WO2004048151A1

WO2004048151A1 - Method and circuit for detecting a failure in an indicator light

Info

Publication number: WO2004048151A1
Application number: PCT/NL2003/000840
Authority: WO
Inventors: Peter De Bruijn; Glenn Raymond Van Maenen
Original assignee: Ecs Electronics B.V.
Priority date: 2002-11-28
Filing date: 2003-11-28
Publication date: 2004-06-10
Also published as: NL1022013C1; AU2003289683A1

Abstract

The invention relates to a method for detecting a failure in an auxiliary indicator light, for instance on a trailer, which is connected to a main indicator light installation, for instance of a tractive vehicle, which main indicator light installation comprises an indicator pulse generator, a number of main indicator lights actuated thereby and a malfunction detection circuit monitoring the status of the main indicator lights, wherein an error signal is generated to the malfunction detection circuit of the main indicator light installation when an error condition is determined in the auxiliary indicator light, and wherein the operation of the indicator pulse generator is monitored and the error signal is provided as long as the indicator pulse generator is in operation. By thus providing the error signal during both the 'on' and 'off' phase of the indicator lights an error condition of the auxiliary indicator light is detected under all conditions by the malfunction detection circuit, irrespective of the moment of checking. The invention further relates to a circuit for performing this method.

Description

METHOD AND CIRCUIT FOR DETECTING A FAILURE IN AN INDICATOR LIGHT

The invention relates to a method for detecting a failure in at least one auxiliary indicator light, for instance on a trailer, which is connected to a main indicator light installation, for instance of a tractive vehicle, which main indicator light installation comprises an indicator pulse generator, at least one main indicator light actuated thereby and a malfunction detection circuit monitoring the status of the or each main indicator light, wherein an error signal is generated to the malfunction detection circuit of the main indicator light installation when an error condition is determined in the auxiliary indicator light. Such a method is known from the European patent 0 501 014.

Vehicles permitted to use the public highway must be provided with indicator lights, with which a change in the travel direction can be indicated. It is of great importance for road safety that such indicator lights function properly, and that the driver of a vehicle is alerted in the case of malfunction of one or more indicators, so that he can carry out replacement or repair. Most motor vehicles are provided for this purpose with a so-called flasher unit into which the indicator pulse generator and the malfunction detection circuit are integrated. This flasher unit further contains provisions for increasing the indicating frequency in the case of malfunction of one of the indicator lights. Because an indicator telltale is situated in the interior, generally on the instrument panel of a motor vehicle, which is operated with the indicating frequency, a malfunction is thus made visible to the driver. The malfunction is also easily detectable from outside the vehicle due to the increased frequency with which still functioning lights are flashing. Situations can be envisaged where extra indicator lights have to be connected onto an indicator light installation, for instance in the case a trailer is coupled to a motor vehicle. Such a trailer is generally provided with a lighting system including indicator lights which is coupled to the electrical system of the tractive vehicle, wherein the indicator lights of the trailer are connected to the indicator light installation of the tractive vehicle. It is important here that malfunctions in the indicators of the trailer are also made known to the driver of the tractive vehicle.

It is not possible however to automatically make use of the already present malfunction detection circuit of the tractive vehicle to detect malfunctions in the indicator lights of the trailer, because the problem arises here that, due to the increase in the total number of lights connected to the flasher unit, the normal number of lights for the vehicle still remains connected to the flasher unit even when one of the lights fails, so that no error condition is detected.

It is of course possible to provide an additional malfunction detection circuit for the extra indicators which, like the malfunction detection circuit of the tractive vehicle, is connected to the indicator pulse generator in order to thus increase the pulse frequency when an indicator light of the trailer malfunctions, but this is a time- consuming and costly business. In the above mentioned European patent 0 501 014 it is therefore proposed to connect one of the indicator lights of the vehicle or the indicator light of the trailer indirectly to the indicator light installation of the tractive vehicle via a driver circuit with a high resistance. In this way only two indicator lights, either the two main indicator lights or a main indicator light and an auxiliary indicator light, are thus always connected directly to the flasher unit, and the presence of an extra indicator remains as it were "concealed" from the indicator light installation and in particular the malfunction detection circuit, so that the indicator light installation will thus function normally. Means are herein present in the driver circuit for monitoring the status of the indicator light connected thereto whereby, when a failure is determined in this indicator, one of the indicators directly connected to the flasher unit, a main indicator light or the auxiliary indicator light, is switched over to the driver circuit. The relevant indicator light thus remains connected to the indicator light installation so that it also remains in operation. The malfunction detection circuit of the indicator light installation will however detect a malfunction since one of the main indicator lights has failed or appears to be disabled, to which this circuit responds by increasing the indicating frequency. In this manner it will be made apparent to the driver of the tractive vehicle that one of the indicators of his vehicle or the trailer has failed.

