Abstract:
The number of electrical devices implemented in motor vehicles and supplied with electricity by the motor vehicle battery rises with each generation. The manufacturers of motor vehicles make great demands on electrical device to be built-in their motor vehicles, especially to the stand-by power consumption of the built-in devices in order to protect the motor vehicle battery from a fast and undesired unloading during the stop of the motor vehicle. The present invention relates to an electrical circuit provided to be implemented in motor vehicle built-in devices for limiting the power consumption during the stand-by thereof, i.e. to reduce the power consumption to almost no consumption. Further, the present invention relates to an electrical motor vehicle built-in device having the aforementioned electrical circuit for limiting the power consumption of the electrical motor vehicle built-in device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is for entry into the U.S. national phase under §371 for International Application No. PCT/IB02/002449 having an international filing date of Jun. 27, 2002, and from which priority is claimed under all applicable sections of Title 35 of the United States Code including, but not limited to, Sections 120, 363 and 365(c). 
   TECHNICAL FIELD 
   The present invention relates to an electrical circuit for controlling the power supply of an electrical device. More particularly, the present invention relates to the aforementioned circuit for offering a stand-by mode of the electrical device and operable with wake-up signals and sleep signals causing the switching of the power supply. 
   BACKGROUND OF THE INVENTION 
   Modern motor vehicles include a large number of electrical devices and electrical units and the number of such electrical devices and units still increases, since buyers of motor vehicle demand these electrical devices and units often in a built-in fashion to provide convenience to the driver of the motor vehicle. 
   The electrical devices and units built-in a motor vehicle all have to be energized by the power supply of the motor vehicle, typically a battery of limited capacity. The battery of a motor vehicle is charged during operation of the engine of the motor vehicle so that the power consumption of electrical devices and units of the motor vehicle represents no serious problem while the engine is running. But the power consumption of electrical devices and units consuming power from the battery when the engine of the motor vehicle is not running causes a decharging of the battery and hence causes a problem. Especially, electrical devices and units operable in a stand-by operation mode have to be designed in a power saving way. The stand-by operation mode allows for example to activate the device or unit being in the stand-by operation mode via a central control unit of the motor vehicle. 
   A motor vehicle built-in unit for mobile communication devices is used to couple a mobile communication device such as a mobile phone, a communication handheld and the like, to the further electrical devices and units included in the motor vehicle. The motor vehicle built-in unit allows to couple to an external speaker, e.g. of a car radio, an external microphone, e.g. implemented in the dashboard, to an antenna e.g. having a power amplifier, a remote control e.g. having keys for operating functions of the mobile phone, to the electric circuits of the motor vehicle e.g. via a data communication bus of the motor vehicle for remote operating and the like. Typically such a motor vehicle built-in unit for mobile communication devices is operable with stand-by operation mode to be energizable within different situations. 
   The manufacturer of motor vehicles impose strict regulations for the requirements to be fulfilled by electrical devices and units to be included in motor vehicles, especially power consumption of electrical devices and units being in a stand-by operation mode is of special interest. The power consumption has to be minimized or has to be below a maximum level or power consumption, in order to ensure that the battery is not drained too much, especially in case the motor vehicle is parked. Certain electrical circuits have been implemented into such devices and units to offer stand-by operation mode. These electrical circuits of the state of the art comprise a large number of active components conventionally requiring to be energized by a voltage regulator. 
   SUMMARY OF THE INVENTION 
   The present invention relates to the aspect of providing a power signal from an external power supply, here the battery of the motor vehicle, to an electric device, wherein the power supply is operable with power-up/power-down signals switching on or off the connection of the external power supply to the mobile device, respectively. 
   The inventive concept relates to an electrical circuit to be implemented into electrical devices energized by external power supplies and to be activated by an external power-on signal operable with a stand-by operation mode. An advantage of the electrical circuit according to an embodiment of the invention is that the electrical circuit does not consume substantial amounts of power. Further, the electrical circuit implements further a voltage level check which prevents damage and faulty operation of the electric device coupled to the electrical circuit by checking if the supplied voltage of the external power supply is within a certain pre-defined voltage level range. 
