Apparatus for monitoring cell voltage

Abstract

The present invention relates to a device for monitoring the cell voltages of individual cells (3) of a fuel cell stack (1) formed by membrane electrode assemblies (4) and bipolar plates (6), comprising a measuring device (7) for each individual cell (3), the measuring device (7) including an optical signal generator (9) controllable by the measuring device (7). The device according to the present invention is characterized in that the measuring device (7) is formed on a flexible circuit board connected to or formed as part of the frame (5) of the framed membrane electrode assemblies (4, 5).

Description

[Technical Field]

[0001] The invention relates to a device for monitoring the cell voltage of an individual cell formed by a membrane electrode assembly and a bipolar plate of a fuel cell stack according to the manner defined in more detail in the preamble of claim 1 . [Background technology]

[0002] Monitoring the cell voltages of individual cells in a fuel cell stack is generally known in the prior art. This is often referred to or abbreviated as Cell Voltage Measurement (CVM). For example, in the case of fuel cell stacks used in vehicles, such CVMs are relatively large and expensive, requiring considerable installation space. Furthermore, to ensure reliable measurements, electrical contacts for typically 200 to 400 individual cells must be identified and possibly traced for each fuel cell stack. Furthermore, the entire configuration is located in a high-voltage environment and must be constructed appropriately safely, for example, in terms of insulation resistance, voltage resistance, and creepage and creep distances. Furthermore, this configuration is typically located inside a housing surrounding the fuel cell stack. In this case, special explosion-proof measures must be taken into account due to the possibility of hydrogen accumulation due to permeation and leakage. Furthermore, the entire configuration is in a harsh environment in terms of electrochemical corrosion.

[0003] To overcome these problems, Patent Document 1 proposes an optical cell voltage monitoring system for fuel cell stacks. An optical device is placed on a measuring instrument fixed between the bipolar plates of each individual cell, generating an optical signal of the measured voltage. These optical signals are then collected by sensors or detectors assigned to each signal source, such as optical couplers, to transmit the voltage measurements of the individual cells detected within the fuel cell stack to the external environment of the fuel cell stack. High-resolution detectors may be used, and the number of detectors can be reduced by using mirrors.

[0004] Nevertheless, this arrangement is relatively large and complex, especially since it is located between and connected to the bipolar plates, and requires high-resolution detectors to process the signals, which are on the one hand prone to errors and on the other hand large and expensive. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] DE102007015735A1 Summary of the Invention [Problem to be solved by the invention]

[0006] The object of the present invention is to provide an improved device for monitoring cell voltages according to the preamble of claim 1, which advantageously develops the above-mentioned prior art. [Means for solving the problem]

[0007] According to the invention, this problem is solved by a device having the features set forth in claim 1, and here in particular the features set forth in the characterizing part of claim 1. Advantageous configurations and developments of the device according to the invention can be found from the dependent claims on which it is based.

[0008] The device according to the present invention contemplates that each individual cell is assigned a measuring device with an optical signal generator, as described in the prior art at the outset. According to the present invention, the measuring device is formed on a flexible circuit board connected to or formed as part of the frame of a framed membrane electrode assembly (MEFA), a so-called MEFA. This MEFA plays a crucial role in today's fuel cell stacks. This configuration is completed during the manufacture of the electrodes, catalyst-coated membranes, and gas diffusion layers, and then inserted between the two bipolar plates during stacking of the fuel cell stack, known as an SMEFA, with special seals, for example. Alternatively, these seals are connected to the bipolar plates accordingly, or are inserted during stacking. Regardless, the flexible board is very simple and efficient in the area of ​​the frame, and can even partially constitute this frame. Such a board can include various functionalities, and in the case of the present invention, it can include at least a measuring device and, together with an optical signal generator controllable by the measuring device.

[0009] This makes the construction very simple and the assembly very efficient. The flexible circuit board requires almost no installation space in the fuel cell stack that would be occupied by other components, so that the device for monitoring the cell voltages of the individual cells can be constructed with almost no installation space restrictions. Furthermore, as is basically known from the prior art, the requirements regarding electrical safety and explosion protection can be met without any problems through the use of an optical signal generator in cooperation with at least one optical sensor in the form of an optical coupler.