The method known from this older patent has the drawback however that it operates with the indicator frequency. During each "on" phase of the indicator lights the operation thereof is monitored and on the basis thereof one of the indicator lights is optionally switched from a direct connection to the pulse generator to a connection via the driver circuit. During the subsequent "off" phase however, the switch-over is reversed and the original situation restored. This is a problem when the check on the status of the indicators takes place not during the "on" phase but at a random moment during indicating, as is increasingly the case in modern cars. In this case no error condition is determined when the check takes place during an "off" phase, when the original situation has been restored and the correct number of operating indicators is directly connected to the pulse generator.

The invention therefore has for its object to provide an improved method of the above described type wherein this drawback does not occur. This is achieved according to the invention in that the operation of the indicator pulse generator is monitored and the error signal is provided as long as the indicator pulse generator is in operation. By thus providing the error signal not only during the on-phase but also during the off-phase of the indicator lights, the malfunction detection circuit detects an error condition of the auxiliary indicator light under all conditions, irrespective of the moment of checking.

This method can be readily applied by connecting one of the main or auxiliary indicator lights via a driver circuit to the indicator pulse generator and the or each other indicator light directly thereto, switching one of the other indicator lights from a direct connection to the indicator pulse generator to a connection via the driver circuit when an error condition is determined in the indicator light connected via the driver circuit to the indicator pulse generator, and by maintaining the switch-over as long as the indicator pulse generator is in operation.

Pulses can be detected at an output of the indicator pulse generator and the switch-over can be maintained as long as these indicator pulses succeed each other with a determined frequency. It is thus easily possible to ensure that an error condition in an indicator light is noticed at all times. A simple manner of achieving a continuous switchover is that the switch-over is maintained for a determined delay period, and the delay period restarts each time an indicator pulse is detected before the period ends. Switch- over periods thus merge as it were into each other up until the moment that the indicator pulse generator is switched off. In order to prevent the switch-over unintentionally being ended, the delay period is here advantageously chosen to be considerably longer than the period of the successive indicator pulses. This delay period is for instance in the order of 0.5 to 5 seconds, preferably in the order of 1 to 2 seconds, and more preferably about 1.5 second.

A simple method of determining an error condition in an auxiliary indicator light is for this error condition to be determined when this indicator light does not take up current while the trailer is still connected to the vehicle. The connection of the trailer to the vehicle can herein advantageously be determined on the basis of a signal from a trailer detection circuit, and in the absence of this signal the connection can be checked by measuring whether another auxiliary indicator light on the trailer is taking up current. In this manner it is possible to determine with certainty if there is an error condition in the auxiliary indicator light. The invention also relates to a circuit for performing the above described method. Such a circuit for detecting a failure in at least one auxiliary indicator light, for instance on a trailer, which is connected to a main indicator light installation, for instance of a tractive vehicle, which main indicator light installation comprises an indicator pulse generator, at least one main indicator light actuated thereby and a malfunction detection circuit monitoring the status of the or each main indicator light, which is provided in conventional manner with means for generating an error signal to the malfunction detection circuit of the main indicator light installation when an error condition is determined in the auxiliary indicator light, is characterized according to the invention in that the malfunction detection circuit further has means for monitoring the operation of the indicator pulse generator, which monitoring means are connected in controlling manner to the error signal generating means and activate these latter as long as the indicator pulse generator is in operation. Preferred embodiments of the failure detection circuit according to the invention form the subject-matter of the dependent claims 11-18.

The invention is now elucidated on the basis of an embodiment, wherein reference is made to the annexed drawing, in which: fig. 1 shows a schematic diagram of an indicator light installation of a tractive vehicle, indicator lights of a trailer connected thereto and a failure detection circuit according to the invention, figs. 2A-2D show a detailed circuit diagram of the failure detection circuit of fig. 1, fig. 3 is a flow diagram showing the most important steps of the method according to the invention, and fig. 4 is a diagram in which the progression of indicator pulses and an error signal is shown.

