Vehicle indicator illumination circuit and method for controlling vehicle indicator illumination circuit

A power supply route for a light source includes a first route that closes when a microcomputer is operated and a second route that opens when the microcomputer is operated. When the microcomputer stops operating, the power supply route of the light source is automatically switched from the first route to the second route. When the microcomputer is operated and a position lamp switch is turned on, current is supplied to the light source through the first route. When the microcomputer stops operating and the position lamp switch is turned on, current is supplied to the light source through the second route.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2011-202128, filed on Sep. 15, 2011 and prior Japanese Patent Application No. 2011-249996, filed on Nov. 15, 2011, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an illumination circuit, which controls the illumination of an indicator provided in a vehicle interior, and a method for controlling an illumination circuit.

A vehicle includes an indicator, which indicates the vehicle state or an abnormality, and an illumination circuit, which controls the illumination of the indicator. Japanese Laid-Open Patent Publication No. 2004-325324 describes an example of such an illumination circuit. As shown inFIG. 7, an illumination circuit includes a micro-processing unit (MPU)71, a transistor72, and an indicator73. The MPU71activates or deactivates the transistor72to control the illumination of the indicator73. The indicator73includes a mark74, which indicates the content of a warning, and a light emitting diode75. When the transistor72is activated, the light emitting diode75is lit to illuminate the mark74. The MPU71provides the transistor72with a pulse width modulation (PWM) signal based on illumination information or darkening information of the indicator73, which is obtained by a sensor, to perform pulse width modulation (PWM) control on the light emitting diode75. When the MPU71illuminates or darkens the light emitting diode75, the MPU71changes the duty ratio of the pulse width modulation (PWM) signal to gradually change the brightness of the light emitting diode75. In this manner, the illumination control generates a fade-in effect, which gradually brightens the light emitting diode75, or a fade-out effect, which gradually darkens the light emitting diode75.

In the illumination circuit of Japanese Laid-Open Patent Publication No. 2004-325324, when an ignition switch deactivates the power supply of the vehicle, the MPU71stops operating. Thus, the indicator73cannot be illuminated. However, it has become desirable that the indicator73be illuminate even when the MPU71stops operating.

One aspect of the present invention is a vehicle indicator illumination circuit including an indicator that indicates a vehicle state or abnormality. A control circuit is operated by an ignition power supply. The control circuit controls power that is supplied to the indicator. A power supply route extends to the indicator. The power supply route includes a first route that closes when the control circuit operates and a second route that opens when the control circuit operates. When the control circuit stops operating, the first route opens and the second route closes, and the power supply route extending to the indicator is automatically switched from the first route to the second route.

DETAILED DESCRIPTION OF THE INVENTION

A vehicle indicator illumination circuit according to a first embodiment of the present invention will now be described with reference toFIGS. 1 to 3.

The vehicle indicator illumination circuit of the first embodiment includes an indicator, a control circuit, and a power supply route. The indicator indicates the vehicle state or an abnormality. The control circuit is powered by an ignition power supply and controls the supply of power to the indicator. The power supply route to the indicator includes a first route, which closes when the control circuit operates, and a second route, which opens when the control circuit operates. When the control circuit stops operating, the first route opens and the second route closes. This automatically switches the power supply route to the indicator from the first route to the second route.

As shown inFIG. 1, a vehicle includes a vehicle indicator illumination circuit1, which illuminates various instruments of the vehicle. The vehicle indicator illumination circuit1illuminates, for example, a manual control device, a telltale, an indicator, and the like in the vehicle so that they can be recognized. The vehicle indicator illumination circuit1includes a microcomputer2, which controls illumination by the vehicle indicator illumination circuit1, a light source3, which emits illumination light, and a rheostat4. The microcomputer2corresponds to a control circuit. An example of the light source3is a light emitting diode (LED). In this case, a first terminal of the light source is an anode, and a second terminal of the light source is a cathode.

