Device for displaying remaining electric energy of battery

A display device has a time-constant circuit and a switching element such as a transistor for supplying the voltage of a battery to the time-constant circuit when a battery check key is pressed. A microcomputer measures a time interval from a time when the voltage of the battery starts being supplied to the time-constant circuit to a time when a voltage of the time-constant circuit reaches a predetermined value, and determines the remaining electric energy of the battery from the measured time interval. The microcomputer supplies data to a display unit which then displays a progressively variable display mark combination representative of the determined remaining electric energy of the battery.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates to a device for displaying the remaining 
electric energy of a battery for use with a phase-locked loop (PLL) radio 
receiver, for example. 
Description of the Prior Art 
Some radio receivers have tuner circuits which incorporate variable 
capacitors. When batteries used as power supplies in such radio receivers 
are consumed, or their voltage drops, sounds reproduced by the radio 
receivers are distorted. Transmitted signals can be reproduced normally 
when the consumed batteries are replaced with new batteries. 
However, when batteries used as power supplies in radio receivers with PLL 
synthesizers are consumed, data items stored in a memory with respect to 
selected frequencies to be received are changed or destructed. Therefore, 
transmitted signals cannot be received properly because the stored data 
items are changed or destructed simply by replacing the consumed batteries 
with new batteries. 
Usually, battery-powered radio receivers are equipped with a display 
circuit for displaying a warning when the battery voltage drops to a level 
which adversely affects the memory of the radio receiver. 
FIG. 1 of the accompanying drawings shows a conventional display device for 
displaying such a warning. A radio receiver 10 has a PLL synthesizer with 
a PLL 11 and a loudspeaker 12. The PLL 11 includes a variable frequency 
divider (not shown) whose frequency dividing ratio N may be varied to 
select the broadcast of a frequency corresponding to the frequency 
dividing ratio N. 
The frequency dividing ratio N is controlled by a microcomputer 20 having a 
memory 21 for storing data relative to frequencies to be received, e.g., 
frequency dividing ratios N corresponding to those frequencies to be 
received. To the microcomputer 20, there is connected a LCD (liquid 
crystal display) 3 for digitally displaying a frequency being received, 
and a plurality of control keys 4 for indicating a desired frequency. 
When the control keys 4 are operated on to indicate a frequency, the 
microcomputer 20 reads a corresponding frequency dividing ratio N from the 
memory 21, and sets the variable frequency divider in the PLL 11 to the 
frequency dividing ratio N read from the memory 21 for selecting the 
desired frequency. 
A battery 5 of 3 V, for example, used as a power supply applies a voltage 
V5 to a power supply terminal VDD of the microcomputer 20. A switching 
transistor Q1 is connected between the battery 5 and a power supply 
terminal VDD of the radio receiver 10. The transistor Q1 has a base 
supplied with a control signal from the microcomputer 20. 
A voltage detector 6 serves to detect the voltage V5 from the battery 5, 
and applies a detected output V6 to a chip-enable terminal CE of the 
microcomputer 20. When the voltage V5 of the battery 5 is equal to or 
higher than a threshold value VTH, e.g., 67% of an initial voltage Vinit, 
the detected output V6 is "1", and when the voltage V5 is lower than the 
threshold value VTH, the detected output V6 is "0". The voltage detector 6 
comprises a one-chip CI dedicated for voltage detection. A pull-up 
resistor R1 is connected between the power supply terminal VDD of the 
microcomputer 20 and the chip-enable terminal CE. A backup capacitor 9 is 
connected across the battery 5, so that it is normally charged by the 
battery 5. In the event that the battery 5 is replaced with a new battery, 
the backup capacitor 9 provides the voltage V5 for a short period of time 
to protect the data stored in the memory 21 during the replacement of the 
battery 5. 
If the voltage V5 of the battery 5 is of a sufficiently high level, i.e., a 
level higher than the threshold value VTH, then the detected output V6 is 
"1". Under this condition, a power supply key of the control keys 4 is 
turned on, the microcomputer 20 turns on the transistor Q1. The voltage V5 
is now applied through the transistor Q1 as an operating voltage to the 
radio receiver 10 to enable the radio receiver 10 to receive a broadcast 
whose frequency has been selected by the control keys 4 as described 
above. 