   The electrical circuit is designed to be composed of passive components requiring no dedicated power supply (voltage regulator) during the stand-by operation. The electrical circuit comprising passive components is clearly economical and even more reliable during its life-time. 
   The objects of the invention are attained by an electrical circuit and an apparatus which are characterized by what is claimed in the accompanying independent claims. Further embodiments of the invention are the subject of the corresponding dependent claims. 
   According to an embodiment of the invention, an electrical circuit for providing an electrical operable connection of an external power supply and an electrical motor vehicle built-in device is provided. The electrical circuit has a main input for coupling to the external power supply, a main output for coupling to the electrical device and a first electrical operable switch interconnected between the main input and the main output. The first electrical operable switch is operable in two switching positions, an open position and a closed position. The first electrical operable switch is conductive in case it is closed and is non-conductive in case it is opened. Further the electrical circuit is provided with at least one wake-up input to receive a wake-up signal and at least one sleep input to receive a sleep signal and the electrical circuit has a bi-stable sub-circuit coupled to the main input and coupled to the first electrical operable switch as well as connected to the at least one wake-up input and to the at least one sleep input The bi-stable circuit is operable such that receiving a wake-up signal results in energizing the bi-stable sub-circuit and receiving a sleep signal results in de-energizing thereof. The energized bi-stable sub-circuit effects to close the first electrical operable switch and the corresponding de-energized bi-stable sub-circuit effects to open the first electrical operable switch. 
   According to an embodiment of the invention, the bi-stable sub-circuit is embodied by a first transistor and a second transistor. The first transistor has an emitter terminal, a collector terminal and a base terminal and is configured with a bridge resistor interconnected between its emitter terminal and its base terminal and a base resistor connected to its base terminal. The second transistor has an emitter terminal, a collector terminal and a base terminal and is configured with a bridge resistor interconnected between its emitter terminal and its base terminal and a base resistor connected to its base terminal. An interposed resistor having a first and a second terminals is interconnected in-between the first and the second transistor. 
   The first transistor is coupled via its emitter terminal to the main input, via its collector terminal to a first terminal of the interposed resistor and via its base resistor to the collector terminal of the second transistor. The second transistor is coupled via its emitter to ground, via its base resistor to a second terminal of the interposed resistor and via its collector to the base resistor of the first transistor. 
   The at least one wake-up input is coupled to a first connection line between the base resistor of the first transistor and the collector terminal of the second transistor, both being coupled. The at least one sleep input is coupled to a second connection line between the base resistor of the second transistor and the second terminal of the interposed resistor, both being coupled. The first electrical operable switch being operable with the bi-stable sub-circuit is coupled to the second connection line. 
   According to an embodiment of the invention, the electrical circuit further implements a third transistor. The third transistor has analogously an emitter terminal, a collector terminal and a base terminal and is analogously configured with a bridge resistor interconnected between its emitter terminal and its base terminal and a base resistor connected to its base terminal. The third transistor is interconnected in-between a connection line coupling the bi-stable sub-circuit and the first electrical operable switch such that the third transistor is coupled via its base resistor to the second connection line via its collector terminal to the first electrical operable switch and via its emitter terminal to ground. 
   According to an embodiment of the invention, the electrical circuit further comprises at least one wake-up transistor. Each wake-up transistor has analogously an emitter terminal, a collector terminal and a base terminal and is analogously configured with a bridge resistor interconnected between its emitter terminal and its base terminal and a base resistor connected to its base terminal. The at least one wake-up transistor is interconnected in-between the at least one wake-up input and the bi-stable sub-circuit such that the at least one wake-up transistor is coupled via its base resistor to the at least one wake-up input, via its emitter terminal to ground and via its collector terminal to the first connection line. 
   According to an embodiment of the invention, the electrical circuit further comprises at least one sleep transistor. Each sleep transistor has analogously an emitter terminal, a collector terminal and a base terminal and is analogously configured with a bridge resistor interconnected between its emitter terminal and its base terminal and a base resistor connected to its base terminal. The at least one sleep transistor is interconnected in-between the at least one sleep input and the bi-stable cub-circuit such that the at least one sleep transistor is coupled via its base resistor to the at least one sleep input, via its emitter terminal to ground and via its collector terminal to the second connection line. 