When a vehicle power supply (ignition switch) is located at an on position and the microcomputer2is thus operating, the microcomputer2performs pulse width modulation (PWM) to control the illumination or darkening of the light source3. In the PWM control, the duty ratio of a PWM signal Sp1output from the microcomputer2is changed to set the brightness of the light source3. IN this manner, the brightness of the light source3is set based on the duty ratio of the PWM signal Sp1. The duty ratio of the PWM signal Sp1is the ratio of an H-level period and an L-level period in a single signal cycle. Examples of the on position of the ignition switch are an ignition on position (IG) and a starter on (ST) position. The PWM signal Sp1corresponds to a control signal.

When the ignition switch is located at an off position and the microcomputer2thus stops operating, the vehicle indicator illumination circuit1directly controls the rheostat4to illuminate or darken the light source3. Accordingly, even when the microcomputer2stops operating, the light source3can be lit by the direct control of the rheostat4(hereinafter simply referred to as direct control). In the direct control, the rheostat4generates a pulsed rheostat signal Srs, and illumination of the light source3is controlled based on the rheostat signal Srs. Examples of the off position of the ignition switch are an IG off position. The rheostat4corresponds to a signal generation circuit, and the rheostat signal Srs corresponds to an external pulse signal.

The vehicle includes an illumination switch6that supplies a first terminal of the light source3with positive illumination voltage ILL+ via a voltage regulation resistor5from an in-vehicle battery. The positive illumination voltage ILL+ is supplied from the in-vehicle battery to the light source3in accordance with an on/off operation of the illumination switch6. Specifically, when the illumination switch6is turned on, the positive illumination voltage ILL+ is supplied from the in-vehicle battery to the light source3. Conversely, when the illumination switch6is turned off, the positive illumination voltage ILL+ is not supplied from the in-vehicle battery to the light source3. An example of the illumination switch6is a position lamp switch operated when illuminating or darkening the headlights of the vehicle.

The vehicle indicator illumination circuit1includes a first transistor Tr1, a second transistor Tr2, and a connection switching circuit7that activates or deactivates the second transistor Tr2. The first transistor Tr1is activated during illumination control (hereinafter referred to as computer control) of the light source3by the microcomputer2. The second transistor Tr2is activated during direct control of the rheostat4. Also, the connection switching circuit7includes a third transistor Tr3, which switches between the computer control and the direct control, and a fourth transistor Tr4, which is activated or deactivated by the rheostat signal Srs during the direct control. The first transistor Tr1corresponds to a first switch unit or a first switching element, and the second transistor Tr2corresponds to a second switch unit or a second switching element. Also, the third transistor Tr3corresponds to a fourth switching element, and the fourth transistor Tr4corresponds to a third switching element.

The first transistor Tr1is formed by an NPN-type bipolar transistor, for example. The first transistor Tr1includes a collector terminal connected to the second terminal of the light source3, an emitter terminal connected to ground, and a base terminal connected to a PWM control terminal9of the microcomputer2via a resistor8. Between the base and the emitter of the first transistor Tr1, a resistor10is connected to determine the voltage between the base and the emitter. A current route that extends through the light source3and the first transistor Tr1is referred to as a first route R1.

The second transistor Tr2is formed by an NPN-type bipolar transistor, for example. The second transistor Tr2includes a collector terminal connected to a connection node P1between the light source3and the first transistor Tr1, an emitter terminal connected to ground, and a base terminal connected to a collector terminal of the fourth transistor Tr4via a series circuit13of two resistors11and12. Between the base and the emitter of the second transistor Tr2, a resistor14is connected to determine the voltage between the base and the emitter. A current route that extends through the light source3and the second transistor Tr2is referred to as a second route R2.

The third transistor Tr3is formed by an NPN-type bipolar transistor, for example. The third transistor Tr3includes a collector terminal connected to a connection node P2between the resistor11and the resistor12, an emitter terminal connected to ground, and a base terminal connected to an operation switching terminal16of the microcomputer2via a resistor15. The collector terminal of the third transistor Tr3is connected to the connection node P2between the resistor11and the resistor12. Between the base and the emitter of the third transistor Tr3, a resistor17is connected to determine the voltage between the base and the emitter.

The fourth transistor Tr4is formed by a PNP-type bipolar transistor, for example. The fourth transistor Tr4includes an emitter terminal, which is connected to the illumination voltage ILL+, and a base terminal, which is connected to a rheostat connection terminal19of the vehicle indicator illumination circuit1via a resistor18.