An indicator light installation 1 (fig. 1) of a (schematically shown) tractive vehicle 2 comprises a control module 3 which is connected to an on-board computer (not shown here) . When a driver of the vehicle operates the direction indicator, this module 3 generates indicator pulses L, R via lines 5L, 5R to a left-hand indicator light 4L or a right-hand indicator light 4R, depending on the direction to be indicated. In addition, module 3 simultaneously generates indicator pulses L, R to both indicator lights 4L, 4R when the driver operates a switch for warning lights. Module 3 further transmits the on-board voltage V of vehicle 2, at this moment generally 12 V, via a line 6.

When vehicle 2 is adapted to pull a trailer 7, there is a control circuit 8 present for operating the indicator light installation 9 of trailer 7. The purpose of this control circuit 8 is to cause the indicator light installation 9 of trailer 7 to operate synchronously with that of vehicle 2, and receives for this purpose the indicator pulses L, R on lines 5L, 5R via lines 22L, 22R. The control circuit must further prevent the operation of indicator light installation 1 of vehicle 2 being influenced by the presence of the indicator light installation 9 of trailer 7. This circuit is therefore also referred to as a "C2 module", after the contact to which a malfunction detection circuit present in control module 3 is connected. The control circuit or C2 module 8 must therefore conceal the presence of the indicator light installation 9 of trailer 7 from the malfunction detection circuit of control module 3. This circuit 8 comprises for this purpose two driver circuits 10L, 10R whereby the indicator lights 11L, 11R of trailer 7 are operated via lines 20L, 20R. These driver circuits 10L, 10R are supplied with the on-board voltage of vehicle 2 via lines 13L, 13R. In order to limit the current take-up thereof, driver circuits 10L, 10R have a high input resistance. The load on control module 3 by driver circuits 10L, 10R is thus minimized. Driver circuits 10L, 10R are activated over lines 24L, 24R by means of control signals L_IN, R_IN from a processor 12, which is supplied with the on-board voltage V via a line 14, and generate possible error signals over lines 25L, 25R to processor 12. Control circuit 8 further comprises a trailer detection line 17, which is in principle conductive when an electrical connection is realized between vehicle 2 and the indicator light installation 9 of trailer 7. This trailer detection line 17 runs, as do the lines 20L, 20R and the lines (not shown) for operating the other lamps of the trailer lighting, to a socket into which is inserted a plug of trailer 7.

The malfunction detection circuit in control module 3 of the indicator light installation 1 is adapted in known manner, when the indicator light installation 1 is in operation and generates indicator pulses L, R to one of the two indicator lights 4L, 4R, to check the operation of the relevant indicator light. To this end the current taken up by indicator light 4L, 4R is measured periodically. If this circuit detects a failure, because indicator light 4L, 4R is not taking up any current, the frequency of the indicator pulses is increased. A telltale light, which is arranged visibly to the driver in the instrument panel of vehicle 2 and is likewise actuated by module 3, will hereby also begin to flash more rapidly, whereby the attention of the driver is drawn to the malfunction. The malfunction is of course also noticeable from outside vehicle 2 due to the increased indicating frequency.

As stated, this manner of detecting malfunctions would be frustrated if the indicator lights 11L, 11R of trailer 7 were connected directly to control module 3 of vehicle 2, since failure of an indicator light 4L, 4R of vehicle 2 would then be camouflaged by the still operating indicator light 11L, 11R of trailer 7. This problem is solved by interposing the buffers 10L, 10R.

Control circuit 8 is shown in detail in figs. 2A- 2D, in which the symbols denote the following:

Control circuit 8 is adapted to make use of the malfunction detection circuit in control module 3 to also make malfunctions in the trailer indicator lights 11L, 11R visible to the driver of vehicle 2 and other traffic. To this end the processor 12 is connected in controlling manner via lines 21L, 21R to two relays 15L, 15R, which are connected in turn to switches 16L, 16R in lines 5L, 5R. Each relay 15L, 15R (fig. 2B) is provided by a diode circuit DI, D2, D3 (fig. 2A) with a voltage V _cc. When processor 12 receives an error signal L_ST, R_ST via line 25L, 25R from one of the driver circuits 10L, 10R which indicates that the trailer indicator light 11L, 11R actuated by the circuit has failed, the processor generates a signal via line 21L, 21R to transistor 22L, 22R which is connected thereto and which is incorporated in the power supply line 41L, 41R of the relay 15L, 15R in question. Transistor 22L, 22R is hereby switched from the blocked state to the transmitting state, whereby the power supply line 1L, 41R carries current and the relevant relay 15L, 15R is energized. This results in switch 16L, 16R being turned and the relevant indicator light 4L, 4R of vehicle 2 being disconnected from control module 3.

