 to 3 (m + 1) are given by the equation (1) ##EQU1## where R 31  to R 3 (m+1) are the resistance values of the resistors 31 to 3(m+1), respectively. 
     When the voltage V s3m  obtained by the equation (1) is set at the maximum value of the input voltage Vg selected by the gth switching signal, the voltage V s3m  corresponds to 100% of the maximum value of the input voltage. 
     Further, the resistance values of the resistors 31 to 3m are chosen so that the voltages V s31  to V s3m  in the equation (1) will be the predetermined ratios of voltage to the maximum value of the input voltage, respectively. 
     As described above, the input voltage V g  selected by the gth switch 11g is outputed from the switch 11g and is compared with the voltages V s31  to V s3m . When V g  &lt; V s31  to V s3m , none of the light emitting diodes 521 to 52m light up. On the contrary, when V g  &gt; V s31  to V s3m , all of the diodes 521 to 52m light up. 
     If the voltage V g  is between the ith voltage V s3i  and the jth voltage V s3j , i.e. V s3i  &lt; V g  &lt; V s3j , the light emitting diodes 521 to 52i light up, while the diodes 52j to 52m do not light up. 
     As seen from the above, the light-emitting state of the respective light emitting diodes shows what is the ratio of the input voltage V g  to the maximum value of the input voltage. In the case where a higher accuracy is required for display, the voltages V s31  to V s3m  are formed so as to satisfy such accuracy. 
     The resistors 511 to 51m are used for adjusting the current flowing through the corresponding light emitting diodes 521 to 52m thereby to make the brightness of the respective light emitting diodes uniform. 
     It is assumed now that the input voltage Vg is between the ith voltage V s3i  and the jth voltage V s3j . One of the light emitting diodes from the first to the ith, for example, the diode 52i has the current I 52i  flowing therethrough which is given by the equation (2) ##EQU2## where r 52i  and r 51i  are the forward resistance of the photodiode 52i and the resistance of the resistor 51i, respectively. 
     The numerator V s3i  of the right side of the equation (2) is variable for the respective light emitting diodes. Therefore, in order to make constant the currents flowing through the respective light emitting diodes, it is necessary to change the resistance values of the resistors 511 to 51m. More particularly, as the voltage which a light emitting diode indicates is closer to the maximum value of the input voltage, the resistor to be connected in series with the light emitting diode must be smaller in resistance. Assuming that the resistance values of resistors 511 to 51m are r 511  to r 51m , these resistance values are expressed as follows: r 511  &gt; r 512  &gt; r 513  &gt; . . . r 51m . 
     The display apparatus heretofore described provides the useful effects as given below. 
     (1) In the conventional display apparatus, the comparing circuit and the indicating circuit are separately provided. On the other hand, the display apparatus according to the present invention is constructed such that the comparison and the display function can be achieved by a single circuit construction. Accordingly, necessary parts are saved. 
     (2) The saving of parts make it possible to increase the reliability of the apparatus and to reduce the cost thereof. 
     (3) Since the switches 111 to 11m are used, it is possible to display a plurality of input data. 
     In the embodiment of the display apparatus heretofore described, as the input data increase, the number of the diodes in light-emitting state is increased. As will be recalled, the display device according to the present invention may be constructed such that, with increase of the input data, the number of the diodes in light-emitting state is conversely decreased. Such a construction is easily realized by reversing the connection of light emitting diodes in the direction so that a direction of current flow through the light emitting diode is reversed. 
     The switching signal production circuit 10 in the above-mentioned circuitry is composed of, for example, a DC power source 14, a resistor R 1 , diodes D 1 , switches SW 1  to SW n . An operator such as an automobile&#39;s driver turns on one of the switches SW 1  to SW n . By turning-on of the switch, the corresponding switches are selected from the groups of the switches 111 to 11n and 121 to 12n. When the present invention is applied to the automobile, another control means may be used for producing the switching signals S 1  to S n . The control means may be registers, ring counters and the like which comprise flip-flops commonly used as digital circuit elements. Semiconductor switches may also be used for the switches SW 1  to SW n . These digital circuit elements may be arranged so as to be operable in response to other control signals. 
     The circuit of the preferred embodiment mentioned above is constructed to be adapted to the automobile in which automobile&#39;s speed, water temperature, residual fuel make up the bulk of the input data. The detecting means for the input data completely differs depending on the variety of the input data to be detected. Accordingly, the variable range of the output voltage and the amplitude thereof also differ depending on the variety of the input data. It is for this reason that the arrangement is employed in which the input voltage applied to the voltage-dividing circuit consisting of resistors 31 to 3 (m + 1) is changed over within the range of E 1  to E n  by means of switches 121 to 12n. It is noted that if the variable range of each of the input voltages V 1  to V n  is identical, the voltages E 1  to E n  may be the same in magnitude. In this case, the switches 121 to 12n become unnecessary.