[0010] According to a particularly preferred development of the device according to the invention, the measuring device can be electrically connected to both adjacent bipolar plates via flexible conductors and / or, particularly preferably, via spring contacts. Such an arrangement via flexible conductors, which together with the measuring device and the bipolar plate form a conductor loop between the flexible substrate, is accordingly simple and allows for the inevitable expansion of the length of the fuel cell stack during operation, even due to pressure and / or temperature changes. The same is possible using spring contacts between the measuring device or a flexible circuit board provided with the measuring device and the adjacent bipolar plate. Furthermore, the spring contact variant also makes installation particularly simple, since no special attention needs to be paid to the contact of the measuring device, which is automatically achieved in any case when stacking the elements of the fuel cell stack, whereas with flexible substrates, these would still need to be connected, for example soldered.

[0011] The measuring device itself may be constructed in a manner known per se. In a particularly preferred embodiment of the device according to the invention, the measuring device in each case comprises a boost converter. Here, such a boost converter can correspondingly increase the relatively low voltage of the individual cells to efficiently activate an optical signal generator, which may include, for example, one or more LEDs. Since the power is provided by each cell itself, no further means are required for connecting the components.

[0012] The cell voltage, which constitutes the physical input parameter for the measuring device, is 0 to typically 1.23 V for each individual cell. Via a corresponding boost converter, preferably configured as part of an integrated circuit, and via a resonant circuit as a pulse generator or frequency generator, via a boost converter as a DC / DC boost converter, this cell voltage, which is typically above 0.6 V, can be correspondingly increased to a voltage level of, for example, 2.4 to 4 V, so that an LED of the optical signal generator, in particular also a multicolor LED, or a plurality of LEDs with different brightness, color, flashing frequency, etc., can be correspondingly activated, and using all of the above, the voltage of the monitored individual cell can be received contactlessly, for example via a CCD or CIS sensor, and correspondingly used to control the fuel cell stack.

[0013] According to a highly advantageous development of the inventive device, the optical signal generator is configured in such a way that it can be controlled in different states by a measuring device. This controllability preferably includes four distinct states. The first of these states, which would ideally be the normal state, can be a state in which the optical signal generator remains off. That is, without an activated optical signal, the cell operates within its specified rated range. For example, if a problem occurs, such as a too low voltage (commonly referred to as a low cell), a too high voltage (a high cell), or even more seriously, a polarity reversal of an individual cell (cell reversal), the optical signal generator is activated accordingly via the measuring device. Typical voltages for a low cell are lower than 600 mV for an operating individual cell, and for a high cell, higher than approximately 825 mV. Cell reversal occurs most often at approximately -600 mV, when the supply voltage of the individual cell is between -10 mV and -800 mV.

[0014] That is, when the optical signal generator is activated, it clearly indicates that there is a problem with each individual cell, and in fact, the fuel cell stack containing the individual cell. Ideally, with the optical signal generator turned on, at least two different states of the optical signal generator are visible, allowing the user to determine whether an individual cell is functioning normally: that the optical signal generator is off, that the optical signal generator is on because the cell is providing too much or too little voltage, or that the optical signal generator is on because an individual cell has reversed its polarity, typically also referred to as "cell reversal." Cell reversal is the most important condition to be indicated, followed by low cell. Too high a voltage, typically also referred to as high cell, is the least serious condition.

[0015] Therefore, the simplest case would be to display the problem no matter what, followed by distinguishing the problem into Problem and Cell Reversal, i.e., combining High Cell and Low Cell into one state, or, particularly preferred, if this is easily possible in terms of effort and installation space, to explicitly display all three states.

[0016] Various possibilities known per se are available for indicating the individual states. Thus, for example, if the light source of the light signal emitter is one or more monochromatic light sources, different flashing frequencies or the like can be used to distinguish the states. However, different colors can also be particularly preferably used.

[0017] In a particularly preferred embodiment of the device according to the invention, the optical signal generator of each measuring device may be constituted by a light-emitting diode (LED) capable of emitting at least two, preferably three, light colors. That is, the light-emitting diode may in particular be constituted as a so-called multicolor light-emitting diode (Multicolor-LED). Depending on the state, the diode may then remain off, corresponding to the normal state of the individual cell, or may emit a first color, e.g., white, corresponding to a reduced voltage state of the individual cell, or a first color, e.g., red, corresponding to a polarity reversal state of the individual cell. Optionally, for example, blue may indicate that the voltage of the individual cell is too high.