   According to an embodiment of the invention, the electrical circuit implements additionally an upper voltage level check circuit. The upper voltage level check circuit comprises a second resistor, a z-diode and a fourth transistor. The fourth transistor has analogously an emitter terminal, a collector terminal and a base terminal and is analogously configured with a bridge resistor interconnected between its emitter terminal and its base terminal and a base resistor connected to its base terminal. The second resistor is coupled to a connection line between the main input and the first electrical operable switch and to a first terminal of the z-diode. The fourth transistor is coupled via its base resistor to a second terminal of the z-diode, via its emitter terminal to ground and via its collector terminal to the second connection line. And the z-diode is adapted to be conductive in case a voltage applied to the main input exceeds a pre-defined upper voltage level. 
   According to an embodiment of the invention, the electrical circuit further comprises a second electrical operable switch interconnected in-between the first electrical operable switch and the main output and implements additionally an lower voltage level check circuit. The lower voltage level check circuit further comprises a third resistor, a z-diode and a fifth transistor. The fifth transistor has analogously an emitter terminal, a collector terminal and a base terminal and is analogously configured with a bridge resistor interconnected between the emitter terminal and the base terminal and a base resistor connected to its base terminal. The second resistor is coupled to a connection line between the first electrical operable switch and the main output and to a first terminal of the z-diode. The fifth transistor is coupled via its base resistor to a second terminal of the z-diode, via its emitter terminal to ground and via its collector terminal to the second electrical operable switch to be operated. And the z-diode is adapted to be conductive in case a voltage applied to the main input and conducted via the first electrical operable switch being conductive exceeds a pre-defined lower voltage level. 
   According to an embodiment of the invention, the first electrical operable switch is a metal-oxide field-effect transistor (MOSFET). Alternatively, the first electrical operable switch is a conventional relay. 
   According to an embodiment of the invention, the second electrical operable switch is a metal-oxide field-effect transistor (MOSFET). Alternatively, the second electrical operable switch is a conventional relay. 
   According to an embodiment of the invention, the electrical circuit further comprises a third z-diode interconnected between the second terminal of the interposed resistor and the base terminal of the second transistor within the second connection line. The z-diode is adapted to be conductive in case a voltage applied to the main input and conducted via the first transistor being conductive exceeds a pre-defined voltage level. 
   According to an embodiment of the invention, the motor vehicle built-in device is a free-hand installation main device for detachably connecting a mobile communication device and the external power supply is a battery of a motor vehicle. 
   According to an embodiment of the invention, a motor vehicle built-in device is provided which is operably connected to an external power supply. The motor vehicle built-in device comprises a plurality of electrical components energized by the external power supply and an electrical circuit for providing an electrical operable connection of an external power supply and the motor vehicle built-in device. The electrical circuit further implements a main input and a main output. The main input is coupled to the external power supply, for example a battery of the motor vehicle including the built-in device. The main output is for example coupled internally to the components of the built-in device to be energized. The first electrical operable switch is operable in two switching positions, an open position and a closed position. The first electrical operable switch is conductive in case it is closed and is non-conductive in case it is open. Further the electrical circuit is provided with at least one wake-up input to receive a wake-up signal and at least one sleep input to receive a sleep signal and the electrical circuit has a bi-stable sub-circuit coupled to the main input and coupled to the first electrical operable switch as well as connected to the at least one wake-up input and to the least one sleep input The bi-stable circuit is operable such that receiving of a wake-up signal results in energizing of the bi-stable sub-circuit and receiving of a sleep signal results in de-energizing thereof. The energized bi-stable sub-circuit effects to close the first electrical operable switch and the corresponding de-energized bi-stable sub-circuit effects to open the first electrical operable switch. 
   According to an embodiment of the invention, electrical circuit embedded in the motor vehicle built-in device is an electrical circuit according to anyone of the aforementioned embodiments of the electrical circuit with respect to the present invention. 
   According to an embodiment of the invention, the motor vehicle built-in device is a free hand installation main device for detachably connecting a mobile communication device. Conventionally, such a free-hand installation is mounted permanently in a motor vehicle and provided connectivity to further installations, devices and units included in the motor vehicle. The external power supply is an accumulator of a motor vehicle. 
