Electronic display device for use in motor cars

An electronic display device for use in a motor car wherein the contents of the display are determined by the respective positions of a manually operated switch and the ignition switch is disclosed. A control circuit detects the respective positions of the two switches and thereby determines the data to be displayed.

BACKGROUND OF THE INVENTION 
This invention relates to electronic display device, for example an 
electronic timepiece for use in motor cars, wherein the number of the 
operating switches is reduced. 
In recent years, digital type electronic display devices utilizing quartz 
such as electronic timepieces have been widely used for motor cars. 
Considering an electronic timepiece as an example, the timepiece is 
constantly driven by receiving a driving current from a battery, but for 
the purpose of decreasing the power consumption of the battery, a display 
device of the timepiece which consumes relatively large power is operated 
only when the accessory contact or an ignition contact of an engine key 
switch is closed. For this reason, it has been impossible to know the time 
displayed by the timepiece when these contacts of the engine key switch 
are open or when an engine key is removed from the engine key switch. 
For this reason, a time display device has been proposed wherein an 
independent manually operated switch is provided for the timepiece so that 
when this switch is closed, the time is displayed for a definite interval 
even when the engine key switch is open. 
In such a display device, however, it is general to further provide 
independent manually operated switches for selecting additional display 
functions (added to a normal display of the time) of the device with the 
result that the operation panel requires a large area and becomes 
complicated, raising the production cost and impairing the appearance and 
ease of operation. Especially, in a digital electronic timepiece which is 
provided with many correction knobs, problems raised by the addition of 
separate manually operated switches are aggravated. 
SUMMARY OF THE INVENTION 
It is an objects of this invention to provide an improved electronic 
display device for use in a motor car capable of decreasing the number of 
the operating switches so as to simplify the construction of the operation 
panel, decreasing the cost and to improve the operating performance. 
Another object of this invention is to provide an electronic display device 
wherein the function of changing the contents of display or rendering the 
display activated which is effected by means of manually operated switches 
is switched over by turning on-off an accessory contact or an ignition 
contact of the engine key switch with a minimized number of the manually 
operated switches. 
According to this invention, there is provided an electronic display device 
for use in a motor car, comprising manually operated switch means for 
changing contents displayed by the display device or performances thereof, 
and a control circuit actuated by a signal generated by an engine key 
switch of the motor car, the control circuit including means responsive to 
the operation of the manually operated switch means for switching over the 
contents displayed by the display device or performances thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of this invention applied to an electronic timepiece 
of a motor car shown in FIG. 1 comprises an integrated circuit 1 such as 
for example Type No. MB 8731 manufactured by Fuji Tsushinki Seizo Corp., 
Japan including a crystal oscillator, a frequency divider, a counter, a 
quick correction circuit, decoder, display drive circuit, etc. The 
integrated circuit 1 is provided with a source terminal V.sub.DD, a reset 
terminal RS, an hour set terminal HS, a minute set terminal MS, a set 
enabling terminal SE and a plurality of display output terminals DP. 
Although not shown, quartz connecting terminal and grounding terminal are 
also provided and a quartz vibrator such as designated in FIG. 2 at 20 is 
connected to the quartz connecting terminal. To the output terminals DP is 
connected a well known fluorescent display device in the form of a tube 
including a plurality of digits each having letter 8 shaped display 
segments each including an anode terminal A. The tube also includes a grid 
terminal G and a filament terminal F. A terminal a is connected to a 
positive terminal of a battery, not shown, and a terminal b is connected 
to the positive terminal through an accessory contact ACC of an engine key 
switch SW.sub.1 via an accessory circuit, such as a car radio receiver, 
and a terminal c is grounded. The terminal b is also connected to an 
ignition contact ON of the engine key switch SW.sub.1. As is well known in 
the art, the engine key switch SW.sub.1 comprises, for example, in 
addition to contact ACC and contact ON, a start contact ST and a movable 
contact having a width sufficient to bridge two adjacent contacts. 
The terminal a is connected to the source terminal V.sub.DD of the 
integrated circuit 1 via a resistor R1 and voltage applied to the source 
terminal is maintained at a constant value by a Zener diode D1. 