[0018] Alternatively, the optical signal generator of each measuring device may comprise at least two light-emitting diodes. The two, preferably three, light-emitting diodes of the optical signal generator may both emit the same color, in which case evaluation would be required for two different optical sensors. Alternatively, in a particularly preferred development of the device according to the invention, the sensors may also emit different colors. In this case, if a multicolor LED is used as the optical signal generator, a single optical sensor is generally sufficient, provided that it has the appropriate evaluation electronics that can distinguish the different light colors generated, for example by Fourier analysis of the collected signal.

[0019] Different light colors can be generated simply and efficiently via multiple separately configured LEDs, and depending on the requirements this can be a simpler and less expensive alternative to using larger multi-colored diodes, although the installation space required for multiple light-emitting diodes means that one or the other alternative can be advantageous in some situations.

[0020] Regardless of both variants, ultimately, a light is generated that, preferably in the on state, indicates a different problem in each individual cell via a different color. This can then be investigated individually by a corresponding light sensor in each individual light signal generator of the multiple measuring devices, or via mirrors and high-resolution sensors in the manner shown in the prior art described at the beginning. However, in practice, it often doesn't matter which individual cell in the fuel cell stack causes the problem, since compensation must typically be performed using a reaction that affects the entire fuel cell stack, or since it is practically impossible to turn off individual cells, the fuel cell stack as a whole must, for example, be turned off to avoid further damage.

[0021] Therefore, in a particularly advantageous and cost-effective development of the device according to the invention, the signals of all optical signal generators of the fuel cell stack are connected to at least one optical sensor via at least one optical guide. That is, in this particularly preferred embodiment of the invention, optical guides can be used. In principle, each individual light-emitting diode or each light source of an individual optical signal generator could be provided with its own optical guide that directs light to a common optical sensor or to a small number of optical sensors. However, in particular, in one advantageous embodiment of the device according to the invention, a particularly simple and efficient construction is achieved by using at least one optical guide strip, in which the optical signal generators direct their own light into one of the longitudinal sides of the optical guide strip and at least one optical sensor is arranged at at least one end of the optical guide strip.

[0022] In this case, for example, a single light guide extending along the fuel cell stack in the stack direction may be sufficient to simultaneously control all signal generators with a single optical sensor, through which the optical signal generators of each measuring device can be guided when activated. Via such a light guide, the single optical sensor can detect a problem within the stack. If the reaction is to turn off the stack, this is entirely sufficient, significantly reducing the costs of individual cell voltage monitoring that were previously incurred.

[0023] According to a particularly preferred development of this idea, the light sensor may be assigned evaluation electronics that are designed to identify the color. For example, a Fourier analysis of the data collected by the light sensor can be used to filter out red light colors if they occur with a sufficient frequency. Also, via a single sensor and possibly activated optical signal generators of several individual cells, it can be detected whether one or all of the individual cells have a "low cell" or "high cell" problem, or whether one or more of these individual cells have a polarity reversal problem.

[0024] This works for both a single multicolor LED that can emit different colors, and multiple LEDs of different colors, both emitting their light into one and the same light guide.

[0025] Furthermore, in one alternative embodiment of the variant having at least two separated LEDs, preferably with light of different colors, there are at least two strip-shaped light guides, for example arranged in parallel, while the individual LEDs of the optical signal generator can also be arranged, for example adjacent to each other and offset transversely to the stack direction. In this case, two or three light guides extending along the stack can guide the light of one LED on the one hand and the light of the other LED on the other hand to one region at the end of the stack appropriately. One sensor per light guide can detect one or the other LED, and thus one or the other state, i.e., the presence of at least one low cell, one high cell, or at least one cell with reversed polarity, without extensive software evaluation.

[0026] Depending on the number of individual cells and the length of the fuel cell stack, it may also be advantageous to place one light sensor at each end of the fuel cell stack, i.e., on each of the two end sides of the light guide, to increase reliability when the light efficiency in the area of ​​the light sensor is low, and this is also possible in the variant using the ribbon-shaped light guide described above.