The rheostat4includes a rheostat transistor20that generates the rheostat signal Srs and a PWM signal generation circuit22. The rheostat transistor20is formed by an NPN-type bipolar transistor, for example. The rheostat transistor20includes a collector terminal connected to a rheostat signal output terminal21, an emitter terminal connected to ground, and a base terminal connected to the PWM signal generation circuit22. This configuration supplies the rheostat signal output terminal21with negative-voltage illumination voltage ILL−.

The PWM signal generation circuit22generates a pulse signal having a duty ratio that is in accordance with an operation amount of a rheostat switch (not shown) and provides the pulse signal to the base terminal of the rheostat transistor20to activate or deactivate the rheostat transistor20. In response, the rheostat4supplies a pulsed rheostat signal Srs having a pulse width in accordance with the duty ratio of the pulse signal to the base terminal of the fourth transistor Tr4via the rheostat signal output terminal21and the rheostat connection terminal19. Thus, the fourth transistor Tr4is activated or deactivated based on the PWM control of the rheostat4. The rheostat4is supplied with power from the in-vehicle battery regardless of the position of the ignition switch and constantly outputs the rheostat signal Srs.

The microcomputer2is supplied with ignition voltage IG+ via an I/F circuit23. When the ignition switch is located at the on position, the ignition voltage IG+ is supplied from the in-vehicle battery to the microcomputer2. As a result, the microcomputer2becomes operable. The ignition voltage IG+ corresponds to an ignition power supply.

The microcomputer2includes an ignition voltage monitoring unit24that monitors the ignition voltage IG+. The ignition voltage monitoring unit24sequentially monitors the value of the ignition voltage IG+ during the operation of the microcomputer2.

The microcomputer2includes a PWM control unit25that performs PWM control when operated to activate or deactivate the first transistor Tr1and illuminate the light source3. The PWM control unit25supplies the PWM signal Sp1from the PWM control terminal9to the base terminal of the first transistor Tr1to switch between activation and deactivation of the first transistor Tr1at high speeds. As a result, current intermittently flows in the first route R1and allows the light source3to be illuminated with the desired brightness. In this manner, the light source3is illuminated when the first transistor Tr1is activated and darkened when the first transistor Tr1is deactivated. The illumination and darkening of the light source3are switched at high speeds, and the light source3appears as if it is intermittently illuminated. Further, the brightness of the light source3is determined in accordance with the duty ratio of the PWM signal Sp1. The brightness of the light source3increases when the duty ratio of the PWM signal Sp1increases. The brightness of the light source3decreases when the duty ratio of the PWM signal Sp1decreases.

The microcomputer2includes a switching control unit26that switches the illumination control of the light source3between the computer control and the direct control. The switching control unit26generates a switching request signal Sch and outputs the switching request signal Sch from the operation switching terminal16. The switching control unit26generates an H-level switching request signal Sch during operation of the microcomputer2. This activates the third transistor Tr3, and current Ia sequentially flows to the fourth transistor Tr4, the resistor12, the third transistor Tr3, and the ground as shown inFIG. 2. Thus, when the fourth transistor Tr4is provided with the rheostat signal Srs, the connection switching circuit7can supply an L-level signal to the base of the second transistor Tr2to deactivate the second transistor Tr2. Accordingly, when the microcomputer2is operating, the first transistor Tr1is activated, and the second transistor Tr2is deactivated.

When the microcomputer2is not operating, the ignition voltage IG+ is not supplied to the microcomputer2, and the switching control unit26generates an L-level switching request signal Sch. This deactivates the third transistor Tr3, and current Ib flows in the fourth transistor Tr4, the resistor11, and the resistor14in this order as shown inFIG. 3. The connection switching circuit7can activate or deactivate the second transistor Tr2via the fourth transistor Tr4in accordance with the rheostat signal Srs. Accordingly, when the microcomputer2is not operating, the first transistor Tr1is deactivated, and the second transistor Tr2is activated.