Furthermore, the terminal a is connected to the reset terminal RS of the 
integrated circuit 1 via a series circuit made up of a push button switch 
SW.sub.4 and a resistor R5, which constitute an operating circuit. The 
source terminal V.sub.DD of the integrated circuit 1 is connected to its 
hour set terminal HS via a push button switch SW.sub.2 and also to its 
minute set terminal MS via a push button switch SW.sub.3. The switches 
SW.sub.2, SW.sub.3 and SW.sub.4 are herein generally called manually 
operated switch means. Across the juncture between push button switch 
SW.sub.4 and resistor R5 and the juncture between a diode D2 and a 
resistor R3 is connected a reverse current preventing diode D5. The set 
enabling terminal SE is connected directly to the accessory terminal b 
without passing through a reverse current preventing diode D2. Although 
not shown in the drawing, the integrated circuit 1 contains a set enabling 
circuit including a gate circuit with its input connected to the set 
enabling terminal SE. The juncture between diodes D2 and D5 is connected 
to the filament terminal F of the fluorescent display device 2 via 
serially connected resistors R3 and R4, and the voltage applied to the 
filament terminal F is maintained at a constant value by a Zener diode D3. 
The grid terminal G of the display device 2 is connected directly to the 
battery terminal a. A diode D4 is connected between the grounding terminal 
c and the Zener diodes D1 and D3. Upon connection of the terminal a to the 
battery, a voltage is impressed upon the source terminal V.sub.DD of the 
integrated circuit 1 whereby the integrated circuit operates to count the 
number of the clock signals to commence the timepiece driving operation. 
Then, an output signal is produced at the display output terminal DP. 
Although a positive bias voltage is applied to the grid terminal G, the 
fluorescent display device 2 may not operate since at this time no 
filament current flows to the filament terminal F. At this phase, if the 
push button switch SW.sub.2 is depressed, the hour display is rapidly 
changed whereas if the push button switch SW.sub.3 is depressed, the 
minute display is rapidly changed, thus rapidly correct the hour and 
minute displays. 
Thus, under a condition in which voltage is not supplied to terminal b by 
the opening of the accessory contact or of the ignition contact of the 
engine key switch, the fluorescent display device 2 may not luminesce 
because no voltage is applied to its filament terminal F which is utilized 
as an operating terminal. Upon depression of the push button switch 
SW.sub.4, the voltage of the battery is applied to the filament or 
operating terminal F even if the engine key switch is being opened, so 
that the display device 2 operates to display hours and minutes. At this 
time, the battery voltage is applied to the reset terminal RS via resistor 
R5. But, since the set enabling terminal RS is at an "L" level, the set 
enabling circuit is disabled thus performing no reset correction. For this 
reason, the time is displayed only during an interval in which the push 
button switch SW.sub.4 is being depressed or during a definite interval 
following the release of the push button switch SW.sub.4. When the 
accessory contact or the ignition contact of the engine key switch is 
closed, the fluorescent display device will operate normally to display 
time. At this time, however, the set enabling terminal SE is at a "H" 
level, the time can be corrected. Namely, if the push button switch 
SW.sub.4 is depressed, it operates to correct the setting, thus resetting 
to zero the minute display of the fluorescent display device 2. In this 
manner, the push button switch SW.sub.4 is used to manually operate the 
display device and to correct resetting. For this reason, according to 
this invention, it is possible to add the function of the manual control 
of the display without using any additional switch exclusively used for 
resetting. 
Although, in the embodiment shown, the push button switch SW.sub.4 was 
connected to the reset terminal RS, when it is connected to the hour set 
terminal HS or minute set terminal MS, the push button switch SW.sub.2 or 
the push button switch SW.sub.3 may be omitted. 
