Light emission device and distance measurement device

Abstract

A light emission device (2) comprises: a laser beam source (11); an electricity storage element (33) for supplying a drive current to the laser beam source (11); a plurality of switching elements (34a, 34b) that are connected in series to the laser beam source (11); and an electrical element (Zener diode (37)) that is provided with respect to the switching element (34a) to which the other switching element (34b) is connected at least on the downstream side, and that is provided between ground and an input unit (Pa) for a switch open / close signal , the electrical element guiding high level noise exceeding the voltage level of the switch open / close signal to ground.

Description

Light emitting device and distance measuring device

[0001] The present invention relates to a light emitting device that emits pulsed light and a distance measuring device that measures the distance to an object using the pulsed light emitted from the light emitting device.

[0002] Distance measurement devices that measure the distance to an object using pulsed laser light whose intensity changes in a pulsed manner have been known. In this type of distance measurement device, the distance to the object is measured based on the time difference between the emission timing of the laser light and the reception timing of the reflected light of the laser light from the object. This type of distance measurement device is described in Patent Document 1 listed below.

[0003] When emitting pulsed laser light as described above, the laser light source is controlled so that the amount of laser light per pulse satisfies the eye-safe standard. However, if an abnormality occurs in the control circuit of the laser light source, the control cannot be performed properly.

[0004] Japanese Patent Application Laid-Open No. 2003-144994 describes a configuration in which a monitor light receiving element monitors the amount of light emitted from a laser light source. In this configuration, the operation of the laser light source is stopped when the output of the monitor light receiving element exceeds a predetermined level.

[0005] Japanese Patent Laid-Open No. 7-229967 Japanese Patent Laid-Open No. 2003-124564

[0006] A configuration for making the laser light source emit pulsed light can be used in which a switch element is connected in series to the laser light source and the switch element is turned on during the pulsed light emission period. However, in this configuration, if a failure occurs in the switch element, the laser light source will not emit pulsed light properly.

[0007] In view of the above problem, an object of the present invention is to provide a light emitting device and a distance measuring device that can more appropriately emit pulsed light from a laser light source while suppressing failure of a switching element.

[0008] A first aspect of the present invention relates to a light-emitting device comprising: a laser light source, a power storage element for supplying a drive current to the laser light source, a plurality of switch elements connected in series to the laser light source, and an electric element disposed between a switch signal input section and ground for each of the switch elements connected downstream from at least one other switch element, the electric element directing high-level noise exceeding the voltage level of the switch signal to ground.

[0009] According to the light emitting device of this aspect, multiple switch elements are connected in series to the laser light source, so that even if a short circuit occurs in one of the switch elements, the other switch elements can properly cause the laser light source to emit pulsed light.

[0010] When multiple switch elements are connected in series to a laser light source, the connection between two adjacent switch elements becomes floating when one switch element is opened. This makes it easy for noise to be superimposed on the input of the switch element on the laser light source side. Therefore, if there is a time lag in the switch opening / closing signals input to these switch elements, ringing occurs when excess charge in the storage element moves back and forth between the storage element and the switch element, generating high-level noise at the input of the switch element on the laser light source side. If this noise exceeds the rated capacity of the switch element, the switch element may malfunction.

[0011] In contrast, in the light-emitting device according to this embodiment, as described above, for at least a switch element connected downstream of another switch element, an electric element is provided that guides high-level noise generated at the input part to which the switch opening / closing signal is input to ground. Therefore, noise caused by the ringing described above is guided to ground, and noise exceeding the rated value is prevented from being generated at the input part. This makes it possible to prevent malfunction of the switching element due to noise.

[0012] In this way, the light emitting device according to this aspect can suppress failure of the switch element and more appropriately perform pulsed emission of the laser light source.

[0013] A second aspect of the present invention relates to a distance measurement device, which includes the light emitting device according to the first aspect, a projection optical system that projects pulsed light emitted from the light emitting device onto a target area, and a light receiving unit that receives light of the pulsed light reflected from an object.

[0014] The distance measurement device according to this aspect includes the light emitting device according to the first aspect, and therefore can more appropriately emit pulses of light from the laser light source while suppressing failure of the switch element, thereby improving the reliability of the distance measurement device.

[0015] As described above, according to the present invention, it is possible to provide a light emitting device and a distance measuring device that can more appropriately perform pulsed emission of a laser light source while suppressing failure of a switch element.

[0016] The effects and significance of the present invention will become more apparent from the following description of the embodiments, however, the embodiments shown below are merely examples of how the present invention can be put into practice, and the present invention is not limited to the embodiments described below.

[0017] FIG. 1 is a block diagram showing the configuration of a distance measurement device according to a first embodiment. FIG. 2 is a circuit diagram showing the configuration of a light-emitting device according to a reference example. FIGS. 3(a) and 3(b) are time charts showing the ON / OFF states of two switch elements when a short circuit occurs in one of the switch elements according to a reference example. FIG. 3(c) is a time chart showing the drive current of a laser light source in the cases of FIGS. 3(a) and 3(b). FIG. 3(d) is a time chart showing the ON / OFF states of switch elements when a short circuit occurs in the switch element according to a comparative example. FIG. 3(e) is a time chart showing the drive current of a laser light source in the case of FIG. 3(d). FIG. 4 is a circuit diagram showing the configuration of a light-emitting device according to the first embodiment. FIG. 5(a) is a graph showing the simulation results of voltages generated at the input sections of two switch elements when there is no time lag between the drive signals input to the two switch elements according to a reference example. FIG. 5(b) is a graph showing a simulation result of the voltage generated at the input of each switch element when there is a time lag between the drive signals input to the two switch elements according to the reference example. FIG. 5(c) is a graph showing a simulation result of the voltage generated at the input of each switch element when there is no time lag between the drive signals input to the two switch elements according to the first embodiment. FIG. 5(d) is a graph showing a simulation result of the voltage generated at the input of each switch element when there is a time lag between the drive signals input to the two switch elements according to the first embodiment. FIG. 6 is a circuit diagram showing the configuration of a light-emitting device according to a second embodiment. FIGS. 7(a) and 7(b) are time charts showing examples of drive signals input to the two switch elements according to the second embodiment. FIG. 7(c) is a time chart showing the drive current of the laser light source in the cases of FIGS. 7(a) and 7(b). FIG. 8 is a flowchart showing a driver abnormality detection process according to the second embodiment. FIG. 9 is a circuit diagram showing the configuration of a light-emitting device according to a third embodiment. 10A to 10D are time charts showing simulation results of the drive signal, the applied voltage (drain voltage) of each switch element, the monitor signal, and the drive current of the laser light source when two switch elements are operating normally, according to the third embodiment.11(a) to 11(d) are time charts showing simulation results of the drive signal of the downstream switch element, the applied voltage (drain voltage) of the downstream switch element, the monitor signal, and the drive current of the laser light source when a short circuit occurs only in the upstream switch element according to the third embodiment. 12(a) to 12(d) are time charts showing simulation results of the drive signal of the upstream switch element, the applied voltage (drain voltage) of the upstream switch element, the monitor signal, and the drive current of the laser light source when a short circuit occurs only in the downstream switch element according to the third embodiment. 13 is a flowchart showing the abnormality detection process of the switch element according to the third embodiment. 14 is a block diagram showing the configuration of a light-emitting device according to the third embodiment. 15 is a block diagram showing the configuration of a light-emitting device according to a modified example of the third embodiment. 16 is a circuit diagram showing the configuration of a light-emitting device according to a modified example 1. 17 is a circuit diagram showing the configuration of a light-emitting device according to a modified example 2.