When the voltage of the in-vehicle battery (illumination voltage ILL+, ignition voltage IG+) fluctuates, the PWM control unit25adjusts the duty ratio of the PWM signal Sp1in accordance with the fluctuation in the voltage of the in-vehicle battery so that the light source3can be illuminated with a predetermined brightness. Specifically, the PWM control unit25monitors the difference of the voltage monitored by the ignition voltage monitoring unit24and a target voltage and adjusts the duty ratio of the PWM signal Sp1in accordance with the difference. In this manner, the PWM control unit25controls the light source3so that the brightness is constant.

The PWM control unit25sets the brightness of the light source3in accordance with the operation amount set by the rheostat4. More specifically, the microcomputer2includes a rheostat signal input unit27that obtains the rheostat signal Srs from the rheostat4. The rheostat signal input unit27is connected via an I/F circuit28to a connection node P3between the resistor18and the rheostat connection terminal19. The rheostat signal input unit27is provided with the rheostat signal Srs via the I/F circuit28. The PWM control unit25controls the duty ratio of the PWM signal Sp1in accordance with the duty ratio of the rheostat signal Srs. This illuminates the light source3with the brightness set by the rheostat4. The rheostat signal input unit27corresponds to an input circuit.

The operation of the vehicle indicator illumination circuit1in the first embodiment will now be described with reference toFIGS. 2 and 3.

First, as shown inFIG. 2, when the ignition switch is switched from the off position to the on position, the microcomputer2is supplied with the ignition voltage IG+ and activated. Here, the PWM control unit25starts supplying the PWM signal Sp1from the PWM control terminal9. Further, the switching control unit26starts supplying an H-level switching request signal Sch from the operation switching terminal16. As a result, the third transistor Tr3is activated, and the connection switching circuit7deactivates the second transistor Tr2. The illumination switch6is still off, and the illumination voltage ILL+ is not supplied. Accordingly, the light source3remains dark.

When the microcomputer2is operated and the illumination switch6is turned on, the illumination voltage ILL+ is supplied to the collector terminal of the first transistor Tr1and the emitter terminal of the fourth transistor Tr4. The base terminal of the fourth transistor Tr4is provided with the pulsed rheostat signal Srs from the rheostat4.

In the fourth transistor Tr4, current flows in accordance with the pulse generation timing of the rheostat signal Srs. This activates the third transistor Tr3and, as shown inFIG. 2, current Ia flows from the fourth transistor Tr4to the third transistor Tr3. No current is supplied to the base terminal of the second transistor Tr2. That is, the first transistor Tr1is activated, and the second transistor Tr2is deactivated.

When the microcomputer2is operated and the illumination voltage ILL+ is supplied, the light source3is illuminated or darkened by the PWM control of the microcomputer2. Since the second transistor Tr2is deactivated, current I1flows through the first route R1as shown inFIG. 2whenever the H-level PWM signal Sp1is generated. That is, the current I1flows intermittently through the first route R1in accordance with the activation or deactivation of the first transistor Tr1. Accordingly, the light source3is illuminated with a brightness that is in accordance with the duty ratio of the PWM signal Sp1.

Further, the PWM control unit25adjusts the duty ratio of the PWM signal Sp1in accordance with fluctuations in the voltage of the in-vehicle battery to control the brightness of the light source3to be constant during PWM control. In this manner, even when the ignition voltage IG+ and the illumination voltage ILL+ vary due to a fluctuation in the voltage of the in-vehicle battery, the duty ratio of the PWM signal Sp1is adjusted so that the brightness of the light source3is constant. Also, the PWM control unit25obtains the rheostat signal Srs from the rheostat4and adjusts the duty ratio of the PWM signal Sp1based on the rheostat signal Srs. Thus, the brightness of the light source3is set based on the operation amount of the rheostat4.

Subsequently, as shown inFIG. 3, when the ignition switch is switched from the on position to the off position and the illumination switch6is turned on, the microcomputer2is no longer supplied with the ignition voltage IG+ and stops operating. Thus, the PWM control unit25is deactivated, and the microcomputer2cannot perform PWM control. Further, the switching control unit26cannot be operated, and an L-level switching request signal Sch is provided from the operation switching terminal16. Thus, the third transistor Tr3is deactivated, and the connection switching circuit7is set to activate the second transistor Tr2.