In a timepiece including a switch which, when operated, displays, for a 
definite time, a calendar, this switch may be used for calendar display as 
well as for manual setting of the time displayed. Furthermore, an 
integrated circuit containing a set enabling circuit provided with a set 
terminal SE was described, it will be clear that such a set enabling 
circuit may be disposed on the outside of the integrated circuit, and that 
fluorescent type display segments may be substituted by well known light 
emitting diodes, liquid crystal elements or the like. Furthermore, 
although in the foregoing embodiment, a reset enabling circuit of the 
control circuit was used to switch the time display correction, and manual 
display operation for momentarily display time while the display device is 
not used to display time, it is also possible to switch between an 
electronic timepiece and other instruments or devices, and to switch the 
display contents of the same display device. In addition, it should be 
understood that the number of the operating switches is not limited to one 
but a number of such switches may be provided, if desired. Instead of 
digital display, analogue display can also be used. 
According to the embodiment described above, by opening and closing of the 
accessory contact or the ignition contact of the engine key switch, the 
displayed contents or display performances which are changed by means of 
manually operated switches can be switched over, thus minimizing the 
number of the manually operated switches and simplifying the panel 
construction. 
FIG. 2 shows another embodiment of this invention comprising a well known 
clock circuit 10 such as for example Type No. MJ002 manufactured by Nippon 
Denso Co., Japan. The clock circuit 10 includes a crystal oscillator 
connected to a quartz vibrator 20, a frequency divider and a decoder and 
sends out timepiece display signals for digitally operating display 
segments. There are also provided a fluorescent display device 30 
including a plurality of digit segments, a voltage measuring circuit 40, 
such as for example Type No. LD130 manufactured by Siliconix, U.S.A, which 
effects an analogue-digital conversion of voltage to produce digital 
signals for operating the display segments, and a display selection 
circuit 50 which selects either one of the time display signal and the 
voltage display signal, the detail thereof being described later with 
reference to FIG. 3. There are also provided a positive terminal B 
connected to the positive pole of the battery, a grounding terminal GND, a 
terminal D which is connected to be applied with the battery voltage when 
the accessory contact or the ignition contact of the engine key switch is 
closed, the terminal D corresponding to the terminal b shown in FIG. 1, 
and a writing terminal L which is connected to receive a voltage when a 
writing switch, not shown, is closed. Protective circuits 60, 70, 80 and 
90 respectively bounded by dot and dash lines are provided for the purpose 
of stabilizing the operation of the circuit but as they are immaterial to 
the operation of the circuit, their detailed description is believed 
unnecessary. 
The clock circuit 10 is provided with a correct time set terminal HS which 
corrects the time by setting to zero the righthand two minute displaying 
digits, a correcting terminal UR which rapidly changes righthand two 
digits for correcting minute display or day display, a correcting terminal 
UL which rapidly changes lefthand two digits for rapidly correcting hours 
or months, a calendar terminal S for displaying a calendar, and a writing 
terminal LI. When these terminals are connected to a voltage source 
terminal VSS, operations assigned to these terminals are performed. The 
fluorescent display device 30 in the form of a tube is provided with a 
grid terminal G connected to the positive terminal B and filament terminal 
F1 connected to terminal D. A filament terminal F2 is normally grounded 
and the anode electrode of respective segments is supplied with a display 
signal from the display selection circuit 50. 
The voltage measuring circuit 40 is provided with an input terminal IN 
which is connected to terminal B to be applied with the battery voltage to 
be constantly measured or monitored. The display selection circuit 50 is 
provided with a control terminal A which is normally at a "H" level, 
whereby a time display signal 2b is supplied as an output 2c to the 
fluorescent display device 30. When the control terminal A is connected to 
a source terminal VSS via transistor Tr5 and switch SW.sub.7 which are 
turned on, it becomes an "L" level, thus producing a voltage display 
signal 2a as output 2c. 
A manually operated push button switch SW.sub.5 is connected between the 
calendar terminal S and the voltage terminal VSS and a noise preventive 
capacitor C is connected across the push button switch SW.sub.5. Likewise, 
one terminal of a push button switch SW.sub.6 is connected to the 
correction terminal UL of the clock circuit 10 via serially connected 
transistor Tr1 and diode D6, while the other contact is connected to the 
voltage terminal VSS. One terminal of a push button switch SW.sub.7 is 
connected to the correction terminal UR via a series circuit comprising a 
transistor Tr2 and a diode D7, while the other terminal is connected to 
the voltage terminal VSS. One terminal of a switch SW.sub.8 is connected 
to the correct time set terminal HS via serially connected transistor Tr3 
and a diode D8 whereas the other terminal is connected to the voltage 
terminal VSS. 