If the voltage V5 of the battery 5 drops below the threshold value VTH, 
then the detected output V6 becomes "0". At this time, the microcomputer 
20 is no longer capable of accepting input signals from the control keys 
4. The microcomputer 20 also turns off the transistor Q1, so that the 
voltage V5 is not supplied from the battery 5 to the radio receiver 10. 
The microcomputer 20 controls the LCD 3 to display a blinking mark 
indicating the voltage drop of the battery 5 or demanding a battery 
replacement, e.g., a mark indicating that the battery is used up, as shown 
in FIG. 2 of the accompanying drawings. 
At this time, the voltage of the battery 5 is of a minimum level required 
to keep the microcomputer 20 energized for thereby protecting the 
frequency dividing ratios N stored in the memory 21 from changes or 
destruction. 
With the display device shown in FIG. 1, therefore, when the voltage V5 of 
the battery 5 drops below the threshold value VTH, the radio receiver 10 
is forcibly de-energized, the remaining voltage of the battery 5 is used 
to keep the frequency dividing ratios N stored in the memory 21, and the 
voltage drop of the battery 5 is displayed by the LCD 3. 
However, the user is notified of the voltage drop by the LCD 3 for the 
first time when the voltage V5 of the battery 5 drops below the threshold 
value VTH. Inasmuch as the user cannot predict the time to replace the 
battery 5 before the voltage drop of the battery 5 is displayed by the LCD 
3, the user is often unable to have a new battery ready for immediate 
replacement of the consumed battery 5. As a result, in the event of the 
voltage drop of the battery 5, the user may not change frequencies to be 
received by the radio receiver 10. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a device for displaying 
progressively varying stepwise levels of the remaining electric energy or 
voltage that is available from a battery. 
According to the present invention, there is provided a display device for 
displaying the remaining electric energy of a battery in an electronic 
device having a memory for storing predetermined data, the display device 
comprising a time-constant circuit, switching means for supplying a 
voltage of the battery to the time-constant circuit in response to a key 
stroke, control means for measuring a time interval from a time when the 
voltage of the battery starts being supplied to the time-constant circuit 
to a time when a voltage of the time-constant circuit reaches a 
predetermined value, and for determining the remaining electric energy of 
the battery from the measured time interval, and display means for 
displaying a progressively variable display mark combination 
representative of the determined remaining electric energy of the battery. 
The display means comprises common display elements for selectively 
displaying the data stored in the memory and the progressively variable 
display mark combination representative of the determined remaining 
electric energy of the battery. The progressively variable display mark 
combination comprises a plurality of display marks selectively energizable 
depending on the remaining electric energy of the battery which is 
determined by the control means. 
The time interval until the time when the voltage of the time-constant 
circuit reaches the predetermined value varies depending on the remaining 
electric energy or voltage of the battery. The remaining electric energy 
of the battery is determined from the time interval, and displayed as the 
progressively variable display mark combination by the display means. 
The above and other objects, features, and advantages of the present 
invention will become apparent from the following description of 
illustrative embodiments thereof to be read in conjunction with the 
accompanying drawings, in which like reference numerals represent the same 
or similar objects.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 3 shows a battery voltage display device according to the present 
invention. As shown in FIG. 3, a radio receiver 10 with a PLL 11, a 
loudspeaker 12, a microcomputer 20 having a memory 21, an LCD 3, a 
plurality of control keys 4, a battery 5, a switching transistor Q1, a 
voltage detector 6, a pull-up resistor R1, and a backup capacitor 9 are 
identical structurally and functionally to those shown in FIG. 1, and will 
not be described in detail below. 
A time-constant circuit 7 comprises a resistor R7 and a capacitor C7. The 
resistor R7, which is an input terminal of the time-constant circuit 7, is 
connected through a transistor Q7 to the battery 5. The transistor Q7 has 
a base supplied with a control signal $20 from an output port Po of the 
microcomputer 20. 
The time-constant circuit 7 applies an output voltage V7 from the capacitor 
C7 to a voltage comparator 8, which is supplied with a reference voltage 
VRE that is derived from a constant voltage produced by a constant-voltage 
circuit (not shown) of the radio receiver 10. The voltage comparator 8 
applies a comparison output V8 to an input port Pi of the microcomputer 
20. 