When the microcomputer2stops operating and the illumination voltage ILL+ is supplied, current flows in the fourth transistor Tr4whenever an H-level rheostat signal Srs is generated. Since the third transistor Tr3is deactivated, current Ib flows from the fourth transistor Tr4via the resistor12, the resistor11, and the resistor14to ground as shown inFIG. 3. In this case, current Ib does not flow to the third transistor Tr3. Thus, the first transistor Tr1is deactivated, and the second transistor Tr2is activated.

When the microcomputer2stops operating and the illumination voltage ILL+ is supplied, the light source3is illuminated or darkened by the direct control of the rheostat signal Srs (PWM control by the rheostat4). In this case, the fourth transistor Tr4is activated, and current Ib flows whenever the H-level rheostat signal Srs is generated. This activates the second transistor Tr2. Whenever the second transistor Tr2is activated, current I2intermittently flows through the second route R2as shown inFIG. 3. Accordingly, the light source3is illuminated with a brightness that is in accordance with the duty ratio of the rheostat signal Srs.

When the illumination voltage ILL+ is supplied and the ignition switch is switched again from the off position to the on position, the illumination control of the light source3is returned from the direct control to the computer control. Here, the PWM control unit25resumes the output of the PWM signal Sp1and starts the computer control. Also, the switching control unit26generates an H-level switching request signal Sch and activates the third transistor Tr3again.

The first embodiment has the advantages described below.

(1) A power supply route of the light source3includes the first route R1, through which the current I1flows, and the second route R2, through which the current I2flows. When the microcomputer2is operated and the ignition switch is located at the on position, the current I1flows to the light source3through the first route R1. When the microcomputer2stops operating and the ignition switch is located at the on position, the current I2flows to the light source3through the second route R2. Thus, even when the microcomputer2stops operating, the current I2flows to the light source3through the second route R2so that the light source3can be illuminated when the ignition switch is located at the on position.

(2) When the operation of the microcomputer2is resumed, the power supply route of the light source3is automatically returned from the second route R2to the first route R1. In this case, the microcomputer2performs PWM control again to illuminate the light source3.

(3) The first to fourth transistors Tr1to Tr4are activated or deactivated in accordance with whether or not the microcomputer2is operated. That is, when the microcomputer2stops operating or resumes operation, activation and deactivation of the first to fourth transistors Tr1to Tr4are switched. Thus, the power supply route to the light source3can be easily switched between the first route R1and the second route R2.

(4) When the power supply route to the light source3is switched from the first route R1to the second route R2, the rheostat signal Srs of the rheostat4is used to adjust the brightness of the light source3. This can control the supply of power to the light source3. The rheostat4is operated by power constantly supplied from the power supply. Thus, when the microcomputer2stops operating, the rheostat4continues to operation. This allows for the illumination of the light source3to be adjusted in accordance with the duty ratio of the rheostat signal Srs.

(5) Power is supplied to the light source3when the ignition switch is located at the on position. Thus, the light source3is illuminated only when necessary.

(6) When the ignition switch is located at the off position, the rheostat4can control the brightness of the light source3in accordance with the duty ratio of the rheostat signal Srs (PWM output). Also, when the ignition switch is located at the on position, the microcomputer2can regulate the duty ratio of the PWM signal Sp1in accordance with the duty ratio of the rheostat signal Srs to control the brightness of the light source3. Thus, when the duty ratio of the PWM signal Sp1is equal to the duty ratio of the rheostat signal Srs, the brightness of the light source3is constant regardless of the position of the ignition switch. The brightness of the light source3is set based on the duty ratio of the rheostat signal Srs regardless of the position of the ignition switch, that is, regardless of the operation of the microcomputer2. The duty ratio of the PWM signal Sp1may differ from the duty ratio of the rheostat signal Srs.