   According to an embodiment of the invention, the motor vehicle built-in device embodying a free hand installation main device at least comprises additionally an interface for exchanging signals between electrical units included in the motor vehicle and the motor vehicle built-in device, an interface for exchanging signal between the apparatus and the mobile communication device connected detachably and a control unit to pass signals in-between the interfaces. The exchanged signals comprises at least one wake-up signal and at least one sleep signal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in greater detail by the means of preferred embodiments with reference to the accompanying drawings, in which 
       FIG. 1  shows a block diagram illustrating a typical state of the art circuit providing connectivity of an electrical device to an external power supply; 
       FIG. 2  shows a block diagram illustrating a circuit providing connectivity of an electrical device to an external power supply according to an embodiment of the invention; and 
       FIG. 3  shows a block diagram illustrating a junction box of a hand-free installation according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   The following description relates to mobile communication devices according to embodiments of the invention. Same or equal parts shown in the figures will be referred to by the same reference numerals. 
     FIG. 1  illustrates a typical state of the art circuit providing connectivity of an electrical device to an external power supply. The depicted circuit is dedicated to provide connectivity of an electrical device to an external power supply, such as the battery of the motor vehicle, wherein the external power supply can be switched on and switched off using a switching on/off input I 0  of the circuit controlling the main switch  31 . A power supply input  30  is coupled to the external power supply, here a battery  35  and further the input  30  is coupled to the output  32  via the interconnected main switch  31 . The output  32  provides the power to the electrical device. 
   A voltage regulator  10  connected to the input  30  is employed to generate a first voltage signal V wake  and a second voltage signal V ref . The first voltage signal V wake  is supplied to further components of the circuit to energize them. The second voltage signal V ref  is used as a reference voltage signal V ref  to be compared with further generated voltage signals as described below. 
   Resistors  20 ,  21  and  22  connected in series and coupled to the input  30  to be energized by the external battery  35  are used as a staged voltage divider. A first test voltage is picked up in-between the resistor  20  and the resistor  21 , whereas a second test voltage is picked up in-between the resistor  21  and the resistor  22 . The first picked up test voltage is supplied to a first operational amplifier  11  operated as comparator to be compared with the reference voltage signal V ref , whereas analogously, the second picked up test voltage is supplied to a second operational amplifier  12  operated as comparator to be compared with the reference voltage signal V ref . Both the first operational amplifier  11  and the second operational amplifier  12  are energized by the first voltage signal V wake  generated by the voltage regulator  10 . 
   The first operational amplifier  11  operated as comparator provides a resulting signal indicating that the operating voltage supplied to the circuit via input  30  is below a certain upper voltage level, whereas the second operational amplifier  12  operated as comparator provides a resulting signal indicating that the operating voltage supplied to the circuit via input  30  is above a certain lower voltage level. 
   The testing of the voltage level of the external power supply coupled to input  30  ensures that this voltage level of the external power supply is within a certain pre-defined voltage level range, defined by the characteristics of the resistors  20 ,  21  and  22  as well as the voltage signal V ref  generated by the voltage regulator  10 . The pre-determined voltage level range is choosen to protect the mobile device, coupled to this circuit, to be energized with a voltage level being too low and with a voltage level being too high which overall results in a protection against damages and miss-operations. 
   The resulting signals of the operational amplifier  11  and  12  are supplied to inputs of a logical OR component  13 . The output of the logical OR component is connected to a reset input R of a RS-flip-flop  14 . That is, the RS-flip-flop  14  can only be operated via the set input S in case the voltage level of the external power supply is within the pre-determined voltage level range described above. The set input S of the RS-flip-flop serves as the switching on/off input I 0  of the described circuit. In case the RS-flip-flop is in an initial mode in which the output Q of the RS-flip-flop is switched off, a pulse signal supplied to the input S switches the output Q on and a following pulse signal switches the output Q back off, i.e. to the initial mode. The output signal of the output Q of the RS-flip-flop  14  serves to operate the main switch  31  via a transistor  15 . 
