Battery rundown protection system for an automobile having a radio telephone apparatus with an automatic answering function

An automobile telephone device uses as its power source a battery for the automobile. This applies to an automobile telephone device with an automatic answering or receiving function. In the automobile telephone device, a voltage monitoring unit monitors the voltage of the battery. When the voltage falls near a limit voltage required for starting the automobile engine, the monitoring unit electrically interrupts a power source line connecting the battery and the telephone device via a proper switch circuit to thereby inhibit excessive consumption of the automobile battery by the automatic answering or receiving function during engine stoppage.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to automobile-mounted telephone devices which 
realize telephone communication with other telephone devices via a radio 
circuit and, more particularly, to apparatus with an automatic answering 
or receiving function which inhibit excessive use of an automobile battery 
by the answering or receiving function. 
2. Description of the Prior Art 
FIG. 1 shows one example of automobile telephone devices having such 
automatic answering or receiving function. As shown in FIG. 1, the 
automobile telephone device mainly includes a transceiver unit 1 and 
control unit 2. Transceiver unit 1 is usually built in the rear trunk of 
the automobile, and includes power supply 11, automatic receiving or 
answering unit 12, first control module 13, radio unit 14, and audio unit 
15. Control unit 2 is usually installed near the seat of the automobile, 
and includes main switch 21, display unit 22, second control module 23, 
key pad unit 24, telephone receiver 25, a telephone transmitter 26 and 
speaker 27. 
The operation of the telephone device shown in FIG. 1 will now be described 
briefly. First, an incoming call signal from a different telephone device 
is received by antenna 4 and applied to first control module 13 via radio 
unit 14. First control module 13 is responsive to that signal to deliver a 
signal to second control module 23 of control unit 2. Thus second control 
module 23 drives speaker 27 to generate a ringing tone. If the handset 
(not shown), which includes telephone receiver 25 and telephone 
transmitter 26, is taken off the hook in response to that ringing tone, 
this operation is detected by second control module 23 which then delivers 
a signal indicative of this detection to first control module 13. First 
control module 13 enables audio unit 15 to operate. Thus telephone 
communication is possible via antenna 4, radio unit 14, audio unit 15, 
telephone receiver 25, and telephone transmitter 26. 
As well known, automatic answering or receiving unit 12 is adapted to 
automatically receive and respond to an incoming call from a different 
telephone device by allowing the caller to hear messages recorded 
beforehand on an endless tape or in an IC memory, and then by recording 
the contents of the caller's business even when the driver of the 
automobile on which the telephone device 12 is mounted leaves the 
automobile. The driver can confirm the contents of the caller's message 
thus recorded when the driver returns to his automobile. The fact that 
there was an incoming call from a different telephone device can be 
indicated by display unit 22. 
In order to operate automatic answering or receiving apparatus 12, as 
mentioned above, first, main switch 21 of control unit 2 is turned on to 
operate power supply 11 of the automobile telephone device. Key pad unit 
24 of control unit 2 is then used to perform a predetermined operation to 
thereby drive the telephone device. 
The automobile telephone device having such automatic receiving or 
answering device 12, if device 12 is set in an active state, as mentioned 
above, consumed currents flow from automobile battery 3 to respective 
elements of the telephone device via power supply 11 each time there is an 
incoming call. This occurs even when the driver has stopped the engine and 
has left his automobile. Therefore, in the worst case, battery 3 voltage 
may drop so greatly that the driver cannot restart the engine. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an automobile 
telephone device which prevents excessive drop of the battery voltage 
which would otherwise impair the function of the automobile even if the 
automatic receiving or answering function is fulfilled. 
In order to achieve this object, according to the present invention, an 
automobile telephone device with an automatic receiving or answering 
function includes voltage monitoring means for detecting a drop in the 
automobile battery voltage below a predetermined voltage, and power source 
switch means for electrically interrupting a power source line connecting 
the automobile battery and the telephone device in accordance with the 
detection output from the voltage monitoring means. 
If the predetermined voltage related to the battery voltage monitored by 
the voltage monitoring means is set to a voltage slightly above a limit 
voltage required for starting the engine, an emergent situation can be 
safely avoided in which the battery voltage drops so greatly that the 
engine cannot be started because the battery power is consumed by the 
automatic answering or receiving function while the engine is stopped.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 2 is a block diagram showing an embodiment of an automobile telephone 
device according to the present invention. In FIG. 2, the same reference 
numeral as that in FIG. 1 is used to denote a unit or element having the 
same function as the unit or element in FIG. 1, for a and a further 
detailed description thereof will be omitted. 
This embodiment includes battery voltage monitoring unit 16 which detects a 
drop in the voltage of battery 3 below a predetermined voltage, and power 
source switch 17 which electrically interrupts power source line 18 in 
accordance with the detection output from monitoring unit 16, in addition 
to similar elements to those in the telephone device of FIG. 1. 
In operation, if main switch 21 is turned on, as mentioned above, power 
(Vcc1, Vcc2) is supplied from power supply 11 to transceiver unit 1 and 
control unit 2 via a feed line (not shown) and the telephone device is 
activated. 