In the first embodiment, in a state in which the illumination switch6is turned on, when the ignition switch is switched from the on position to the off position, and the illumination control for the light source3is switched from the direct control to the PWM control (computer control), the rising timing of the PWM signal Sp1may be shifted from the rising timing of the switching request signal Sch, as shown inFIG. 4. When the rising timing of the PWM signal Sp1is later than the rising timing of the switching request signal Sch as shown inFIG. 4, a period in which the light source3momentarily darkens is generated. Also, when the rising timing of the PWM signal Sp1is advanced from the rising timing of the switching request signal Sch (not shown), interference occurs between the PWM driving. This flickers the illumination.

In a second embodiment, a microcomputer2ain a vehicle indicator illumination circuit1acontrols the rising timing of the PWM signal Sp1and the rising timing of the switching request signal Sch.

Differences from the first embodiment will mainly be described. Like or same reference numerals are given to those components that are the same as the corresponding components shown inFIGS. 1 to 4. Such components will not be described.

As shown inFIG. 5, the microcomputer2aincludes an output timing control unit29that controls the rising timing of the PWM signal Sp1and the rising timing of the switching request signal Sch. The output timing control unit29synchronizes the PWM signal Sp1with the switching request signal Sch in the computer control. That is, the output timing control unit29controls the PWM control unit25and the switching control unit26so that the rising timing of the PWM signal Sp1conforms to the rising timing of the switching request signal Sch. The output timing control unit29corresponds to an output synchronization circuit.

In the first embodiment, when the illumination control is switched from the direct control to the computer control, the PWM signal Sp1rises immediately after the rheostat signal Srs rises. This shortens the darkening period of the light source3as compared with the prior art and flickers the illumination. Thus, when the illumination control is switched from the direct control to the computer control, the output timing control unit29synchronizes the PWM signal Sp1and the switching request signal Sch with the rheostat signal Srs.

As shown inFIG. 6, the output timing control unit29controls the output timing of the PWM signal Sp1and the switching request signal Sch so that the rising timing of the PWM signal Sp1conforms to the rising timing of the switching request signal Sch. This prevents flickering in the illumination of the light source3when the illumination control is switched from the direct control to the computer control.

The output timing control unit29also synchronizes the PWM signal Sp1and the switching request signal Sch with the cycle of the PWM control of the rheostat signal Srs. More specifically, when the illumination control is switched from the direct control to the computer control, the output timing control unit29outputs the PWM signal Sp1and the switching request signal Sch at timing determined from the cycle of the PWM control of the rheostat signal Srs to synchronize these signals with the rheostat signal Srs. This further prevents flickering in the illumination of the light source3when the illumination control is switched from the direct control to the computer control.

In this manner, when the illumination voltage ILL+ is supplied and the illumination control for the light source3is switched from the direct control to the computer control, the output timing control unit29synchronizes the output timing of the PWM signal Sp1with the output timing of the switching request signal Sch. Thus, the timing at which the PWM signal Sp1illuminates the light source3accurately conforms to the timing at which the connection switching circuit7deactivates the second transistor Tr2. This prevents flickering in the illumination of the light source3.

In this case, the PWM signal Sp1and the switching request signal Sch are synchronized with the rheostat signal Srs and output from the microcomputer2a. This prevents the illumination cycle of the light source3from being changed drastically when the illumination control is switched from the direct control to the computer control. Thus, flickering in the illumination of the light source3can be prevented.

The second embodiment has the advantages described below.

(1) When the illumination voltage ILL+ is supplied and the illumination control for the light source3is switched from the direct control to the computer control, the output timing control unit29synchronizes the rising timing of the PWM signal Sp1with the rising timing of the switching request signal Sch. Thus, timing at which current flows to the light source3conforms to the timing at which the connection switching circuit7deactivates the second transistor Tr2. This prevents flickering in the illumination of the light source3when the illumination control is switched from the direct control to the computer control.

(2) When the illumination control for the light source3is switched from the direct control to the computer control, the PWM signal Sp1and the switching request signal Sch are output in synchronization with the rheostat signal Srs. Thus, when the illumination control for the light source3is switched from the direct control to the computer control, the cycles of the PWM signal Sp1, the switching request signal Sch, and the rheostat signal Srs conform to one another near the switching timing. This further restricts flickering in the illumination of the light source3.

(3) Since the computer control is the PWM control, the brightness of the light source3can be easily regulated simply by switching of the duty ratios of the PWM signal Sp1, the switching request signal Sch, and the rheostat signal Srs.