This indicator light 4L, 4R is thus "concealed" from control module 3, and a decrease in current is detected by the malfunction detection circuit and an error signal generated which results in an increase in the indicating frequency. By turning the switch 16L, 16R the relevant indicator light 4L, 4R is then powered by a line 18L, 18R from the corresponding driver circuit 10L, 10R of control circuit 8. As shown in the detailed diagram of fig. 2A, control circuit 8 also comprises a voltage divider 19 whereby the on-board voltage on line 6 is reduced to a suitable value V _DD for supplying power to processor 12. In the practical embodiment of control circuit 8 the driver circuits 10L, 10R, which are shown separately in fig. 1, are further combined into a single processor which is supplied with the on-board voltage V via line 6. This processor has two sets of inputs IN _{l f} IN ₂ and IN ₃, IN ₄, to which control lines 24L, 24R respectively are connected, and two sets of outputs OUT _{l t} OUT ₂ and OUT ₃, OUT ₄, to which the lines 20L, 20R to trailer indicator lights 11L, 11R are connected (fig. 2C) . If there is a "high" signal on input IN _n, the corresponding output OUT _n also becomes "high" as a sign that the indicator light 11 connected thereto must light up. If the load of this output OUT _n is then too low because indicator light 11 has failed, a corresponding status output ST _n is then made "low", whereby the error signal L_ST or R_ST is placed on line 25L or 25R.

The detail diagram of fig. 2D further shows a diode circuit 40 with three diodes D4, D5, D6 which are connected to AGND. This ensures that circuit 8 also continues to function properly when the earth GND falls away, for instance through wire breakage. Making malfunctions in the trailer indicator lights 11L, 11R visible by a switch-over as described above is per se known. In known systems, however, the relay is released again after each indicator pulse, whereby the initial situation is restored. The underlying idea here is that the status of indicator lights 4L, 4R is checked by conventional malfunction detection circuits during the "on" phase of each indicator pulse. It has been found however that, particularly in the case of modern vehicles, in the module 3 actuated by the on-board computer this check is not linked directly to the indicator pulses, and can therefore also be carried out during an "off" phase. However, because the conventional control circuits reverse the switch-over of indicator light 4L, 4R again during the "out" phase, no malfunction would be detected at that moment during a check, and the indicating frequency would thus not be increased.

In order to nevertheless also be able to detect malfunctions in trailer indicator lights 11L, 11R under these conditions with the control module 3 of vehicle 2, control circuit 8 is adapted according to the present invention to also generate a failure report during the "off" phase of the indicator pulses. In the shown embodiment this is achieved in that a signal is supplied to the relevant transistor 22L, 22R, and relay 15L, 15R is thus energized for a delay period T which is longer than the period P of the indicator pulses. The switch-over of indicator light 4L, 4R of vehicle 2 is thus also maintained during the "off" phase of each indicator pulse and a malfunction therefore also detected when the malfunction detection circuit carries out the check during this "out" phase.

Various aspects are shown in fig. 4. When at time t ₀ the control module 3 generates a first indicator pulse, consisting of an "on" phase 26 and an "off" phase 27, driver circuit 10L, 10R checks the operation of the relevant trailer indicator light 11L, 11R. In the case of a malfunction an error signal L_ST, R_ST will be sent after a very short time Δt to processor 12, which then sends an energizing signal 29 to the relevant transistor 22L, 22R, thereby energizing relay 15L, 15R. Processor 12 comprises a clock which ensures that signal 29 is maintained for a determined delay period t _τ that is chosen to be considerably longer than the period P of the indicator pulses. If at the moment in time t₁ = t₀ + P a subsequent indicator pulse is now detected, and it is found during checking by driver circuit 10L, 10R that the malfunction still exists, a new delay period t ₂ is then started by the clock.