   The main disadvantage of the described state of the art circuit is that a couple of active components, i.e. components requiring power, consume power of the battery  35  which is the main power supply of the circuit and the device coupled to output  32 . The active components, here voltage regulator  10 , operational amplifiers  11  and  12 , logical OR component  13  and RS-flip-flop  14 , consume power even in case the device coupled to output  32  is switched off via main switch  31 . Especially in the case of batteries used as main power supplies coupled to input  30 , the reduction of power consumption may be essential since the capacity of such power supplies is limited. Further disadvantages are the complexity of the active components and the costs of them. The complexity may result in a greater number of defects during the expected life time of the circuit which is especially in view of the high costs of the components not efficient. 
   The following circuit realizes the above presented features of the state of the art circuit while overcoming the disadvantages thereof. 
     FIG. 2  illustrates a circuit providing connectivity of an electrical device to an external power supply according to an embodiment of the invention. The depicted circuit can be embedded in a free-hand installation providing connectivity of an electric device, e.g. mobile device, especially a mobile phone, a mobile communication handheld and further electric mobile devices, to the electronics of a motor vehicle, especially a car. Such a free-hand installation has a mechanical holder or a support for taking a mobile phone and one or several electrical contacts to couple the mobile phone to several electrical units or components of the motor vehicle, respectively, such as a power supply, an ignition, an external antenna, a light, a data bus embedded in the motor vehicle for controlling electrical installations, an on-board multifunctional processing device, a microphone in the interior of the motor vehicle, a car radio and/or the speaker of the car radio. The free-hand installation is energized by the battery of the motor vehicle and is operable with a stand-by operation mode. During the stand-by operation mode the free-hand installation electronic of the motor vehicle is allowed to put the free-hand installation into operation by transmitting a dedicated wake-up signal causing to activate the functionality of the free-hand installation. 
   The depicted circuit according to an embodiment of the invention has a main input P 1  and a main output P 2 . The main input P 1  is coupled to the power supply PS, whereas the output P 2  couples the depicted circuit to the electrical device to be energized by the power supply PS. The main switch Sw 1  is interconnected in-between the main input P 1  and the main output P 2  to switch either on or off the power supplied to the main output P 2  or the coupled device, respectively. At least one input I 1  offers the possibility to feed in a pulse signal causing to close the main switch Sw 1 . Here, three inputs I 1 , I 2  and I 3  can receive pulse signals, each causing to switch on the main switch Sw 1 . Further at least one input Pd offers the possibility to feed in a pulse signal causing to open the main switch Sw 1 . An additional embedded circuit Cr 1  serves to detect overvoltage and an additional embedded circuit Cr 2  serves to detect lowervoltage. 
   In the following, the components of the circuit and their configuration are described in detail. The circuit is based on passive components, i.e. a plurality of configured transistors, resistors, Z-diodes and electrical operable main switches. 
   Each of the transistors comprised in the circuit is configured with resistors, i.e. a base terminal of a configured transistor has interconnected a resistor and the emitter terminal of the configured transistor is interconnected to the base via a further resistor. The configuration of the transistors is designed such that the transistors operate as electrical switches. In the following, when the transistors become conductive the conductive state will be termed as switched on, whereas when the transistor is not conductive the non-conductive state will be termed as switched off. 
   The circuit has two operation states, a power-down state and a power-up state. In the power-down state, which is also the initial circuit state, the main switch Sw 1  is opened. In the power-up state the main switch Sw 1  is closed, i.e. the main switch is conductive. The detailed operation of the circuit is described in the following in view of a wake-up functionality transferring the power-down state into the power-up state and a sleep functionality transferring the power-up state into the power-down state 
   Wake-up Functionality 
   A wake-up signal is supplied to the wake-up input I 1  connected to the base terminal of the configured transistor T 3  causing to switch on the configured transistor T 3 . The wake-up signal can be a pulse signal having a pulse level adapted to switch on the configured transistor T 3 . The emitter terminal of the configured transistor T 3  is coupled to ground of the circuit whereas the collector terminal of the configured transistor T 3  is coupled to the base terminal of the configured transistor T 1 , which in turn is coupled via its emitter terminal to the main input P 1  and via its collector terminal through a resistor R 1  to the base terminal of configured transistor T 2 . A configured transistor T 2  switched on results in a switching on of the configured transistor T 1 . The configured transistor T 2  is further coupled via its emitter terminal to ground and via its collector terminal to the base terminal of the configured transistor T 1  such that the switched on configured transistor T 1  results in a switched on transistor T 2  which in turn holds the switched on status of the configured transistor T 1  even in case no wake-up signal is supplied any more to the input I 1 . 