(4) The voltage of the in-vehicle battery (ignition voltage IG+) is monitored, and the duty ratio of the PWM control is switched in accordance with fluctuations in the voltage of the in-vehicle battery. Thus, the brightness of the light source3is adjusted to be constant. Accordingly, even when the voltage of the in-vehicle battery fluctuates, the brightness of the light source3can be adjusted to be constant by adjusting the duty ratio of the PWM control.

(5) The rheostat signal Srs is provided to the microcomputer2a, and the duty ratio of the PWM signal Sp1is set in accordance with the duty ratio of the rheostat signal Srs. Thus, the brightness of the light source3can be adjusted to the brightness selected with the rheostat4.

(6) The rheostat4is constantly supplied with power from the in-vehicle battery. Thus, the rheostat4is constantly operated regardless of the power supply state of the vehicle, and the vehicle indicator illumination circuit1acan be constantly provided with the rheostat signal Srs.

(7) The vehicle indicator illumination circuit1aincludes versatile circuits mainly configured by transistors.

The bipolar first to fourth transistors Tr1to Tr4may be replaced by field-effect transistors (FETs).

In the above embodiments, as a condition for illuminating the light source3, power is supplied to the light source3when the position lamp switch is turned on. However, the condition may be changed. For example, a sensor that detects the illuminance around the vehicle may be used, and the light source3is illuminated when the illuminance detected by the sensor becomes than a threshold value. Alternatively, the occurrence of a vehicle abnormality (such as an abnormality in any of various in-vehicle control systems) or detection of various warning signals, such as one indicating that a seatbelt is not fastened, may be the condition for illuminating the light source3.

In the above embodiments, the illumination of the single light source3is controlled. However, a plurality of light sources3may be connected in parallel, and the light sources3may be simultaneously illuminated and darkened. Each of the light sources3is arranged in correspondence with illuminating surfaces of different switches, warning lamps, or indicators.

In the above embodiments, the rheostat4is operated by the power of the in-vehicle battery. However, a dedicated cell for the rheostat4(primary cell or rechargeable cell) may constantly be used as the power supply.

In the above embodiments, the light source3is illuminated when the illumination switch6is turned on. However, the light source3may be constantly illuminated. In this case, the light source3is constantly connected to a power supply such as a battery (B+) and not to an illumination power supply.

Instead of the rheostat4, a power supply such as a battery may be connected to a negative terminal. In this case, when the microcomputer2or2astops operating and the third transistor Tr3is deactivated, the power supply route to the light source3is switched from the first route R1to the second route R2.

The illumination switch6is not limited to the position lamp switch and may be changed to another switch arranged in the vehicle.

The rheostat4does not have to be constantly active using the in-vehicle battery as a power supply and may be operable only when the illumination voltage ILL+ is supplied, for example.

The connection switching circuit7may deactivate the second transistor Tr2when provided with an L-level switching request signal Sch and may activate the second transistor Tr2when provided with an H-level switching request signal Sch, for example.

An external control device is not limited to the rheostat4as long as it can control the second transistor Tr2from outside.

The connection switching circuit7is not limited to a circuit including the third transistor Tr3and the fourth transistor Tr4and may be changed to another circuit as long as it can validate or invalidate the direct control.

The computer control is not limited to PWM control and may be changed to another control.

The direct control is not limited to the PWM control and may be constant pulse width control, for example.

The light source3is not limited to the LED and may be another member such as a lamp.

The first to forth transistors Tr1to Tr4are not limited to bipolar transistors and may be transistors of another type or different switch members.

The on position of the ignition switch may include an accessory (ACC) on position.

The vehicle indicator illumination circuit1is not limited to the circuit configuration of the above embodiments and may be changed to another configuration.

When the illumination control of the light source3is switched from the direct control to the computer control, the vehicle indicator illumination circuit1does not have to perform both of the process for synchronizing the output timing of the PWM signal Sp1with the output timing of the switching request signal Sch and the process for synchronizing the PWM signal Sp1and the switching request signal Sch with the rheostat signal Srs. The vehicle indicator illumination circuit1only needs to perform at least the former process.