[0018] However, the drawings are for illustrative purposes only and do not limit the scope of the present invention.

[0019] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0020] First Embodiment Fig. 1 is a block diagram showing the configuration of a distance measurement device 1. Fig. 1 shows a so-called flash type distance measurement device 1.

[0021] The distance measurement device 1 includes an optical system including a laser light source 11, a projection optical system 12, a light receiving optical system 13, and an image sensor 14.

[0022] The laser light source 11 is, for example, configured by a laser diode, and emits laser light (projected light) of a predetermined wavelength. When the distance measurement device 1 is mounted on a vehicle, the emission wavelength of the laser light source 11 is set, for example, to an infrared wavelength band (e.g., 905 nm). The emission wavelength of the laser light source 11 can be changed as appropriate depending on the usage mode of the distance measurement device 1. The laser light source 11 may be configured by a plurality of laser diodes. Furthermore, the laser light source 11 may be configured by a laser emitter other than a laser diode.

[0023] The projection optical system 12 guides the projection light emitted from the laser light source 11 to the distance measurement area A10 at a predetermined spread angle. The projection optical system 12 projects the projection light with a uniform intensity distribution onto the distance measurement area A10. The projection optical system 12 may be composed of a single lens or may include multiple lenses. The projection optical system 12 may also include a concave mirror or the like.

[0024] The light receiving optical system 13 collects the reflected light of the laser light reflected by an object present in the distance measurement area A10 and focuses the light on the light receiving surface 14a of the image sensor 14. The light receiving optical system 13 may be composed of a single lens or may include multiple lenses. The light receiving optical system 13 may also include a concave mirror or the like.

[0025] The image sensor 14 receives reflected light using a plurality of pixels arranged on the light-receiving surface 14a and outputs a detection signal corresponding to the intensity of the received reflected light. A large number of pixels are arranged in a matrix on the light-receiving surface 14a. For example, an avalanche photodiode is arranged in each pixel. Other photodetection elements may also be arranged in each pixel.

[0026] A filter that transmits the wavelength band of the projected light and blocks light of other wavelength bands may be disposed between the light receiving optical system 13 and the image sensor 14. This makes it possible to prevent unwanted light of wavelengths different from the projected light from entering the light receiving surface 14a of the image sensor 14. Furthermore, when the laser light source 11 emits infrared light, the image sensor 14 may have detection sensitivity only in the infrared wavelength band. This makes it possible to prevent unwanted visible light from being detected by the image sensor 14.

[0027] The distance measurement device 1 includes a control unit 21, a light source driving unit 22, a signal processing unit 23, and a distance calculation unit 24 as a circuit configuration.

[0028] The control unit 21 includes an arithmetic processing circuit and memory, and is configured by, for example, an FPGA, an MPU, a ROM, a RAM, etc. The control unit 21 outputs a control signal to the light source drive unit 22 and controls the laser light source 11 via the light source drive unit 22. The light source drive unit 22 causes the laser light source 11 to emit pulsed light with a predetermined intensity and pulse width in accordance with the control signal. The control unit 21 also outputs the control signal output to the light source drive unit 22 to the distance calculation unit 24 at the same timing as the output to the light source drive unit 22.

[0029] The signal processing unit 23 performs amplification and noise removal processing on the detection signal of each pixel output from the imaging element 14 , and outputs the processed detection signal to the distance calculation unit 24 .

[0030] The distance calculation unit 24 is a circuit that includes an arithmetic processing circuit and a memory and performs distance calculations. The distance calculation unit 24 calculates the distance to an object within the ranging area A10 for each pixel based on the timing at which the distance calculation unit 24 receives a pulse emission control signal from the control unit 21 and the timing at which the distance calculation unit 24 receives a detection signal for each pixel of the image sensor 14 from the signal processing unit 23. The distance calculation unit 24 then generates distance image data for one screen (one frame) in which the distance calculated for each pixel is mapped to the position of each pixel, and outputs the generated distance image data to the display unit of the distance measurement device 1 or an external device to which the distance measurement device 1 is connected.

[0031] 1 , the light emitting device 2 is configured with the laser light source 11, the light source driving unit 22, and the control unit 21. Here, the control unit 21 controls the light emitting device 2 as well as the distance measuring device 1. The light receiving optical system 13 and the image sensor 14 configure the light receiving unit 3.

[0032] 2 is a circuit diagram showing the configuration of a light emitting device 2 according to a reference example, in which the control unit 21 is omitted.

[0033] 1 , the light emitting device 2 according to the reference example includes a light source drive unit 22 including a DC power supply 31, a resistor 32, a storage element 33, switch elements 34a and 34b, drivers 35a and 35b, and a pulse generation circuit 36. The pulse generation circuit 36 ​​may be built into the control unit 21.

[0034] The storage element 33 is made of a capacitor and is connected to the DC power supply 31 via the resistor 32. The storage element 33 may be made of a single capacitor, or may be configured with a plurality of capacitors connected in parallel. Charge is accumulated in the storage element 33 according to the time constant of the circuit made up of the resistor 32 and the storage element 33. The storage element 33 supplies a drive current to the laser light source 11 in response to both of the switch elements 34 a and 34 b being conductive.

[0035] As described above, the laser light source 11 is a laser diode. The laser light source 11 emits laser light when a drive current is supplied from the power storage element 33. A light source may be configured by connecting a plurality of laser light sources 11 in series or in parallel between the power storage element 33 and the switch element 34a.