The circuit shown in FIG. 2 operates as follows. More particularly, when 
the accessory contact or the ignition contact of the engine key switch is 
closed to apply voltage to the terminal D, this voltage is applied to the 
filament terminal F1 via diode D9 so that current flows through the 
filament of the display device to cause the fluorescent display device 30 
to luminesce. Under this condition, since both transistors Tr4 and Tr5 are 
in their OFF state, the control terminal A of the display selection 
circuit 50 bears a "H" level, while the time display signal from the clock 
circuit 10 is supplied to the fluorescent display device 30 via the 
display selection circuit 50, thereby displaying hours and minutes. Upon 
closure of switch SW.sub.5, the calender terminal becomes an "L" level so 
that the hour and minute displays are extinguished and substituted by a 
calendar display representing months and days. The circuit is constructed 
such that even when the switch SW.sub.5 is opened the display of the 
calender will continue for several seconds. Where voltage is being 
impressed upon terminal D, transistors Tr1, Tr2 and Tr3 become ON state 
because their base electrodes are positively biased. For this reason, upon 
closure of switch SW.sub.6, the voltage of the correction terminals 
becomes low level thus rapidly correcting the time, whereas when the 
switch SW.sub.7 is closed, the voltage of the correction terminal UR 
becomes the low level "L", thus rapidly correcting minutes. Similarly, 
when switch SW.sub.8 is closed, the voltage of the correct time set 
terminal becomes the low level, thus adjusting the time to a correct time, 
Upon closure of switch SW.sub.7, the voltage of the emitter electrode of 
transistor Tr5 becomes the low level "L". But since transistor Tr4 is in 
its OFF state, no base current flows through this transistor thus 
maintaining it in the OFF state. Accordingly, the voltage of the control 
terminal A of the display selection circuit 50 will be continuously 
maintained at the "H" level. Under these conditions, transistors Tr1, Tr2 
and Tr3 permit setting of the corresponding corrections and possess the 
function of a set enabling circuit. 
When the accessory contact or the ignition contact of the engine key switch 
is opened to remove voltage from contact D, no current flows through the 
filament. Thus the fluoroscent display device 3 stops to display even when 
a display signal is applied thereto. Since transistors Tr1, Tr2 and Tr3 
are OFF, even when switches SW.sub.6, SW.sub.7 and SW.sub.8 are closed, 
the correction terminals UL and UR and the correct time setting terminals 
HS will not become "L" level. 
At this time, the base voltage of the transistor Tr4 becomes the low level, 
so that this transistor is in its ON state. Under these conditions, when 
switch SW.sub.8 is closed, the base voltage of transistor Tr6 becomes the 
"L" level, thus turning ON the same, with the result that the voltage of 
the terminal B is applied to the filament terminal F1, thus operating the 
fluorescent display device 30. Since the clock circuit 10 is operating 
constantly, the fluorescent display device 30 displays hours and minutes. 
Closure of switch SW.sub.8 does not turn ON transistor Tr5 because diode 
D10 is connected reversely so that the emitter potential of the transistor 
will not be changed to the low level "L". Consequently, the control 
terminal A of the display selection circuit 50 is still maintained at the 
"H" level. Under these conditions, when voltage is impressed upon terminal 
D, the switch SW.sub.8 functions to change the displayed time to the 
correct time when it is desired to change the hour and minute displays to 
the correct time, whereas when voltage is not impressed upon the terminal 
D the switch SW.sub.8 functions to change the display from no display to 
the hour and minute displays. Transistors Tr3 and Tr6 operates depending 
upon the presence and absence of voltage on terminal D to constitute a 
transfer circuit for switching the abovedescribed functions of the switch 
SW.sub.8. 