   The configured transistors T 1  and T 2  are interconnected such that a single wake-up signal supplied via the configured transistor T 3  to the interconnection of the base terminal of configured transistor T 1  and collector terminal of configured transistor T 2  results in transition of a stable switched off status into a stable switched on status of the both configured transistors T 1  and T 2 . The sub-circuit comprising the configured transistors T 1  and T 2  represents a bi-stable circuit Crbs. A signal picked up from this bi-stable circuit Crbs is used in the following to operate the main switch Sw 1 . In order to operate the main switch Sw 1 , the base terminal of the configured transistor T 2  is coupled to the base terminal of a further configured transistor T 5 , of which the emitter terminal is connected to ground and the collector terminal operates the main switch Sw 1 , i.e. the main switch Sw 1  is closed to be conductive. The main switch Sw 1  is for example a MOS-FET (metal-oxide semiconductor field-effect transistor), a relay or any other electrical operable switch, in particular providing the possibility to pass though high currents. 
   Comprehensively, a wake-up signal supplied to wake-up input I 1  switches on configured transistor T 3 , which in turn switches on the configured transistors T 1  and T 2  as well as additionally the configured transistor T 5 . The properties of the bi-stable sub-circuit Crbs comprising the configured transistors T 1  and T 2 , respectively, ensures that the configured transistor T 1  remains switched on and hence also the configured transistors T 2  and T 5  even in case the wake-up signal is no longer supplied to configured transistor T 3 . 
   The number of wake-up inputs is not limited to the aforementioned wake-up input I 1 . An unlimited number of wake-up inputs can be included in the presented circuit according to an embodiment of the invention. The  FIG. 2  illustrates two further wake-up inputs I 2  and I 3 . Each of the wake-up inputs have a configured transistor, here configured transistor T 3   a  and T 3   b , respectively, equivalent to the configured transistor T 3  and configured analogously. The configured transistor T 3   a  and T 3   b  are connected in parallel to the configured transistor T 3 , i.e. the wake-up inputs I 2  and I 3  are coupled to the respective base contacts of the configured transistor T 3   a  and T 3   b  and the collector contacts thereof are coupled to the base terminal of configured transistor T 1 . 
   Sleep Functionality 
   A sleep or power down signal is supplied to the sleep input Pd, respectively, connected to the base terminal of the configured transistor T 4  causing to switch on the configured transistor T 4 . The sleep signal can be a pulse signal having a pulse level adapted to switch on the configured transistor T 4 . The emitter terminal of the configured transistor T 4  is coupled to ground whereas the collector terminal of the configured transistor T 4  is coupled to the base terminal of the configured transistor T 2  and hence also to the base terminal of the configured transistor T 5 . The configured transistor T 4  switched on causes that the both base contacts of the configured transistors T 2  and T 5  are tied to ground resulting in switching off of the configured transistors T 2  and T 5 . This results further in opening of the main switch Sw 1  in reaction to the configured transistor T 5  switched off, i.e. transition of the power-up state of the circuit into the power-down state, and in switching off of configured transistor T 1  in reaction to the configured transistor T 2  switched off. 
   The number of sleep inputs is not limited to the aforementioned sleep input Pd. An unlimited number of sleep inputs can be included in the presented circuit according to an embodiment of the invention. The  FIG. 2  illustrates only the described sleep input Pd. Further sleep inputs can be realized in that further configured transistors are connected in parallel to the configured transistor T 4 . These further configured transistors are equivalent to the configured transistor T 4  and configured analogously. Each further configured transistor providing further sleep inputs is coupled to the base terminal of the configured transistor T 2 . 