[0036] The switch element 34a is switched between a conductive state and a non-conductive state in response to a signal from the driver 35a. The switch element 34b is switched between a conductive state and a non-conductive state in response to a signal from the driver 35b. The switch elements 34a and 34b are, for example, field effect transistors (FETs). The switch elements 34a and 34b may be configured with other switch elements that are switched between a conductive state and a non-conductive state in response to signals from the drivers 35a and 35b.

[0037] The switch elements 34a and 34b switch the laser light source 11 between an emitting state and a non-emitting state in response to signals from the drivers 35a and 35b. That is, when both the switch elements 34a and 34b are in a conductive state, a drive current is supplied from the storage element 33 to the laser light source 11, causing the laser light source 11 to emit light. When at least one of the switch elements 34a and 34b is in a non-conductive state, the supply of drive current to the laser light source 11 is cut off, causing the laser light source 11 to turn off.

[0038] The drivers 35a and 35b drive the switch elements 34a and 34b, respectively, in response to a pulse signal input from the pulse generation circuit 36. The pulse generation circuit 36 ​​outputs a pulse signal of a predetermined duration in response to receiving a pulse emission control signal from the control unit 21. The drivers 35a and 35b set the switch elements 34a and 34b to a conductive state for a period corresponding to the duration of the pulse signal.

[0039] When the switch elements 34a and 34b are FETs, the drivers 35a and 35b supply drive signals to the gates of the FETs during the rising period of the pulse signal. As a result, the switch elements 34a and 34b are turned on during the period corresponding to the pulse signal, and a drive current is supplied to the laser light source 11. In this way, the laser light source 11 emits pulsed light.

[0040] 3A and 3B are time charts showing the ON / OFF states of the two switch elements 34a and 34b in the reference example configuration shown in FIG. 2 when a short circuit occurs in one of the switch elements. Here, the upstream switch element 34a (SW1) in FIG. 2 short-circuits immediately after a pulsed drive signal is input from the driver 35a. The downstream switch element 34b (SW2) operates normally. FIG. 3C is a time chart showing the drive current of the laser light source 11 in the cases of FIGS. 3A and 3B.

[0041] 3(a) to 3(c), in the configuration of the reference example, two switch elements 34a and 34b are connected in series to the laser light source 11, so even if a short circuit occurs in one of the switch elements (here, switch element 34a), the other switch element (here, switch element 34b) opens and closes appropriately with a time width corresponding to the pulse signal input from the pulse generating circuit 36. As a result, the time width of the drive current of the laser light source 11 is maintained at the normal time width.

[0042] Fig. 3(d) is a timing chart showing the ON / OFF states of the switch element when a short circuit occurs in the switch element according to a comparative example, and Fig. 3(e) is a timing chart showing the drive current of the laser light source 11 in the case of Fig. 3(d).

[0043] In the comparative example, only one switch element is disposed between the laser light source 11 and the ground. That is, in the comparative example, for example, the downstream switch element 34b and the driver 35b are omitted from the configuration of FIG.

[0044] The solid line in Fig. 3(d) shows the open / closed state of the switch element in the comparative example when no short circuit occurs in the switch element. The solid line in Fig. 3(e) shows the drive current of the laser light source 11 in this case. In this case, the switch element opens and closes appropriately with a time width corresponding to the pulse signal input from the pulse generating circuit. As a result, the time width of the drive current of the laser light source 11 is maintained at the normal time width.

[0045] The dashed line in Fig. 3(d) shows the open / closed state of the switch element in the comparative example when a short circuit occurs in the switch element. In this case, the short circuit occurs immediately after a pulsed drive signal is input from the driver. The dashed line in Fig. 3(e) shows the drive current of the laser light source 11 in this case.

[0046] As shown in Fig. 3(e), the configuration of the comparative example has only one switch element, and therefore, when a short circuit occurs in the switch element, the switch element remains conductive even after the time width corresponding to the pulse signal input from the pulse generating circuit 36. Therefore, current continues to flow from the storage element 33 to the laser light source 11 even after the normal time width. As a result, an extra amount of laser light, equivalent to the drive current shown by hatching in Fig. 3(e), is emitted from the laser light source 11. In the configuration of the comparative example, this extra amount of light may cause the eye-safety standard to not be met.

[0047] In contrast, in the configuration of the reference example, two switch elements 34a and 34b are provided as described above. Therefore, even if one of the switch elements is short-circuited, the duration of the drive current of the laser light source 11 is maintained at the normal duration, as shown in FIG. 3(c). Therefore, even if one of the switch elements is short-circuited, the laser light source 11 can emit pulses of an appropriate light intensity. Therefore, the intensity of the laser light emitted in one pulse can be set as high as possible within a range that satisfies the eye-safety standard. Furthermore, because the two switch elements 34a and 34b prevent short circuits, the capacity of the storage element 33 can be increased, thereby reducing the voltage value of the DC power supply 33 and suppressing power consumption of the circuit. This allows for a wider range of distance measurement while achieving low power consumption.

[0048] However, in the configuration of the reference example, as shown in FIG. 2 , two switch elements are connected in series to the laser light source 11. Therefore, when the switch elements 34a and 34b are opened, the connection between the two switch elements 34a and 34b becomes floating. This makes it easy for noise to be superimposed on the input of the switch element on the laser light source side. Therefore, if there is a time lag in the drive signals (switch opening / closing signals) input to these switch elements 34a and 34b, the excess charge in the storage element 33 moves back and forth between the storage element 33 and the switch element 34b, causing ringing. This generates high-level noise at the input Pa of the switch element 34a on the laser light source 11 side. If this noise exceeds the rated capacity of the switch element 34a, the switch element 34a may malfunction.

[0049] In this embodiment, a configuration for further solving such problems is provided in the light emitting device 2. This configuration will be described below.

[0050] Fig. 4 is a circuit diagram showing the configuration of the light emitting device 2 according to the embodiment 1. As in Fig. 2, the control unit 21 is not shown in Fig. 4.

[0051] As shown in Fig. 4, the light-emitting device 2 according to the first embodiment includes a Zener diode 37 connected between the input terminal Pa of the upstream switch element 34a and ground. The Zener diode 37 blocks the drive signal output from the driver 35a from reaching ground and directs high-level noise exceeding the voltage level of the drive signal to ground. The voltage at which the Zener diode 37 begins to conduct (breakdown voltage or Zener voltage) is set to be higher than the voltage level of the drive signal but lower than the rated voltage of the switch element 34a. This prevents the switch element 34a from failing due to high-voltage noise even if ringing occurs, as described above.