Closure of switch SW.sub.7 turns ON the transistor Tr7 to operate the 
luminous display device 30 while at the same time it changes the emitter 
voltage of transistor Tr5 to the "L" level, thus turning it ON wth the 
result that the emitter voltage of transistor Tr5 becomes the "L" level, 
thus changing the voltage of the control electrode A of the display 
selection circuit 50 to the low level "L". Consequently, the voltage 
display signal produced by the voltage measuring circuit 40 is selected by 
the display selection circuit 50 and supplied to the fluorescent display 
device 30 to digitally display the value of the battery voltage. 
Consequently, it is possible to accurately measure and display the battery 
voltage when the battery is not charged while the engine of the motor car 
is stopped. At this time, depending upon the presence and absence of the 
voltage impressed upon terminal D the switch SW.sub.7 functions to rapidly 
change the hour and minute display to the minute display or to change the 
display mode from no display to battery voltage display. Transistors Tr2, 
Tr6, Tr4 and Tr5 constitute a transfer circuit which switches the 
operation of the switch SW.sub.7. When switch SW.sub.7 is closed, although 
the emitter electrode of transistor Tr3 is connected to the VSS voltage 
terminal through diodes D8 and D10 due to the forward voltage drop of 
diode D10, the emitter voltage will not be decreased sufficiently so that 
transistor Tr3 maintains its OFF state. Irrespective of the presence and 
absence of the voltage on terminal D, the voltage at the writing terminal 
LI of the clock circuit 10 is maintained at the level " L" by the voltage 
at the terminal L at the time transistor Tr7 becomes ON. 
While in the foregoing embodiment the switches SW.sub.7 and SW.sub.8 were 
used as transfer switches, switches SW.sub.5 and SW.sub.6 can also be used 
as transfer switches. Generally, when the engine is rotating and while the 
car is running, the voltage is applied on terminal D, the items to be 
displayed under these conditions include, in addition to hours and minutes 
described above, a calender, rapid correction of display, correct time, 
car speed, number of revolutions of the engine, cooling water temperature, 
pressure of the lubricant, charging current of the battery and so forth. 
No voltage on terminal D corresponds to engine stop, the items to be 
displayed include transfer from no display to so-called manual hour, 
minute display wherein hours, minutes and seconds are displayed for a 
definite interval, and battery voltage. Items to be displayed in each case 
include running distance and the remaining quantity of the fuel. The 
switches can be transferred to perform any one of various combinations of 
these items. Since a calender is normally used except correction of time, 
and setting of the correct time, it is advantageous to use a mode of 
operation wherein the time is automatically displayed for a definite 
interval after operation of the switches. 
One example of one component of the display selection circuit 50 is 
illustrated in FIG. 3, which comprises an AND gate circuit G1 with one 
input connected to receive the voltage display signal 2a from the voltage 
measuring circuit 40 and the other input connected to the control terminal 
A via an inverter INV.sub.1, an AND gate circuit G2 with one input 
connected to receive the time display signal 2b from the clock circuit 10 
and the other input connected to the control terminal A, and an OR gate 
circuit G3 having its inputs connected to outputs of the AND gate circuits 
G1 and G2 and its output connected to the fluorescent display device 30 
for supplying thereto the output 2c. The one component of the above 
construction is provided for each of 7 display segments of each digit in 
the fluorescent display device 30. 
In operation, when the control terminal A is at the "H" level, the time 
display signal 2b is allowed to pass through the AND gate circuit G2 and 
the OR gate circuit G3, thereby being delivered out as the output 2c. On 
the other hand, when the control terminal A is rendered "L" level, this 
"L" level is inverted by the inverter INV.sub.1 so that the voltage 
display signal 2a is allowed to pass through the AND gate circuit G1 and 
the OR gate circuit G3, thereby being transmitted as the output 2c. 