The LCD 3 has a display unit or mark 30 (see FIG. 4) as indicating the 
remaining voltage of the battery 5. As shown in FIG. 4, the display mark 
30 comprises a plurality of display elements 31, 32, 33, 34, 35. The 
display element 31 is in the shape of an elongate rectangular frame 
representing the profile of a general battery, and includes a projection 
on one end, indicating the positive terminal of the battery. The display 
elements 32.about.35 are each of an elongate rectangular shape, and are 
positioned closely within the display element 31. In the LCD 3, the 
display elements 31 35 actually comprise transparent electrodes for 
driving the liquid crystal of the LCD 3. 
A battery check key 4a, in addition to the control keys 4, is also 
connected to the microcomputer 20. 
When the battery check key 4a is pressed at a time to (see FIGS. 6A through 
6D), the control signal S20 from the microcomputer 20 becomes "1" for a 
predetermined period of time from the time to (see FIG. 6A), turning on 
the transistor Q7 from the time to on. As indicated by the solid-line 
curve in FIG. 6B, the voltage V5 of the battery 5 is supplied through the 
transistor Q7 to the time-constant circuit 7 from the time to on. 
Therefore, the output voltage V7 across the capacitor C7 gradually 
increases from the time t0 on, as shown in FIG. 6C. 
The voltage comparator 8 compares the voltage V7 with the reference voltage 
VRE. When the voltage V7 is equal to or higher than the reference voltage 
VRE at a time t1, as shown in FIG. 6D, the voltage comparator 8 supplies a 
comparison output V8 of "1" to the input port Pi of the microcomputer 20. 
The magnitude of the voltage V5 of the battery 5 varies with the remaining 
electric energy of the battery 5 as indicated by the solid- and 
broken-line curves in FIG. 6B. Therefore, the rate of change of the 
voltage V7 across the capacitor C7 also varies with the remaining electric 
energy of the battery 5. The higher the voltage VS, i.e., the greater the 
remaining electric energy of the battery 5, the higher the rate at which 
the voltage V7 increases. Thus, as the remaining electric energy of the 
battery 5 is greater, the time t1 of the positive-going edge of the 
voltage V8 is earlier, i.e., closer to the time t0, with the time interval 
.tau. between the times t0, t1 being shorter. 
The time interval .tau. has different values .tau.1.about..tau.4 41 44 for 
different values of the voltage V5 of the battery 5, as follows: 
1. When the voltage V5 is 90% of the initial voltage Vinit of the battery 
5, the time interval .tau. has a value 1. 
2. When the voltage V5 is 85% of the initial voltage Vinit of the battery 
5, the time interval .tau. has a value .tau.2. 
3. When the voltage V5 is 80% of the initial voltage Vinit of the battery 
5, the time interval has a value .tau.3. 
4. When the voltage V5 is 67% of the initial voltage Vinit of the battery 
5, the time interval .tau. has a value .tau.4. 
The microcomputer 20 measures the time interval .tau., and determines the 
remaining electric energy or voltage of the battery 5 based on the 
measured time interval .tau.. Then, the microcomputer 20 drives or 
energizes the display elements 31.about.35 of the display mark 30, as 
follows: 
(1) When .tau..ltoreq..tau.1, the microcomputer 20 energizes the display 
elements 31.about.35. 
(2) When .tau.1&lt;.tau..ltoreq..tau.2, the microcomputer 20 energizes the 
display elements 31, 33.about.35. 
(3) When .tau.2&lt;.tau..ltoreq..tau.3, the microcomputer 20 energizes the 
display elements 31, 34, 35. 
(4) When .tau.3&lt;.tau..ltoreq..tau.4, the microcomputer 20 energizes the 
display elements 31, 35. 
(5) When .tau.4.ltoreq..tau., the microcomputer 20 energizes the display 
element 31 so that it blinks. 
Consequently, when the voltage V5 of the battery 5 is equal to or higher 
than 90% of the initial voltage Vinit (i.e., in the case (1) above), all 
the display elements 31.about.35 of the display mark 30 are energized, as 
shown in FIG. 5A. 
When the voltage V5 of the battery 5 is in the range between 90% and 85% of 
the initial voltage Vinit (i.e., in the case (2) above), the display 
elements 31, 33.about.35 of the display mark 30 are energized, as shown in 
FIG. 5B. 
When the voltage V5 of the battery 5 is in the range between 85% and 80% of 
the initial voltage Vinit (i.e., in the case (3) above), the display 
elements 31, 34, 35 of the display mark 30 are energized, as shown in FIG. 
5C. 