[0052] Fig. 5(a) is a graph showing the simulation results of the voltages generated at the input parts Pa and Pb of the two switch elements 34a and 34b when there is no time lag between the drive signals input to the two switch elements 34a and 34b in the configuration of the reference example shown in Fig. 2. Fig. 5(b) is a graph showing the simulation results of the voltages generated at the input parts Pa and Pb of the two switch elements 34a and 34b when there is a time lag between the drive signals input to the two switch elements 34a and 34b in the configuration of the reference example shown in Fig. 2.

[0053] 5(a) and 5(b), the solid line indicates the drive signal input to the input port Pa of the upstream switch element 34a, and the dashed line indicates the drive signal input to the input port Pb of the downstream switch element 34b. The rated voltage of the switch elements 34a and 34b is 6 V. In FIG. 5(b), the drive signal input to the input port Pa is delayed by 2 nsec from the drive signal input to the input port Pb. The time widths of the drive signals are the same.

[0054] 5A, when there is no time lag between the drive signals input to inputs Pa and Pb, noise is superimposed on input Pa near the end of the drive signal. However, because the voltage level of this noise is significantly lower than the rated voltage (6 V) of switch element 34a, this noise will not cause a malfunction in switch element 34a.

[0055] In contrast, as shown in Fig. 5(b), when there is a time lag between the drive signals input to input terminals Pa and Pb, significantly higher noise than in Fig. 5(a) is superimposed on input terminal Pa near the end of the drive signal. As described above, this noise is caused by ringing between switch element 34b and power storage element 33. The voltage level of this noise exceeds the rated voltage (6V) of switch element 34a. Therefore, there is a risk that this noise will cause failure of switch element 34a.

[0056] Fig. 5(c) is a graph showing the simulation results of the voltages generated at the input parts Pa and Pb of the two switch elements 34a and 34b when there is no time lag between the drive signals input to the two switch elements 34a and 34b in the configuration of embodiment 1 shown in Fig. 4. Fig. 5(d) is a graph showing the simulation results of the voltages generated at the input parts Pa and Pb of the two switch elements 34a and 34b when there is a time lag between the drive signals input to the two switch elements 34a and 34b in the configuration of embodiment 1 shown in Fig. 4.

[0057] In Figures 5(c) and 5(d), the solid line indicates the drive signal input to the input port Pa of the upstream switch element 34a, and the dashed line indicates the drive signal input to the input port Pb of the downstream switch element 34b. The rated voltage of the switch elements 34a and 34b is 6 V. In Figure 5(d), as in Figure 5(b), the drive signal input to the input port Pa lags behind the drive signal input to the input port Pb by 2 nsec. The breakdown voltage of the Zener diode 37 shown in Figure 4 is set to a voltage value (here, 5.1 V) included in the range between the maximum value of the drive signal and the rated voltage (6 V) of the switch elements 34a and 34b.

[0058] As shown in Fig. 5(c), if there is no time lag between the drive signals input to inputs Pa and Pb, noise is superimposed on input Pa near the end of the drive signal, just as in the case of Fig. 5(a). However, because the voltage level of this noise is significantly lower than the rated voltage (6 V) of switch element 34a, this noise will not cause a malfunction in switch element 34a.

[0059] Furthermore, as shown in Fig. 5(d), when there is a time lag between the drive signals input to the input terminals Pa and Pb, higher noise is superimposed on the input terminal Pa near the end of the drive signal compared to the case of Fig. 5(c). However, in the configuration of embodiment 1, the action of the Zener diode 37 suppresses the maximum voltage level of the noise to around 5.5 V compared to the reference example shown in Fig. 5(b). Therefore, in the configuration of embodiment 1, this noise does not cause failure of the switch element 34a.

[0060] As described above, according to the configuration of the first embodiment, by connecting the Zener diode 37 between the input terminal Pa of the switch element 34a and the ground, it is possible to suppress noise caused by ringing, thereby preventing the switch element 34a from failing.

[0061] <Effects of First Embodiment> According to the first embodiment, the following effects can be achieved.

[0062] As shown in Figure 4, the light emitting device 2 has two switch elements 34a and 34b connected in series to the laser light source 11, so even if a short circuit occurs in one of the switch elements, the other switch element can properly cause the laser light source 11 to emit pulsed light.

[0063] 4, for the switch element 34a connected downstream to another switch element 34b, a Zener diode 37 (electrical element) is provided to guide high-level noise generated at the input part Pa to which the drive signal (switch opening / closing signal) is input to ground. Therefore, as shown in the simulation result of FIG. 5(d), high-level noise generated by ringing is guided to ground, and it is possible to prevent noise exceeding the rated value from being generated at the input part Pa. This makes it possible to prevent the switch element 34a from breaking down due to noise.

[0064] As described above, according to the light emitting device 2 of the first embodiment, it is possible to more appropriately perform pulsed emission of the laser light source 11 while suppressing failure of the switch elements 34a and 34b.

[0065] Furthermore, with an extremely simple configuration in which the Zener diode 37 is connected between the input section Pa and the ground, it is possible to suppress the occurrence of high-level noise at the input section Pa, and to avoid failure of the switch element 34a.

[0066] 4, the distance measurement device 1 can suppress failure of the switch elements 34a and 34b and can more appropriately emit pulses of light from the laser light source 11. This improves the reliability of the distance measurement device 1.

[0067] Furthermore, as described above, the light intensity of the laser light source 11 can be set as high as possible within the range that satisfies the eye-safe standard while reducing the voltage of the DC power supply 31, thereby achieving low power consumption while widening the range of distances that can be measured, thereby improving the quality of the distance measurement device 1.

[0068] Second Embodiment In a second embodiment, the light emitting device 2 is provided with a configuration for monitoring the drive signals output from the drivers 35a and 35b.

[0069] FIG. 6 is a circuit diagram showing the configuration of a light emitting device 2 according to the second embodiment.

[0070] The light emitting device 2 according to the second embodiment further includes monitor circuits 40a and 40b in addition to the configuration shown in Fig. 4. The configuration other than the monitor circuits 40a and 40b is the same as the configuration shown in Fig. 4.