FIG. 4 illustrates a block diagram showing another embodiment of this 
invention wherein similar elements to those shown in FIG. 1 are designated 
by the same reference characters. The circuit shown in FIG. 4 comprises a 
monostable multivibrator 11 which produces a "H" level output for several 
seconds when applied with an input pulse, a monostable multivibrator which 
produces a "H" level output for a small interval when supplied with an 
input pulse, and flip-flop circuits 13 and 14. A manually operated push 
button switch SW.sub.9 is connected between a voltage terminal V.sub.DD 
and a voltage terminal VSS. 
In operation, where voltage is present on terminal D, since the inputs to 
an exclusive OR gate circuit G4 are "H" and "L" levels, it produces an "H" 
level output. One input to an AND gate circuit G5 is at the "H" level, and 
this input signal is inverted by an inverter INV.sub.2 and applied to one 
input of an AND gate circuit G6 as an "L" level input. In response to the 
signal applied to terminal D, the monostable multivibrator 12 produces, 
for a small interval, a "H" level output which is applied to the reset 
terminals R of the flip-flop circuits 13 and 14 for resetting them. When 
reset, the flip-flop circuit 13 produces an "L" level output at its Q 
output terminal and a "H" level output at its Q output terminal which is 
fed back to its driving D terminal. As a consequence, both inputs to the 
AND gate circuit G5 becomes the "H" level and it is enabled to produce a 
"H" level output. On the other hand, since the output of the monostable 
multivibrator 11 is at the "L" level, the AND gate circuit G6 is disabled 
to produce an "L" level output. As a consequence, one input to an OR gate 
circuit G7 becomes "H" level so that it produces a "H" level output. 
The flip-flop circuit 14 is normally applied with a "H" level input at its 
driving D terminal, and therefore it is reset to produce an "L" level 
output on its Q output terminal and a "H" output on it Q output terminal. 
When this flip-flop circuit 14 is reset, both inputs to the AND gate 
circuit G8 become the "H" level, whereby this AND gate circuit is enabled 
to produce a "H" level output. Since one input to AND gate circuit G9 is 
at the "L" level, it is disabled to produce an "L" level output. 
Consequently, a "H" level input is applied to one input of an OR gate 
circuit G10, whereby it produces a "H" level output which is applied to 
the base electrode of a transistor Tr8 via a buffer amplifier A. This 
turns ON transistor Tr8 to apply the battery voltage at terminal B upon 
the filament terminal F1 to operate the fluorescent display device 30, 
thus displaying hours and minutes. 
When the push button switch SW.sub.1 is depressed, the monostable 
multivibrator circuit 11 produces a "H" level output for several seconds, 
but since the other input to the AND gate circuit G6 is at the "L" level, 
this AND gate circuit G6 is disabled, thus producing an "L" level output. 
Since both inputs to the exclusive OR gate circuit G4 are at the "H" 
level, it produces an "L" level output, whereby the AND gate circuit G8 
produces "L" level output which in turn causes the OR gate circuit G10 to 
produce an "L" level output. Thus, transistor Tr8 is turned off to 
deenergize the luminous display device 30. 
When switch SW.sub.9 is closed, the inputs to the clock terminals of both 
flip-flop circuits 13 and 14 also become the "H" level and the operations 
thereof are reversed so that the output at the Q output of the flip-flop 
circuit 13 becomes the "H" level, whereas that at the Q output terminal 
becomes the "L" level. Likewise the output at the Q output terminal of the 
flip-flop circuit 14 becomes the "H" level and that at the output terminal 
Q becomes the "L" level. Since one input to the AND gate circuit G5 is at 
the "L" level, it is disabled to produce an "L" level output. This also 
causes the OR gate circuit G7 to produce an "L" level output, thus causing 
the AND gate circuit G9 to produce an "L" level output. Both inputs to the 
AND gate circuit G8 are at the "L" level, thus disabling the same. When 
the push button switch SW.sub.9 is released, the output of the exclusive 
OR gate circuit G4 becomes the "H" level. But, as the "L" level output of 
the Q output terminal of the flip-flop circuit 14 is supplied to one input 
of the AND gate circuit G8, this AND gate circuit G8 is disabled to 
produce an "L" level output. Consequently, the fluorescent display device 
30 continues its inoperative condition. 