[0027] Further advantageous configurations of the device according to the invention can be seen from the embodiment shown in more detail with reference to the figures. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell stack. [Figure 2] 1 shows a part of a fuel cell stack with a device according to the invention; [Figure 3] 2 is a view similar to that of FIG. 1 according to a particularly preferred embodiment of the device according to the invention; [Figure 4] 1 is a diagram of one possible configuration of the device according to the invention, based on a fuel cell stack and part of the device in a first possible embodiment. [Figure 5] 3 is a diagram of one possible configuration of the device according to the invention, based on a fuel cell stack and part of the device in a second possible embodiment. [Figure 6] 3 is a diagram of one possible configuration of the device according to the invention, based on a fuel cell stack and part of the device in a third possible embodiment. [Figure 7] 10 is a diagram of one possible configuration of the device according to the invention, based on a fuel cell stack and part of the device in a fourth possible embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0029] The diagram in Figure 1 shows a fuel cell stack, generally designated 1. Between two end plates, each designated 2, there are a number of individual cells, designated 3, although not all of the individual cells are shown here, and not all of the individual cells shown are designated by numerals. The construction of such a fuel cell stack 1 is known to those skilled in the art. The fuel cell stack 1 shown here is a low-temperature fuel cell comprising individual PEM cells, i.e. cells with catalyst-coated proton-conducting membranes.

[0030] The diagram in FIG. 2 shows an enlarged view of a portion of a fuel cell stack. The central individual cell 3, only the upper portion of which is shown, contains a so-called membrane electrode assembly 4, which includes a catalyst-coated membrane on the one hand and gas diffusion layers and electrodes on the other. This membrane electrode assembly is here bonded to a frame 5. This configuration is also called a framed membrane electrode assembly or membrane electrode frame assembly (MEFA). The MEFAs 4 and 5 may be provided with their own seals (not shown here), in which case they are called SMEFAs. Alternatively, seals may also be inserted during stacking, or they may be located on bipolar plates 6 arranged adjacent to the MEFAs 4 and 5, respectively. Two of these bipolar plates 6 are shown in the diagram in FIG. 2. The bipolar plates 6 are equipped on one side with a flow field (not shown here) for distributing a hydrogen-containing gas, and on the other side with a flow field (not shown here) for distributing an oxygen-containing gas to the two adjacent individual cells. Typically, a flow field for the cooling medium is arranged inside the bipolar plate 6 between them. All this is known to those skilled in the art of fuel cells. The bipolar plate 6 may be made of metal or of plastic with conductive fillers or plastic material coated to make it conductive. All this is of secondary importance to the present invention and will not be discussed further.

[0031] Connected to or as part of the frame 5 is a flexible circuit board (not shown here), which is shown here on the frame 5 and carries a measuring device designated 7. This measuring device 7 includes various components, such as a boost converter and a device for detecting the voltage of an individual cell 3, and is connected to the component frame 5. Preferably via elastic electrical contacts 8, the measuring device 7, which is arranged on the flexible circuit board connected to or formed by the frame 5, is electrically connected to both adjacent bipolar plates 6, namely, on one side to the positive surface and on the other side to the negative surface of the corresponding bipolar plate 6. This allows the voltage of the individual cell 3 of the fuel cell stack 1 assigned to the measuring device 7 to be monitored.

[0032] With regard to the operation of the fuel cell stack 1, it is important to distinguish between different voltage states: on the one hand, a normal state, a state in which the cell voltage drops, referred to as a "low cell," a state in which the voltage rises, referred to as a "high cell," and a state in which an electrical polarity reversal of an individual cell 3 has occurred. This state is often referred to by the English term "cell reversal." With regard to the start-up of the fuel cell stack 1, it is important to note that either all of its individual cells 3 are functioning normally or that one or more of the individual cells exhibit one of the serious states described immediately above, with the low cell and high cell states being less serious than the polarity reversal state.

[0033] These states can then be detected by a measuring device 7. Unlike conventional devices for monitoring the voltage of the individual cells 3 of the fuel cell stack 1, the measuring device 7 of the type described here is integrated into the frame 5, which has the advantage that it can be integrated directly into the cells during their manufacture, without the need for a later electrical connection. To transmit the signal safely in hazardous areas in terms of explosion protection measures in relation to possible hydrogen leakage from the fuel cell stack 1, the measuring device 7 is provided with an optical signal generator 9. Here, the optical signal generator 9 can indicate the voltage state of the individual cells 3 mentioned above, in particular by remaining off at normal voltage and, in the simplest case, emitting light when one of the other states occurs.