[0071] The monitor circuit 40a generates a monitor signal for monitoring the state of the drive signal input from the driver 35a to the input section Pa. The monitor circuit 40a includes two resistors 41a and 42a that divide the drive signal, and outputs the voltage at the junction of these resistors 41a and 42a as a monitor signal. The output monitor signal is supplied to the control section 21.

[0072] The monitor circuit 40b generates a monitor signal for monitoring the state of the drive signal input from the driver 35b to the input section Pb. The monitor circuit 40b includes two resistors 41b and 42b that divide the drive signal, and outputs the voltage at the junction of the resistors 41b and 42b as a monitor signal. The output monitor signal is supplied to the control section 21.

[0073] The resistors 41a and 41b are set to the same resistance value. The resistors 42a and 42b are also set to the same resistance value. The resistors 41a and 42a constituting the monitor circuit 40a are set so that the voltage of the monitor signal does not exceed the operating voltage (e.g., 3.3 V) of the control unit 21 (CPU). Similarly, the resistors 41b and 42b constituting the monitor circuit 40b are set so that the voltage of the monitor signal does not exceed the operating voltage (e.g., 3.3 V) of the control unit 21 (CPU). The resistance values ​​of the resistors 41a and 42a and the resistance values ​​of the resistors 41b and 42b are set to high values ​​so that the voltage levels of the drive signals input to the input units Pa and Pb are maintained appropriately.

[0074] The control unit 21 determines whether the drive signals are appropriate based on the monitor signals input from the monitor circuits 40a and 40b, and detects whether the drivers 35a and 35b are operating normally.

[0075] 7A and 7B are time charts showing examples of drive signals input to the two switch elements 34a and 34b, respectively, and FIG. 7C is a time chart showing the drive current of the laser light source 11 in the cases of FIGS. 7A and 7B.

[0076] 7(a) shows the drive signal input to the upstream switch element 34a, and FIG. 7(b) shows the drive signal input to the downstream switch element 34b. In this example, the rise period of the drive signal input to the downstream switch element 34b is longer than normal due to some abnormality. The drive signal input to the upstream switch element 34a is normal. The vertical axes in FIGS. 7(a) and 7(b) represent the voltage value of each drive signal.

[0077] In the configuration of Fig. 6, as in the first embodiment, two switch elements 34a, 34b are connected in series between the laser light source 11 and ground, so even if an abnormality occurs in one drive signal as shown in Fig. 7(b), a drive current with an appropriate pulse width is supplied to the laser light source 11 by the action of the switch element to which the other drive signal is input, as shown in Fig. 7(c). This allows the laser light source 11 to emit pulses appropriately.

[0078] However, in the example of Fig. 7(b), it is assumed that a failure has occurred in the downstream driver 35b, and if operation continues in this state, there is a risk that pulse emission will not be performed properly and eye-safe standards will not be met if a failure occurs in the upstream driver 35b or the upstream switch element 34a. Therefore, if a state such as that shown in Fig. 7(b) occurs, it is necessary to stop emission of light, notify of an abnormality, etc.

[0079] FIG. 8 is a flowchart showing an abnormality detection process for detecting an abnormality in the drivers 35a and 35b (drive signals) based on the monitor signals generated by the monitor circuits 40a and 40b.

[0080] At the timing of pulse emission (S11), the control unit 21 acquires, for each monitor signal, the duration during which the monitor signal maintains a predetermined level or higher (S12). Here, the predetermined level is set to a level that is not affected by noise. Next, the control unit 21 determines whether the duration of each acquired monitor signal is equal to or greater than a threshold value (S13). Here, the threshold value is set to be wider than the specified duration when the monitor signal is normal by an allowable width within which normality is permitted.

[0081] If neither time width is equal to or greater than the threshold (S13: NO), the control unit 21 determines that both drivers 35a, 35b are normal and terminates the process. On the other hand, if at least one of the time widths is equal to or greater than the threshold (S13: YES), the control unit 21 determines that at least one of the drivers 35a, 35b is malfunctioning (S14) and executes abnormality processing (S15).

[0082] In step S15, the control unit 21 stops the pulse emission by, for example, ceasing the output of the control signal to the pulse generating circuit 36. Alternatively, the control unit 21 outputs to an external device (such as a device on the vehicle) that an abnormality has occurred in the drivers 35a, 35b, thereby notifying the external device of the abnormality.

[0083] In steps S13 and S14, the control unit 21 may further detect which driver has a malfunction based on which monitor signal is equal to or greater than a threshold value. In this case, the control unit 21 may further output information identifying the driver in which the malfunction was detected to an external device, and notify the external device of the information.

[0084] If the operating clock of the control unit 21 (CPU) is 500 MHz, the resolution of the time width is 2 nsec. In this case, if a tolerance of about 5 LSB is assumed for the normal time width, the threshold value in step S13 is set 10 nsec wider than the normal time width. Therefore, in this case, if the drive signal is wider than the normal time width by 10 nsec or more, a driver failure can be detected.

[0085] 6, the light emitting device 2 includes two switch elements 34a and 34b and a Zener diode 37. Therefore, similar to the first embodiment, failure of the switch elements 34a and 34b can be suppressed, and pulsed light emission from the laser light source 11 can be performed more appropriately.

[0086] Furthermore, a failure (abnormality in the drive signal) of the drivers 35a, 35b is detected by the monitor signals output from the monitor circuits 40a, 40b, and abnormality processing is performed. This makes it possible to prevent abnormalities in the pulsed light emission of the laser light source 11 due to a failure of the drivers 35a, 35b. Therefore, it is possible to more reliably satisfy the eye-safety standard and perform appropriate pulsed light emission.

[0087] In the above-described second embodiment, the monitor circuits 40a and 40b for detecting failures (abnormalities in the drive signals) of the drivers 35a and 35b are disposed in the light emitting device 2. In contrast, in the third embodiment, the monitor circuits for detecting failures of the switch elements 34a and 34b are disposed in the light emitting device 2.

[0088] FIG. 9 is a circuit diagram showing the configuration of a light emitting device 2 according to the third embodiment.

[0089] The light emitting device 2 according to the third embodiment further includes a monitor circuit 50 in addition to the configuration shown in Fig. 4. The configuration other than the monitor circuit 50 is the same as the configuration shown in Fig. 4.

[0090] The monitor circuit 50 monitors failures of the switch elements 34a, 34b based on the potential at the connection point P1 between the adjacent switch elements 34a, 34b. The monitor circuit 50 includes two resistors 51, 52 that divide the potential at the connection point P1, and outputs the potential at the connection point of these resistors 51, 52 as a monitor signal. The output monitor signal is supplied to the control unit 21.