When the switch SW.sub.9 is again depressed, the flip-flop circuit 14 
continues to maintain its present state whereas the flip-flop circuit 13 
reverses again its state with the result that its Q output becomes the "L" 
level and the Q output becomes the "H" level. Consequently, both inputs to 
the AND gate circuit G5 become the "H" level, whereby this AND gate 
circuit is enabled to produce a "H" level output which causes the OR gate 
circuit G7 to produce a "H" level output. Hence, both inputs to the AND 
gate circuit G9 become the "H" level, whereby this AND gate circuit is 
enabled to produce a "H" level output. This causes the OR gate circuit G10 
to produce a "H" level output, thus turning ON transistor Tr8 to resume 
the display of the fluorescent display device 30. Thereafter, each time 
the switch SW.sub.9 is operated, the display mode and the non-display mode 
are repeated alternately. 
Where the accessory contact or the ignition contact of the key switch is 
being closed and where the display of the luminous display device 30 is 
too bright to the eyes it is advantageous to select either one of the 
display mode and the non-display mode by mere operation of the push button 
switch SW.sub.9. 
Where no voltage is impressed upon the terminal D, both inputs to the 
exclusive OR gate circuit G4 are at the "L" level so that it produces an 
"L" level output. Furthermore, as one input to the AND gate circuit G5 is 
at the "L" level, it is disabled, while this "L" level input being 
inverted by the inverter INV.sub.2 into a "H" level signal. However, as 
the other input to the AND gate circuit G6 is at the "L" level, it is 
disabled to produce an "L" level output and the OR gate circuit G7 
produces an "L" level signal. Thus, both AND gate circuits G8 and G9 
produce "L" level outputs, thus turning OFF transistor Tr8. Consequently, 
the fluorescent display device 30 is rendered inoperative. 
When switch SW.sub.9 is depressed under these conditions, the input to the 
monostable multivibrator 11 becomes the "H" level causing it to produce a 
"H" output for several seconds. Consequently, both inputs to the AND gate 
circuit G6 become the "H" level, thus enabling the same to produce a "H" 
level output. As a consequence, the OR gate circuit G7 produces a "H" 
level output. Thus, both inputs to the AND gate circuit G9 become the "H" 
level and this AND gate circuit G9 is enabled to produce a "H" level 
output causing the OR gate circuit G10 to produce a "H" level output. This 
turns ON transistor Tr8 for energizing again the luminous display device 
30. However, since the output of the monostable multivibrator 11 changes 
to the "L" level, the fluorescent display device returns to the 
inoperative state after displaying only several seconds. Thereafter, each 
time the push button switch SW.sub.9 is operated the fluorescent display 
device becomes operative for several seconds. Accordingly, where the 
accessory contact or the ignition contact of the key switch is being 
opened so that the luminescent display device is in its inoperative mode, 
it is possible to know the time by merely depressing the switch SW.sub.9. 
In the presence of the voltage at terminal D, the flip-flop circuits 13 and 
14 are both reset by the output of the monostable multivibrator 12, so 
that the component circuits are returned to the original states described 
above irrespective of their present states, thus assuring correct 
operation thereof. 
In the embodiment described above, in response to the presence or absence 
of the voltage at terminal D1 the switch SW.sub.9 functions to display or 
not to display hours and minutes, and to change the mode to display hours 
and minutes to non-display mode and vice versa. The monostable 
multivibrator and the flip-flop circuits constitute a transfer circuit 
that transfers the functions of the switch SW.sub.9. With the construction 
where the switch SW.sub.9 is connected directly to one input of the AND 
gate circuit G6 without using the monostable multivibrator 11, it is 
possible to display hours and minutes only while the switch SW.sub.9 is 
held closed. 
While, in the foregoing embodiments, luminous display tube was used as the 
display device, it will be clear that luminous elements such as a light 
emitting diode, a plasma display device and a liquid crystal element can 
also be used. 
As shown in the embodiments described with reference to FIGS. 2 and 4, 
since one switch can provide two functions, it is possible to decrease the 
number of switches on the panel board, thus providing the low cost display 
device which is convenient to use.