[0034] The detection and evaluation of the signal of the optical signal generator can basically be carried out by methods known from the prior art, for example by means of an array of detectors or by converting the light into a high-resolution detector. All of these are basically conceivable, but are relatively extensive in terms of the installation space and costs required. In many cases, especially in vehicle applications, it is sufficient to know that there is a corresponding problem with at least one individual cell 3 of the fuel cell stack 1. In this case, in case of doubt, a reaction is necessary by switching off the entire fuel cell stack 1 or by changing the medium supply of the fuel cell stack 1 accordingly.

[0035] The simplest variant of this configuration is now correspondingly shown in the diagram of FIG. 3 on the basis of a fuel cell stack 1 similar to the one in FIG. 1. Each individual cell 3 shown here is equipped with a measuring device 7 with an optical signal generator 9. A light guide 10, which is configured as a strip-shaped light guide, for example with a rectangular cross-section, extends along the entire fuel cell stack 1 in the stack direction s, so that the optical signal generators 9 of all measuring devices 7 of all individual cells 3 direct their light transversely into the longitudinal side of the light guide 10. Here, an optical sensor 11 is arranged at at least one end side, or optionally at both end sides, preferably in the region of the end side facing the end plate 2 of the fuel cell stack 1 or terminating in the region of the end plate 2. In principle, one optical sensor 11 is sufficient. However, if the number of individual cells 3 is considerably large and thus the length of the fuel cell stack 1 in the stack direction s is large, it may be advantageous to also provide a further optional optical sensor 11 in the area of ​​the second end plate 2 so as to obtain reliable results even if just one individual cell 3 generates its own signal via the signal generator 9 of the measuring device 7 that cannot be reliably detected by the just one optical sensor 11 because it is relatively far from this just one optical sensor 11 along the stack direction s.

[0036] As already mentioned above, it can be advantageous if it is known here whether a low cell, high cell and / or cell reversal problem has been detected via the optical sensor 11. For this purpose, there are basically various possibilities, which are correspondingly shown and explained in the following diagrams of Figures 4 to 7. In each case, a section of an end plate 2 with three individual cells 3 and their measuring devices 7 is shown.

[0037] In the diagram of FIG. 4, each measuring device 7 comprises a light-emitting diode 12 as an optical signal generator 9. The light-emitting diode 12 is configured as a multicolor LED, capable of exhibiting different colors. When each individual cell 3 is at normal voltage, the multicolor LED remains off. The multicolor LED emits a first color, e.g., yellow, for a low cell, a second color, e.g., blue, for a high cell, and a third color, e.g., red, for a cell with the opposite polarity, i.e., cell reversal. The emitted light collected by the light guide 10 and guided to the area of ​​the sensor 11 is then collected by one or, optionally, two light sensors 11 arranged on the two end plates 2 and evaluated accordingly by evaluation electronics 13. The evaluation electronics 13 may, in particular, perform a Fourier analysis to evaluate the different colors of light detected by the sensor 11. If the light is only monochromatic, e.g., yellow, it can signal the problem of one or more low cells via the evaluation electronics 13. If the light only contains red light, it can signal the problem of one or more cell reversals accordingly. If the light only contains blue light, it can signal the problem of one or more high cells. If the light contains all three colors, it can again signal the problem of both low cells and cell reversals. This requires a multicolor LED 12 in terms of hardware and an appropriate evaluation in the evaluation electronics 13 in terms of software.

[0038] Instead of, or essentially also in addition to, different colored lights, different flashing frequencies or sequences, i.e., specific flashing pattern orders, can be used to make it possible to detect various conditions occurring in at least one of the individual cells 3 in the fuel cell stack 1 via at least one optical sensor 11.