[0091] The potential of connection point P1 is pulled up to potential V1 via resistor 53. Potential V1 is set to, for example, about 5 V. Furthermore, the connection point of resistors 51 and 52 is connected to a voltage line of potential V2 via diode 54. Potential V2 is set to the operating voltage (for example, 3.3 V) of control unit 21 (CPU). Diode 54 constitutes a protection circuit that protects control unit 21 (CPU) by preventing a voltage exceeding the operating voltage (such as a voltage due to intrusive noise) from being input to control unit 21 (CPU).

[0092] The resistors 51 and 52 are set so that the voltage of the monitor signal does not exceed the operating voltage of the control unit 21 (CPU). The resistance values ​​of the resistors 51 and 52 are set to high values ​​so that, when the switch elements 34a and 34b are in a conductive state, the drive current of the laser light source 11 does not flow into the resistors 51 and 52 as much as possible. For example, the resistors 51 and 52 are set to 200 Ω and 20 kΩ, respectively.

[0093] The control unit 21 detects whether the switch elements 34 a and 34 b are operating normally based on the monitor signals input from the monitor circuit 50 .

[0094] 10(a) to 10(d) are time charts showing simulation results of the drive signal, the applied voltage (drain voltage) of the switch elements 34a and 34b, the monitor signal, and the drive current of the laser light source 11 when the switch elements 34a and 34b are operating normally.

[0095] 10A, the drive signal input to the upstream switch element 34a is indicated by a solid line, and the drive signal input to the downstream switch element 34b is indicated by a dashed line. Also, in FIG. 10B, the applied voltage (drain voltage) of the upstream switch element 34a is indicated by a solid line, and the applied voltage (drain voltage) of the downstream switch element 34b is indicated by a dashed line.

[0096] 10(a) to 10(d), the resistors 51, 52, and 53 in the monitor circuit 50 shown in Fig. 9 are set to 100 kΩ, 200 Ω, and 20 kΩ, respectively, and the voltages V1 and V2 are set to 5 V and 3.3 V, respectively. In addition, the breakdown voltage of the Zener diode 37 is set to 5.1 V, and the voltage of the DC power supply 31 is set to 40 V.

[0097] As shown in Figures 10(a) to (d), under the above conditions, even when the monitor circuit 50 was connected to the connection point P1, no abnormalities occurred in the waveform of the drive current of the laser light source 11, and a drive current with an appropriate time width was obtained.

[0098] 10(c), the monitor signal fluctuates during the period corresponding to the pulse light emission even when no failure occurs in the switch elements 34a, 34b. Therefore, in order to detect failures in the switch elements 34a, 34b by simple processing without erroneous determination, it is preferable to set a mask period ΔT during a period including the pulse light emission period and perform failure detection for the switch elements 34a, 34b during periods other than the mask period ΔT.

[0099] 11(a) to 11(d) are time charts showing simulation results of the drive signal of the downstream switch element 34b, the applied voltage (drain voltage) of the downstream switch element 34b, the monitor signal, and the drive current of the laser light source 11 when a short circuit occurs only in the upstream switch element 34a.

[0100] The simulation conditions are the same as those in Figures 10(a) to 10(d). Here, a short circuit occurs in the upstream switch element 34a at timing T1 indicated by the arrow in Figure 11(c).

[0101] 11(a) to 11(d), even if a short circuit occurs in the upstream switch element 34a, the downstream switch element 34b operates to obtain a drive current of a duration corresponding to the drive signal for the laser light source 11. Furthermore, under the above conditions, even if the monitor circuit 50 is connected to the connection point P1, no abnormality occurs in the waveform of the drive current for the laser light source 11, and a drive current of an appropriate duration is obtained.

[0102] 11C, in response to the short circuit of the upstream switch element 34a, the monitor signal suddenly increased from the normal voltage level (near 300 mV) shown in FIG. 10C to 3.3 V. Therefore, by setting a first threshold between the normal voltage level (near 300 mV) and the increased voltage (3.3 V) and comparing this first threshold with the monitor signal, it is possible to detect a failure (short circuit) of the upstream switch element 34a.

[0103] 12(a) to 12(d) are time charts showing simulation results of the drive signal of the upstream switch element 34a, the applied voltage (drain voltage) of the upstream switch element 34a, the monitor signal, and the drive current of the laser light source 11 when a short circuit occurs only in the downstream switch element 34b.

[0104] The simulation conditions are the same as those in Figures 10(a) to 10(d). Here, a short circuit occurs in the downstream switch element 34b at timing T2 indicated by the arrow in Figure 12(c).

[0105] 11(a) to 11(d), even if a short circuit occurs in the downstream switch element 34b, the upstream switch element 34a operates to obtain a drive current of a duration corresponding to the drive signal for the laser light source 11. Furthermore, under the above conditions, even if the monitor circuit 50 is connected to the connection point P1, no abnormality occurs in the waveform of the drive current for the laser light source 11, and a drive current of an appropriate duration is obtained.

[0106] 12(c), in response to the short circuit of the downstream switch element 34b, the monitor signal suddenly dropped from the normal voltage level (near 300 mV) shown in Fig. 10(c) to 0 V. Therefore, by setting a second threshold between the normal voltage level (near 300 mV) and the voltage after the drop (0 V) and comparing this second threshold with the monitor signal, it is possible to detect a failure (short circuit) of the downstream switch element 34b.

[0107] FIG. 13 is a flowchart showing an abnormality detection process for detecting an abnormality in the switch elements 34 a and 34 b based on the monitor signal generated by the monitor circuit 50 .

[0108] The control unit 21 monitors the voltage value Vm of the monitor signal during periods other than the mask period ΔT (S21). If the voltage value Vm exceeds the first threshold value Vth1 (S22: YES), the control unit 21 determines that a failure has occurred in the upstream switch element 34a (S23) and executes abnormality processing (S26). As described above, the first threshold value Vth1 is set to a voltage value between the voltage value of the monitor signal generated during periods other than the mask period ΔT during normal operation and the voltage value of the monitor signal generated when a failure occurs in the switch element 34a. In the example of FIG. 11(c), the first threshold value Vth1 may be set to, for example, approximately 2 V.