   Voltage Level Check 
   The circuit illustrated in  FIG. 2  further includes sub-circuits to ensure that the voltage level of the external power supply PS is within a pre-defined voltage level range (V ZD2  to V ZD1 ). In case the voltage level of the external power supply PS exceeds the pre-defined upper voltage level V ZD1  of the voltage level range a sub-circuit Cr 1  protects an electrical device coupled to the output P 2  by opening the main switch Sw 1 . In case the voltage level of the external power supply PS falls below the pre-defined lower voltage level V ZD2  a sub-circuit Cr 2  protects an electrical device coupled to the output P 2  by opening a main switch Sw 2  connected in series to the main switch Sw 1  and interconnected in-between main switch Sw 1  and output P 2 . 
   Upper Voltage Level Check 
   The sub-circuit Cr 1  comprises a configured transistor T 6 , a z-diode D 1  and a resistor R 2  both connected in series to the base terminal of the configured transistor T 6 , wherein the z-diode D 1  is interposed between base terminal and resistor R 2 . In turn, the resistor R 2  is further coupled to the input P 1 , whereas the collector terminal of the configured transistor T 6  is coupled to the base terminal of configured transistor T 2  and the base terminal of configured transistor T 5 , respectively. The z-diode D 1  is interposed such that in case of the voltage level applied to the input P 1  by the external power supply PS exceeds a pre-defined voltage level, this is the upper voltage level V ZD1 , the z-diode D 1  becomes conductive and the configured transistor T 6  is switched on which causes to ground the base terminal of the configured transistor T 2  and the base terminal of the configured transistor T 5 , respectively, (in analogy to a sleep signal supplied to the sleep input Pd). The switching off of configured transistor T 2  causes also to switch off the configured transistor T 1 . The upper voltage level V ZD1  is determined by the characteristics of the z-diode D 1 . 
   Once the pre-defined upper voltage level V ZD2 , is detected to be exceeded the circuit is set into the power-down state. The circuit being again in the power-up state can be accomplished by supplying a wake-up signal to one of the implemented wake-up inputs (here, inputs I 1 , I 2  and I 3 ). In case of overvoltage detection the wake-up functionality is disabled. 
   Lower Voltage Level Check 
   The sub-circuit Cr 2  comprises a configured transistor T 7 , a z-diode D 2  and a resistor R 3  both connected in series to the base terminal of the configured transistor T 7 , wherein the z-diode D 2  is interposed between base terminal and resistor R 3 . In turn, the resistor R 3  is further coupled to an interconnection of the main switch Sw 1  and the main switch Sw 2 , whereas the signal of the collector terminal of the configured transistor T 7  is supplied to the main switch Sw 2  for operating this. The z-diode D 2  is interposed such that in case of the voltage level applied to the input P 1  by the external power supply PS and conducted through a closed main switch Sw 2  exceeds a pre-defined voltage level, this is the lower voltage level V ZD2 , the z-diode D 2  becomes conductive and the configured transistor T 7  is switched on which causes to close the main switch Sw 2  such that power is conducted from the input P 1  through the main switch Sw 1  and main switch Sw 2  to the output P 2 . In case the voltage level applied to the z-diode D 2  falls below the lower voltage level V ZD2  the configured transistor T 7  is switched off and hence the main switch Sw 2  is opened causing the interruption of the connection between input P 1  and input P 2 . The lower voltage level V ZD2  is determined by the characteristics of the z-diode D 2 . The sub-circuit Cr 2  operates independently from the circuit described above. 
   Memory Functionality 
   The voltage level range for example is 6.5 V to 16 V, i.e. the lower voltage level V ZD2 =6.5 V and the upper voltage level V ZD2 =16 V. Further the components of the presented circuit for example are operable at a minimum voltage level of about 2 V. This means, that the main switch Sw 1  is already operable with applied wake-up and sleep signals to the respective inputs in case the external power supply PS supplies a voltage level above this minimum voltage level and the main switch Sw 1  can be closed. Since the lower voltage check sub-circuit Cr 2  closes the main switch Sw 2  only in case the voltage level of the external power supply  35  supplied to the circuit input P 1  exceeds the lower voltage level V ZD2  no current is fed to output P 2 . Since further the sub-circuit Cr 2  is operated independently the main switch Sw 2  is closed in the moment a sufficient voltage level is supplied to input P 1  by the external power supply  35 . A power-up state is preserved even if the voltage level of the external power supply  35  is below the pre-defined voltage level range. 