[0039] This configuration can be modified to the point where the multicolor LED 12 can be omitted entirely. In the configuration of FIG. 5, which should be understood as essentially similar to the diagram of FIG. 4, each of the optical signal generators 9 is provided with, for example, two differently colored light-emitting diodes 14, 15. This can potentially be a lower-cost variant compared to the use of multicolored LEDs, provided sufficient installation space is available. In this variant, both differently colored LEDs 14, 15 inject their own light into the light guide 10 in the same manner as described above. Detection via at least one sensor 11 and evaluation in the evaluation electronics 13 are then performed analogously. That is, the two different LEDs 14, 15 shown here illustratively can communicate three states, with both being in the "off" state. This can be, for example, a normal function where both LEDs 14, 15 are off; a high cell or low cell problem where one of the LEDs, e.g., LED 14, is on; or a cell reversal problem where, e.g., LED 15, is on. It goes without saying that this configuration can be correspondingly expanded by a third LED to further distinguish between High Cell and Low Cell states in the signal reaching the at least one photosensor 11.

[0040] Furthermore, the diagram of FIG. 6 shows a further variant. Instead of arranging the LEDs 14, 15, for example, adjacent to each other in the stack direction as optical signal generators in each measuring device 7, it is also possible to arrange the LEDs 14, 15 laterally offset relative to the stack direction and to direct their own light into the two parallel light guides 10, 16, as can be seen in the diagram of FIG. 6. In this case, the LEDs 14, 15 may be of different colors, or larger LEDs of the same color may be used. In this case, for example, the LEDs 14 shown at the top of the diagram of FIG. 6 indicate a low cell or a high cell when activated, and the LED 15 of the optical signal generator 9, located at the bottom in the diagram of FIG. 6, in the region of the second light guide 16 indicates a cell reversal. Then, via the optical sensor 11, in a manner known per se, a low cell problem can be directly indicated and transmitted to a corresponding control mechanism without the need for further evaluation of the light color, and via the optical sensor 17 on the end side of the other light guide 16, a problem of one or more cell reversals can be transmitted accordingly.

[0041] The configuration shown in Figure 6 can also be correspondingly expanded, as already basically explained above, beyond the two LEDs 14, 15 by a third LED 18 and, in this case also, correspondingly, via a further light guide 19 and a further light sensor 20. This is correspondingly shown in the diagram of Figure 7, which apart from that should be understood as being similar to the diagrams of Figures 4 to 6. With this configuration, one of the interesting states can be correspondingly displayed in each individual light guide 10, 16, 19.

[0042] Overall, the configuration is particularly simple in all its variants and is obtained by several light sensors 11, 17, 20, on whose side it is only necessary to recognize the presence of light and possibly the light color, without making high demands, for example, regarding high pixel resolution or the like.

[0043] These configurations are basically suitable for all types of fuel cell stacks 1, in particular for PEM fuel cells, and are particularly preferred for the use of such types of fuel cell stacks 1 in vehicles, since here it is necessary to meet on the one hand the constraints on installation space and on the other hand the very high cost pressures in assembling and manufacturing the fuel cell stacks 1.

[0044] The device for monitoring the cell voltages in the described possible embodiment variants makes this ideally possible.

Claims

1. 1. An apparatus for monitoring the cell voltages of individual cells (3) of a fuel cell stack (1) formed by a membrane electrode assembly (4) and a bipolar plate (6), the apparatus comprising a measuring device (7) for each of the individual cells (3), the measuring device (7) including an optical signal generator (9) controllable by the measuring device (7), The measuring device (7) is formed on a flexible circuit board connected to or formed as part of the frame (5) of the framed membrane electrode assembly (4, 5); and 1. The device according to claim 1, wherein the optical signal generator (9) of each measuring device (7) is constituted by one light-emitting diode (12), the light-emitting diode (12) being adapted to emit light in at least two light colors.

2. 2. The device according to claim 1, characterized in that the measuring device (7) is conductively connected to two adjacent bipolar plates (6) via flexible conductive elements and / or spring contacts (8).

3. 3. Device according to claim 1 or 2, characterized in that the measuring device (7) comprises a boost converter.

4. 4. The device according to claim 1, wherein the optical signal generator (9) is configured in such a way that it can be controlled by the measuring device (7) in at least three different states.

5. 5. The device according to claim 1, wherein the optical signal generators (9) of all measuring devices (7) are connected to at least one optical sensor (11) via at least one optical conductor (10, 16, 19).

6. 6. The device according to claim 5, characterized in that evaluation electronics (13) are provided for evaluating the data of the at least one light sensor (11), said evaluation electronics (13) being configured to evaluate the detected signals for the occurrence of certain colors and / or flashing frequencies.

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