[0109] If the voltage value Vm does not exceed the first threshold value Vth1 (S22: NO), the control unit 21 further determines whether the voltage value Vm is less than the second threshold value Vth2 (S24). As described above, the second threshold value Vth2 is set to a voltage value between the voltage value of the monitor signal generated during a period other than the mask period ΔT during normal operation and the voltage value of the monitor signal generated when a failure occurs in the switch element 34b. In the example of FIG. 12(c), the second threshold value Vth2 may be set to, for example, around 0.15 V.

[0110] If the voltage value Vm is not less than the second threshold value Vth2 (S24: NO), the control unit 21 determines that no failure has occurred in any of the switch elements at this time and ends the process. In this case, the control unit 21 again executes the processes from step S21 onward to monitor whether or not there is a failure in the switch elements 34a, 34b. If the voltage value Vm is less than the second threshold value Vth2 (S24: YES), the control unit 21 determines that a failure has occurred in the downstream switch element 34b (S25) and executes abnormality processing (S26).

[0111] In step S26, the control unit 21 stops the pulse emission by, for example, ceasing the output of the control signal to the pulse generating circuit 36. Alternatively, the control unit 21 outputs to an external device (such as a device on the vehicle) that an abnormality has occurred in the switch elements 34a, 34b, thereby notifying the external device of the abnormality.

[0112] Fig. 14 is a block diagram showing the configuration of a light emitting device 2 according to embodiment 3. For convenience, the configuration of the circuit portion upstream of the switch element 34a is omitted in Fig. 14. The configuration of the circuit portion upstream of the switch element 34a is the same as that in Fig. 9.

[0113] The light emitting device 2 includes a control unit 21, a monitor circuit 50, an ADC (A / D converter) 25, and a communication unit 26 as circuit components.

[0114] The ADC 25 samples the monitor signal (analog signal) input from the monitor circuit 50 at a predetermined sampling frequency, converts it into a digital signal, and outputs the converted digital signal (monitor signal) to the control unit 21. The control unit 21 executes the process of FIG. 13 using the monitor signal (digital signal) input from the ADC 25. If the control unit 21 detects a fault in either of the switch elements 34a, 34b, it stops the operation of the pulse generation circuit 36 ​​and outputs information reporting the fault to an external device via the communication unit 26. This causes the external device to report the fault.

[0115] 14 may be applied to the light emitting device 2 in the second embodiment. In this case, the monitor circuit 50 in FIG. 14 is replaced with monitor circuits 40 a and 40 b, and monitor signals output from the monitor circuits 40 a and 40 b are supplied to the control unit 21 via ADCs.

[0116] 9, the light emitting device 2 includes two switch elements 34a and 34b and a Zener diode 37. Therefore, similar to the first embodiment, it is possible to suppress failure of the switch elements 34a and 34b and to more appropriately emit pulsed light from the laser light source 11.

[0117] Furthermore, a failure in the switch elements 34a, 34b is detected by the monitor signals output from the monitor circuit 50, and abnormality processing is performed. This makes it possible to prevent abnormalities in the pulsed light emission of the laser light source 11 due to a failure in the switch elements 34a, 34b. Therefore, it is possible to more reliably satisfy the eye-safety standard and perform appropriate pulsed light emission.

[0118] Furthermore, the monitor circuit 50 monitors failures of the switch elements 34a, 34b based on the potential at the connection point P1 between the adjacent switch elements 34a, 34b. Specifically, the monitor circuit 50 includes voltage-dividing resistors 51, 52 arranged between the connection point P1 and ground, and supplies a voltage of a predetermined level V1 to the connection point P1 while outputting a voltage divided by the voltage-dividing resistors 51, 52 as a monitor signal. This allows failures of the switch elements 34a, 34b to be properly detected by the monitor signal, as shown in Figures 11(a) to 12(d).

[0119] 13, the control unit 21 performs control based on the monitoring results of the monitor circuit 50 during periods other than the pulse emission period (mask period ΔT). In this way, by excluding the mask period ΔT, during which the monitor signal fluctuates when the switch elements 34a and 34b are normal, from the monitoring target, it is possible to accurately detect failures in the switch elements 34a and 34b through simple determination processing.

[0120] <Modification of Third Embodiment> The configuration of the circuit section shown in FIG. 14 may be modified as shown in FIG.

[0121] In the configuration of FIG. 15 , a comparator 27 and a switch 28 are added to the configuration of FIG. 14 . The comparator 27 is composed of a comparison circuit and a logic circuit, and performs the same processing as steps S22 and S24 of FIG. 13 on the monitor signal (analog signal) input from the monitor circuit 50. The comparator 27 outputs a detection signal to the switch 28 when the monitor signal is greater than the first threshold Vth1 or less than the second threshold Vth2. This opens the switch 28, and the operation of the pulse generating circuit 36 ​​stops. At this time, the control unit 21 sets the comparator 27 to an inactive state during the mask period ΔT. Therefore, during the mask period ΔT, no detection signal is input from the comparator 27 to the switch 28.

[0122] According to this configuration, the switch 28 is opened by analog processing, so that if a failure occurs in at least one of the switch elements 34 a, 34 b, the pulse generating circuit 36 ​​can be stopped more quickly than in the configuration of Fig. 14. Therefore, it is possible to more reliably prevent the laser light source 11 from emitting pulses in an inappropriate light emission state.

[0123] 15, the switch 28 may be located at another position as long as it can stop the pulsed emission of the laser light source 11 when a failure occurs in the switch elements 34a, 34b. For example, the switch 28 may be located between the pulse generating circuit 36 ​​and the drivers 35a, 35b.

[0124] Furthermore, in the processing of FIG. 13, the judgments of steps S22 and S24 are performed during the entire period other than the mask period ΔT, but this judgment does not necessarily have to be performed during the entire period other than the mask period ΔT, and the judgments of steps S22 and S24 may be performed during a period other than the mask period ΔT that corresponds to a predetermined timing.

[0125] <Other Modifications> The configurations of the light emitting device 2 and the distance measuring device 1 can be modified in various ways in addition to the configurations shown in the first to third embodiments.

[0126] For example, in the above-described first to third embodiments, two switch elements 34a and 34b are connected in series to the laser light source 11, but three or more switch elements may be connected in series to the laser light source 11. In this case, although the circuit size increases as the number of switch elements increases, it is possible to more reliably prevent malfunction of the laser light source 11 due to short-circuiting of the switch elements.