If key pad unit 24 is used to perform a predetermined operation to thereby 
activate automatic answering or receiving unit 12, as mentioned above, 
second control module 23 detects this fact and delivers an activation 
command to first control module 13 of transceiver unit 1 to activate 
automatic answering or receiving unit 12. 
When automatic answering or receiving 12 is active voltage, monitoring unit 
16 monitors the battery voltage, and maintains power source switch 17 on 
when the battery voltage is above a predetermined voltage. The 
predetermined voltage is slightly above a limit voltage required for 
starting the engine. When the battery voltage does not reach the 
predetermined voltage, voltage monitoring unit 16 switches off power 
source switch 17. 
Therefore, if the battery voltage is above the predetermined voltage, and 
the output from voltage monitoring unit 16 maintains the power source 
switch 17 turned on, consumed currents flow from battery 3 via power 
source switch 17 and power supply 11 to the respective elements of the 
telephone device each time there is an incoming call. Automatic answering 
or receiving unit 12 is set usually when the driver leaves his automobile, 
and the engine is stopped. Therefore, battery 3 is not charged and its 
output voltage continues to lower. 
If the battery voltage falls below the predetermined voltage, voltage 
monitoring unit 16 detects this fact and delivers a control signal to 
power source switch 17 to turn power source switch 17 off. Thus power 
source line 18 connecting battery 3 and power supply 11 is electrically 
interrupted, so that the supply of power (Vcc1, Vcc2) from power supply 11 
to the respective elements of the telephone devices is stopped. 
Voltage monitoring unit 16 operates only when automatic receiving or 
answering unit 12 is set by a signal CS from first control module 13 and 
the telephone device is in a "standby" state. This is done to avoid 
interruption of the telephone communication due to turning off of power 
source switch 17 during the telephone communication. If the battery 
voltage falls below the predetermined voltage during telephone 
communication, voltage monitoring unit 16 performs the above operation to 
turn off power source switch 17 after telephone communication has been 
terminated, namely, after the conditions for the "standby" state are met. 
According to such embodiment, a drop in battery 3 voltage is minimized to 
thereby prevent an inability to start the automobile engine due to 
excessive fall of the battery voltage. 
FIG. 3 illustrates the specific structure of voltage monitoring unit 16 and 
power source switch 17 which are the essential elements of the embodiment. 
As shown in FIG. 3, voltage monitoring unit 16 may include, for example, 
comparator 161, voltage dividing circuit 162 which appropriately divides 
the battery voltage and applies the resulting voltage as a comparison or 
monitor voltage to a non-inverting input (+) terminal of comparator 161, 
zener diode 163 which forms a predetermined constant voltage on the basis 
of a voltage extracted from battery 3 and applies the constant voltage as 
a reference voltage to an inverting input (-) terminal of comparator 161 
for voltage comparing (monitoring) purposes, and transistor switch 164 
which is turned on in response to the signal CS from first control module 
13 to ground (correctly reduce) the reference voltage. Power source switch 
17 may include first transistor switch 171 which is turned on when the 
output from comparator 161 becomes high logically, namely, when the 
voltage from dividing circuit 162 falls below the reference voltage set by 
zener diode 163, and second transistor switch 172 which electrically 
interrupts power source line 18 connecting battery 3 and power supply 11 
in accordance with the on operation of first transistor switch 171. 
With voltage monitoring unit 16 of FIG. 3, the signal CS is applied as an 
active signal to voltage monitoring unit 16 to disable same (correctly, to 
reduce the reference voltage for voltage monitoring purposes to ensure the 
on state of second transistor switch 172 in power source switch 17) on 
condition that automatic receiving or answering device 12 is not set or 
that the telephone device is not in the "standby" state or is in telephone 
communication, for example. In other words, when automatic answering or 
receiving device 12 is set and the telephone device is in the "standby" 
state, signal CS is not applied as an active signal, so that the voltage 
monitoring operation of voltage monitoring unit 16 is normally maintained. 
As mentioned above, such structure of voltage monitoring unit 16 is 
effective to avoid inconvenience such as interruption of power supply 
during the telephone communication or compulsive interruption of the 
telephone communication. 
FIG. 4 illustrates a specific structure of another voltage monitoring unit 
16 in which instead of zener diode 163, stabilized IC power source circuit 
165, generally referred to as a 3-terminal regulator, is used as means for 
forming a reference voltage for voltage comparing or monitoring purposes. 
Other circuit elements are similar to those in FIG. 3. The circuit shown 
in FIG. 4 also performs a voltage monitoring operation and a power supply 
protection during telephone communication as does the circuit shown in 
FIG. 3. 
The circuits of FIGS. 3 and 4 are only illustrated as an example. Any other 
circuits which may realize the interruption of the power supply path and 
power supply protection during telephone communication or the like on the 
basis of the voltage monitoring operation may be employed, of course. 
Excessive consumption of the battery due to the use of the automatic 
answering or receiving function is effectively prevented so long as a 
circuit is used which has at least a function to satisfactorily interrupt 
the power supply path on the basis of the monitored battery voltage.