In this manner the signal 29 persists and relay 15L, 15R remains energized as long as new indicator pulses are detected before the end of delay period T. In this situation the malfunction detection circuit of control module 3 will therefore always detect a malfunction, irrespective of whether the check takes place during an "on" phase 26 or an "off" phase 27 of the indicator pulse. After the last indicator pulse has ended, when the signal 28 on line 22L, 22R thus remains low for a longer period of time, the signal on line 21L, 21R still remains high for a time and then becomes low 30, whereby relay 15L, 15R is de-energized again and indicator light 4L, 4R is again connected directly to control module 3.

The operation of control circuit 8, which is achieved by programming in suitable manner the processor 12 and the driver circuits 10L, 10R likewise embodied as processors, is summarized in the flow diagram of fig. 3.

Once the program has been started in block 30, a check is made in block 31 whether an indicator pulse L, R is present on one of the lines 22L, 22R. If this is not the case, the indicator light installation 1 of vehicle 2 is then not active, and the program returns immediately to the start. If an indicator pulse is however detected, a check is then made in block 32 whether the on-board voltage is present on line 6. If this is not the case, an error message is then generated by energizing relay 15L, 15R, so that indicator lights 4L, 4R of the vehicle are powered via driver circuits 10L, 10R (block 38) . The delay period T is herein started (block 39), whereafter the program returns to the start.

If the supply voltage is found to be present, a check is then made in block 33 whether an indicator pulse is present on both lines 22L, 22R. This forms an indication of the use of warning lights, which results in the program returning to block 30 and waiting there. If this is not the case, then direction is being indicated and only one of the indicator lights 4L, 4R and 11L, 11R is thus being actuated.

A check is then made in block 34 whether an indicator light 4L, 4R of vehicle 2 is already connected to driver circuit 10L, 10R. If this is the case, because evidently a malfunction in one of the indicator lights 11L, 11R has already been determined, the delay period T is restarted (block 39) and the program returns to the start. If neither of the indicator lights 4L, 4R has yet been connected via buffer 10L, 10R, a check is then made whether an error signal L_ST, R_ST from the relevant driver circuit 10L, 10R is present on the line 25L, 25R (block 35) . If such an error signal is not detected, the relevant indicator light 11L, 11R is then operating normally and the program returns to the start.

If there is however an error signal L_ST, R_ST, the following step is then to check whether the indicator light installation 9 of trailer 7 is connected to that of vehicle 2. For this purpose the presence of a signal TD on line 17 is checked (block 36) . If the line 17 is found not to be connected to the on-board voltage or to the earth, it is then possible to check in another manner whether trailer 7 is indeed connected to vehicle 2. A check is made for this purpose in block 37 whether the other trailer indicator light 11L or 11R is connected to control circuit 8. If this is not the case, then there is evidently no trailer 7 present.

When the check in block 36 or block 37 shows that trailer 7 is indeed connected to the indicator light installation 1 of vehicle 2, it is then evidently the case that there is a malfunction in the relevant indicator light 11L, 11R, and one of the relays 15L, 15R is energized to supply power to the corresponding indicator light 4L, 4R of vehicle 2 via the associated driver circuit 10L, 10R (block 38). The delay period T is then started (block 39), whereafter the program returns to the start.

The circuit according to the invention and the manner in which it is used thus makes it possible to make malfunctions in the indicator lights of a trailer visible via the control module of the indicator light installation of the tractive vehicle, even when the module is adapted for malfunction detection at a random moment during the indicator pulse . Although the invention has been elucidated above on the basis of one embodiment, it will be apparent that the invention is not limited thereto. Instead of the shown circuit, wherein during normal use the trailer indicator lights are connected via a driver circuit and the indicator lights of the vehicle are connected directly to the control module, use could thus also be made of a circuit wherein the indicator lights of the vehicle are connected via the buffer during use with a trailer, and the trailer indicator lights are then connected directly to the control module. Generating of the error signal and maintaining thereof for a longer period could also be provided in a manner other than described and shown here. The scope of the invention is therefore defined solely by the appended claims.

Claims

1. Method for detecting a failure in at least one auxiliary indicator light, for instance on a trailer, which is connected to a main indicator light installation, for instance of a tractive vehicle, which main indicator light installation comprises an indicator pulse generator, at least one main indicator light actuated thereby and a malfunction detection circuit monitoring the status of the or each main indicator light, wherein an error signal is generated to the malfunction detection circuit of the main indicator light installation when an error condition is determined in the auxiliary indicator light, characterized in that the operation of the indicator pulse generator is monitored and the error signal is provided as long as the indicator pulse generator is in operation.