[0127] In this case, the monitor circuit 50 shown in the third embodiment may be disposed at each connection point between adjacent switch elements. For example, as shown in FIG. 16, when four switch elements 34a to 34d are disposed between the laser light source 11 and ground, monitor circuits 50a to 50c and ADCs 25a to 25c may be disposed at three connection points P1 to P3, respectively. The configuration of the monitor circuits 50a to 50c is the same as that of the monitor circuit 50 shown in the third embodiment. In this embodiment, drivers 35a to 35d are disposed individually for the four switch elements 34a to 34d.

[0128] In this case, Zener diodes 37a to 37c are connected between the ground and the input sections Pa to Pc of the switch elements 34a to 34c, which are connected to the ground side of other switch elements. As a result, similar to the first to third embodiments, when a time lag occurs in the drive signal, it is possible to prevent the switch elements 34a to 34c from failing due to high-level noise generated at the input sections Pa to Pc.

[0129] Note that monitor circuits do not necessarily have to be provided at all of the connection points P1 to P3, as shown in FIG. 16 . For example, monitor circuit 50a may be provided only at connection point P1, and monitor circuits 50b and 50c at the other connection points P2 and P3 may be omitted. In this case, the monitor signal output from monitor circuit 50a will be in the state shown in FIG. 11(c) if a short circuit occurs in switch element 34a, and in the state shown in FIG. 12(c) if all of switch elements 34b to 34d are short-circuited. In other words, if one of switch elements 34b to 34d is normal and the remaining two are faulty, the monitor signal will not be in the state shown in FIG. 12(c).

[0130] However, even in this case, it is possible to detect based on the monitor signal that at least one of the four switch elements 34a to 34d is normal and the other three are in a faulty state. Therefore, it is possible to stop the pulsed light emission of the laser light source 11 before all of the switch elements 34a to 34d fail, and it is possible to prevent pulsed light emission in a state that does not satisfy the eye-safety standard.

[0131] 16, it is possible to detect which of the switch elements 34a to 34d has failed based on the state of the monitor signals from the monitor circuits 50a to 50c. To ensure the reliability of pulsed light emission more reliably, it is preferable to increase the number of connection parts P1 to P3 to which the monitor circuits 50 are applied as much as possible.

[0132] Furthermore, in the above-described first to third embodiments, a Zener diode 37 is used as the electrical element that guides high-level noise exceeding the voltage level of the drive signal (switch opening / closing signal) to ground, but other elements may also be used as this electrical element. For example, as shown in FIG. 17 , a varistor 38 may be used as this electrical element. In this case, too, the voltage at which the varistor 38 begins to conduct (varistor voltage) is set higher than the voltage level of the drive signal and lower than the rated voltage of the switch element 34a. This makes it possible to prevent the switch element 34a from breaking down due to high-voltage level noise, even if ringing occurs due to a time lag between the two drive signals, as described above.

[0133] In the above-described first to third embodiments, the pulse signal output from one pulse generating circuit 36 ​​is input to two drivers 35 a, 35 b, but a pulse generating circuit may be provided for each driver 35 a, 35 b, and a pulse signal may be input from each pulse generating circuit to each driver individually. In this case, the two pulse generating circuits are controlled by the control unit 21 to output pulse signals simultaneously.

[0134] In addition, in the above embodiments 1 to 3, drivers 35a and 35b are arranged for each switch element 34a and 34b, but a configuration in which a drive signal output from one driver is input to each switch element 34a and 34b may also be used.

[0135] Furthermore, both the monitor circuits 40 a and 40 b shown in the second embodiment and the monitor circuit 50 shown in the third embodiment may be applied to the light emitting device 2 .

[0136] In addition, in the above-described first to third embodiments, the power storage element 33, the laser light source 11, and the switch elements 34a and 34b are arranged in this order along the direction of the drive current flow, but the arrangement order is not limited to this. For example, the power storage element 33, the switch elements 34a and 34b, and the laser light source 11 may be arranged in this order along the direction of the drive current flow.

[0137] In the above-described first to third embodiments, the light emitting device 2 is mounted on a so-called flash type distance measuring device 1 that simultaneously irradiates the entire distance measuring area A10 with light. However, the light emitting device 2 is not limited to this, and may be mounted on a distance measuring device that scans a line beam in the short side direction or a distance measuring device that scans a point beam in two-dimensional directions.

[0138] In addition, the embodiments of the present invention can be modified in various ways as appropriate within the scope of the technical ideas set forth in the claims.

[0139] REFERENCE SIGNS LIST 1 distance measuring device 2 light emitting device 3 light receiving section 11 laser light source 12 projection optical system 21 control section 33 power storage element 34a, 34b, 34c, 34d switch element 37, 37a, 37b, 37c zener diode (electrical element) 38 varistor (electrical element) 40a, 40b monitor circuit 50, 50a, 50b, 50c, 50d monitor circuit 51, 52 resistor (voltage dividing resistor) Pa, Pb, Pc input section P1, P2, P3 connection section

Claims

1. A light emitting device comprising: a laser light source; a storage element for supplying a drive current to said laser light source; a plurality of switch elements connected in series to said laser light source; and an electric element arranged between an input section of a switch opening / closing signal and ground for a switch element that has another switch element connected at least downstream thereof, and that directs high-level noise exceeding the voltage level of said switch opening / closing signal to ground.

2. A light emitting device according to claim 1, wherein the electrical element is a Zener diode.

3. A light emitting device according to claim 1, wherein the electrical element is a varistor.

4. The light emitting device according to any one of claims 1 to 3, further comprising a monitor circuit for monitoring a failure of the switch element.

5. A light emitting device according to claim 4, wherein the monitor circuit monitors for failure of the switch element based on the potential of the connection part of the adjacent switch element.

6. A light emitting device according to claim 5, wherein the monitor circuit includes a voltage dividing resistor disposed between the connection section and ground, and supplies a predetermined level of voltage to the connection section while outputting a voltage divided by the voltage dividing resistor as a monitor signal.

7. A light emitting device according to any one of claims 4 to 6, further comprising a control unit that determines whether the switch element has failed based on the monitoring results of the monitor circuit, and the control unit makes a determination based on the monitoring results of the monitor circuit during a predetermined period other than the pulse light emission period.

8. A light emitting device according to any one of claims 1 to 3, further comprising a monitor circuit for monitoring the presence or absence of an abnormality in the switch opening / closing signal input to the input section of each of the switch elements.

9. A distance measuring device comprising: a light emitting device according to any one of claims 1 to 8; a projection optical system that projects pulsed light emitted from the light emitting device onto a target area; and a light receiving unit that receives reflected light of the pulsed light from an object.