   According to another embodiment of the invention, this memory effect can be prevented by interconnecting an additional z-diode in-between resistor R 1  and the configured transistor T 2 , i.e. in-between resistor R 1  and common connecting point of the configured transistors T 2 , T 4 , T 5  and T 6 , respectively. Therefore, the additional z-diode is adapted to the lower voltage level V ZD2 , i.e. its characteristics are adapted to the lower voltage level V ZD2 , preventing the switching of the configured transistor T 2  at a voltage level of the external power supply PS below the lower voltage level V ZD2 . 
   The comparison of the state of the art circuit presented in  FIG. 1  and the circuit according to an embodiment of the invention presented in  FIG. 2  shows clearly the aforementioned advantages of the circuit in accordance with the invention concept. In case of the power-down state of the inventive circuit shown in  FIG. 2  no power is consumed by the circuit, especially when parasitic effects of the components have not to be taken into account. Even when parasitic effects are considered a current consumption in the range of a few micro amperes is achievable. The parasitic effects can be kept under control by selecting the used and installed components. All components are passive components which have to be energized only during the power-up state of the circuit. Additionally, the functionality of the circuit is realized by a small number of single components which reduces dramatically the costs and improves the reliability of the circuit especially in view of a long life-term. 
   The number of wake-up inputs as well as the number of sleep inputs can be fitted easily to the respective requirements by connecting configured transistors in parallel to the corresponding configured transistors implemented therefor. 
     FIG. 3  illustrates a junction box of a hand-free installation according to an embodiment of the invention. The junction box at least implements a circuit according to an embodiment of the invention, for example the circuit depicted in  FIG. 2 . The junction box  200  provides the connectivity of a holder of a mobile device such as a mobile phone  100  depicted to an external power supply  211  and further electronic devices. Especially, wake-up and sleep signals  212  are supplied to the junction box  200 . The junction box includes at least an aforementioned electric circuit  210  according to an embodiment of the invention. The power supply connection is coupled to the circuit  210  and the wake-up and sleep signals are applied to the respective inputs. The wake-up signals and/or the sleep signals can be provided via a data bus system to which one or several interfaces are coupled generating such signal in accordance with data information transmitted via the data bus. The wake-up signal can also be obtained from the holder, eg a wake-up signal is supplied to the junction box upon insertion of the mobile device into the holder. 
   During the stand-by operation mode of the junction box  200  the implemented electrical circuit  210  according to an embodiment of the invention causes to be operable with receiving of a wake-up signal via one of the implemented wake-up inputs while further electrical units and circuits implemented in the junction box providing the functionality of the junction box are separated from the power supply such that these further electrical units and circuits are not able to consume power of the external power supply. The wake-up inputs of the electrical circuit  210  but also the sleep inputs thereof can be adapted to different kind of wake-up and sleep signals, respectively, e.g. different signal levels, signals having a different duration of time etc. 
   The junction box may implement further circuits or interfaces, respectively, necessary to connect the mobile device  100  to further electric, such as an ignition, an external antenna, a light, a data bus embedded in the motor vehicle for controlling electrical installations, an on-board multifunctional processing device, a microphone in the interior of the motor vehicle, a car radio and/or the speaker of the car radio. 
   A multi-wired cable  201  connects the junction box to a holder  110  of the mobile phone  100 . The connecting cable  201  may include at least a power line coupled to the output of the circuit  210  which is energized in accordance with the wake-up and sleep signals supplied to the circuit  210 . The holder  110  takes detachably the mobile phone  100 , wherein the holder  110  has at least a multiple plug connector which connects holder  110  with the mobile phone  100   
   The present invention has been described in view of a hand-free installation for mobile communication device. It is to be understood that the problem which is overcome by the inventive concept also relates to a broad number of different electrical devices and the presented solutions in the way of embodiments of the electrical circuit can be implemented in these different electrical devices. 
   While the invention has been particularly shown and described with respect to embodiments as examples, it will be understood by those skilled in this area of technology that changes in form and details my be made therein, including in the form of other embodiments, without departing from the scope and spirit of the invention, which is defined by the appended claims.