When the voltage V5 of the battery 5 is in the range between 80% and 67% of 
the initial voltage Vinit (i.e., the case (4)), the display elements 31, 
35 of the display mark 30 are energized, as shown in FIG. 5D. 
When the voltage V5 of the battery 5 is lower than 67% of the initial 
voltage Vinit (i.e., the case (5)), the display element 31 of the display 
mark 30 is energized so that it blinks, as shown in FIG. 5E. At this time, 
the detected output V6 from the voltage detector 6 is "0". Thus, the 
microcomputer 20 turns off the transistor Q1, preventing the voltage V5 of 
the battery 5 from being applied to the radio receiver 10, and does not 
accept any input signals from the control keys 4. 
As described above, the voltage V5 of the battery 5 is supplied to the 
time-constant circuit 7, and the remaining electric energy of the battery 
5 is determined from the time interval .tau. that is required for the 
voltage V7 across the capacitor C7 to reach the reference voltage VRE, and 
is also displayed as a progressively varying stepwise mark on the LCD 3. 
Accordingly, the user is able to know the rate at which the remaining 
electric energy of the battery 5 decreases, and to have a new battery 
ready for replacement based on the displayed rate of decrease of the 
remaining electric energy of the battery 5. The battery voltage display 
device according to the present invention is thus highly advantageous when 
incorporated in a circuit that requires frequent battery replacement, such 
as a small-size portable radio receiver which may be carried everyday and 
used on commuter trains, for example, by the user. 
When the ambient temperature is low, the electric energy stored in the 
battery 5 is also reduced. The battery voltage display device according to 
the present invention is also capable of displaying such a 
temperature-dependent reduction in the voltage V5 of the battery 5. 
Another advantage of the battery voltage display device according to the 
present invention is that the cost thereof is not much higher than the 
conventional battery voltage display device shown in FIG. 1 because an 
additional circuit arrangement included in the circuit shown in FIG. 3 is 
relatively simple. 
In the above embodiment, the display mark 30 for indicating the remaining 
electric energy of the battery 5 is independently displayed on the LCD 3. 
However, the display unit of the LCD 3 for displaying received radio 
frequencies may also be used to display the voltage of the battery 5. For 
example, as shown in FIG. 7A, the LCD 3 has a display mark 81 composed of 
seven display segments arranged in the pattern of "8" and also similar 
display marks 82, 83, 84. The LCD 3 additionally has a display mark 85 for 
displaying the unit "kHz" of received radio frequencies, and a display 
mark 86 for displaying the indicia "BATT" meaning a battery. 
While the radio receiver 10 is receiving a broadcast, the frequency thereof 
is digitally displayed by the display marks 81.about.85. When the battery 
check key 4a is pressed, the lower four display segments of each of the 
display marks 81.about.84 and the display mark 86 are selectively 
energized or blinked depending on the remaining electric energy of the 
battery 5, thus displaying the remaining electric energy or voltage of the 
battery 5. 
The battery voltage display device has been described as being incorporated 
in a radio receiver. However, the principles of the present invention are 
also applicable to other electronic devices. FIG. 8 shows a display unit 
for use on a clock with a program timer. The display unit or LCD 3 has a 
plurality of display marks 91, 92, 93, 94 each composed of seven display 
segments arranged in the pattern of "8", the display marks 91, 92 
displaying hours and the display marks 93, 94 displaying minutes. The LCD 
3 additionally has a display mark 95 for displaying the indicia "CLOCK", 
and a display mark 96 for displaying the indicia "BATT" meaning a battery. 
The display marks 91.about.94 therefore display time. When a battery check 
key on the clock is pressed, the lower four display segments of each of 
the display marks 91.about.94 and the display mark 96 are selectively 
energized or blinked depending on the remaining electric energy of the 
battery of the clock, thus displaying the remaining electric energy or 
voltage of the battery. 
In the display units shown in FIGS. 7A through 7G and 8, no new display 
elements need to be added to the LCD 3 for the display of the remaining 
electric energy of the battery 5. Therefore, the display area of the LCD 3 
is made relatively small, and the number of signal lines interconnecting 
the microcomputer 20 and the LCD 3 is not increased. 
Having described preferred embodiments of the invention with reference to 
the accompanying drawings, it is to be understood that the invention is 
not limited to that precise embodiments and that various changes and 
modifications could be effected by one skilled in the art without 
departing from the spirit or scope of the invention as defined in the 
appended claims.