2. Method as claimed in claim 1, characterized in that one of the main or auxiliary indicator lights is connected via a driver circuit to the indicator pulse generator and the or each other indicator light is connected directly thereto, one of the other indicator lights is switched from a direct connection to the indicator pulse generator to a connection via the driver circuit when an error condition is determined in the indicator light connected via the driver circuit to the indicator pulse generator, and the switch-over is maintained as long as the indicator pulse generator is in operation.

3. Method as claimed in claim 2, characterized in that pulses are detected at an output of the indicator pulse generator and the switch-over is maintained as long as the indicator pulses succeed each other with a determined frequency.

4. Method as claimed in claim 3, characterized in that the switch-over is maintained for a determined delay period, and the delay period restarts each time an indicator pulse is detected before the period ends.

5. Method as claimed in claim 4, characterized in that the delay period is chosen to be considerably longer than the period of the successive indicator pulses.

6. Method as claimed in claim 4 or 5, characterized in that the delay period is in the order of 0.5 to 5 seconds.

7. Method as claimed in claim 6, characterized in that the delay period is in the order of 1 to 2 seconds, preferably about 1.5 second.

8. Method as claimed in any of the foregoing claims, characterized in that an error condition in the auxiliary indicator light is determined when this indicator light does not take up current while the trailer is still connected to the vehicle.

9. Method as claimed in claim 8, characterized in that the connection of the trailer to the vehicle is determined on the basis of a signal from a trailer detection circuit, and in the absence of this signal the connection is checked by measuring whether another auxiliary indicator light on the trailer is taking up current.

10. Circuit for detecting a failure in at least one auxiliary indicator light, for instance on a trailer, which is connected to a main indicator light installation, for instance of a tractive vehicle, which main indicator light installation comprises an indicator pulse generator, at least one main indicator light actuated thereby and a malfunction detection circuit monitoring the status of the or each main indicator light, which failure detection circuit has means for generating an error signal to the malfunction detection circuit of the main indicator light installation when an error condition is determined in the auxiliary indicator light, characterized in that the failure detection circuit further has means for monitoring the operation of the indicator pulse generator, which monitoring means are connected in controlling manner to the error signal generating means and activate these latter as long as the indicator pulse generator is in operation.

11. Failure detection circuit as claimed in claim

10, characterized in that the error signal generating means comprise a driver circuit and switch-over means connected thereto, one of the main or auxiliary indicator lights is connected via the driver circuit to the indicator pulse generator and the or each other indicator light and is connected directly thereto, the switch-over means switch one of the other indicator lights from a direct connection to the indicator pulse generator to a connection via the driver circuit when an error condition is determined in the indicator light connected via the driver circuit to the indicator pulse generator, and the switch-over means remain energized by the monitoring means as long as the indicator pulse generator is in operation.

12. Failure detection circuit as claimed in claim

11, characterized in that the switch-over means are connected to a clock, whereby they remain energized for a determined delay period, and the monitoring means are adapted to restart a delay period each time an indicator pulse is detected before the period ends.

13. Failure detection circuit as claimed in claim

12, characterized in that the delay period is considerably longer than the period of the indicator pulses detected by the monitoring means.

14. Failure detection circuit as claimed in claim 12 or 13, characterized in that the delay period is in the order of 0.5 to 5 seconds.

15. Failure detection circuit as claimed in claim 14, characterized in that the delay period is in the order of 1 to 2 seconds, preferably about 1.5 second.

16. Failure detection circuit as claimed in any of the claims 10 to 15, characterized by means for measuring a current taken up by the auxiliary indicator light and means for determining a connection between the tractive vehicle and the trailer, wherein the failure detection circuit is adapted to indicate an error condition when the measuring means measure no current take-up while a connection of the trailer to the vehicle has been determined.

17. Failure detection circuit as claimed in claim 16, characterized in that the means for determining a connection of the trailer to the vehicle comprise a trailer detection circuit which generates a connecting signal, and means for measuring a current taken up by another auxiliary indicator light on the trailer in the absence of this signal.

18. Failure detection circuit as claimed in any of the claims 10 to 17, characterized in that the switch-over means, the monitoring means, the clock, the current measuring means and/or the trailer detection circuit form part of or are controllably connected to a microprocessor.