Abstract:
A control circuit responsive to a flash activating signal of a camera for the synchronous or alternate activation of at least two flash devices is disclosed. Each flash device includes a flash generator including a voltage source and at least one flash unit. An input of the control circuit is adapted to couple with the camera and receive the flash activating signal. Outputs of the control circuit are adapted to electrically couple to control inputs of the flash generators. The control circuit includes a switching device adapted to drawn a supply voltage for the control circuit from the respective voltage sources in the flash generators.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a National Stage Application of PCT/EP2008/006503, filed Jul. 30, 2008, which designates the United States and claims the priority of German Patent Application No. 20 2007 012 203.9, filed on Aug. 29, 2007. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention concerns a control circuit for the synchronous or alternate activation of at least two flash devices each consisting of a flash generator and at least one flash means, by flash activating signals of a camera at the input of the control circuit via outputs of the control circuit electrically connected to the control inputs of the flash generators. 
         [0003]    Control circuits of this kind are used in particular in fields of professional photography in connection with studio flash systems, the use of such control circuits not being confined to the fields mentioned. Usually the flash systems comprise several flash means which are supplied via corresponding flash generators to activate flashes. The control inputs of the flash generators are in turn connected to the outputs of the control circuit, so that for activating flashes, for example by a camera, by corresponding flash request signals at the input of the control circuit flash activating signals are generated at the control circuit outputs. 
         [0004]    The maximum number of flashes that can be activated per unit of time is limited by the recharging time of the individual flash generators. After activation of a flash and the associated discharge of the storage elements of the respective flash generator, a length of time passes for recharging the storage elements until the flash generator is ready for activating a flash again. 
         [0005]    From German patent document DE 4433686C1 is known a studio flash system having a plurality of function elements which comprise one or more electronic control means for the control of several flash devices. It is known that different configurations of a flash system consisting of power supply units, storage elements, flash heads and electronic control means can be controlled electronically. In this case, for example an electronic control means is supplied by a power supply unit which controls several flash devices composed of storage elements and flash heads. 
         [0006]    The voltage supply of the known electronic control means is via external or internal power supply units or other voltage sources, for example batteries or accumulators. A drawback is that the electronic control means have to be supplied with electricity via an additional power supply unit or, in the case of an integrated power supply unit, via additional lines. If batteries or accumulators are used, the control means must first be equipped with full batteries or charged up before use. If they are not used for a long time, it must be assumed because of automatic discharge that the electronic control means is not ready for use after some time of not being used because of discharged batteries or accumulators. A further drawback arises from the fact that the electronic control means must have an interface configured and adapted to the respective configuration of power supply units, storage elements and flash heads. 
       SUMMARY OF THE INVENTION 
       [0007]    It is therefore the object of the present invention to provide an easy-to-handle, reliable, universal control circuit for the synchronous or alternate activation of at least two flash devices. 
         [0008]    The object is achieved according to the invention by the control circuit described hereinbefore by the fact that the control circuit is assigned switching means which are constructed and designed in such a way that the supply voltage of the control circuit is drawn from the respective flash generators. Via the outputs of the control circuit connected to the control inputs of the flash generators, both activation of the flash devices and supply of the control circuit itself with voltage take place. Thus in a surprisingly simple manner supply of the control circuit is guaranteed without using further power supply units or voltage sources. Further, the control circuit is universally usable, as the control means can be connected to various flash generators of which the control inputs can vary within a wide range with respect to both the amounts of signal levels and the amounts of input impedances. 
         [0009]    An appropriate development of the invention is characterised in that the switching means are connected to at least two outputs of the control circuit for obtaining a supply voltage. This ensures that the supply voltage is provided for the control circuit even in the event that sufficient voltage for operating the control circuit cannot be obtained for a certain length of time at one of the outputs during activation of a flash device. In this case the supply voltage of the control circuit is obtained via a non-activating flash device. 
         [0010]    A preferred embodiment is distinguished in that the switching means connected to the outputs of the control circuit for obtaining the supply voltage in each case have at least one current-limiting resistor connected in series with a diode. The current-limiting resistor ensures that the current consumed by the control circuit does not exceed an upper limit value, so that possible erroneous activations of the flash generators are reliably prevented. 
         [0011]    In a further advantageous embodiment of the invention, in particular the current-limiting resistors have larger dimensions than the quantity of the input impedance of the respective flash generator control inputs, so that the direct current (DC) voltage provided at the control input by the flash generator via an internal resistor falls insignificantly due to extraction of a current for the operating voltage supply of the control circuit. Accidental activation due to a fall in DC voltage at the control inputs of the flash generators is therefore reliably avoided. 
         [0012]    Advantageously, the control circuit is arranged in a protected manner in a watertight housing, so that the circuit is protected in particular from moisture-related leakage currents, and therefore error-free operation of the control circuit is ensured. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Further preferred or appropriate features and arrangements of the invention are apparent from the subsidiary claims and the description. Particularly preferred embodiments are described in more detail with the aid of the attached drawings. The drawings show: 
           [0014]      FIG. 1  a schematic view of the basic construction of a flash system by way of example, and 
           [0015]      FIG. 2  a schematic view of an embodiment of the control circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  shows a schematic view of the basic construction of a flash system  35  by way of example. The flash system  35  comprises a camera  12 , a control circuit  11  and flash devices  18  which each consist of a flash generator  16  and a flash means  17 . The camera  12  is connected to the input  10  of the control circuit  11  by lines  13  for the provision of flash activating signals. The outputs  14  of the control circuit  11  are each connected by further lines  13  to control inputs  15  of the flash generators  16  which, in conjunction with a flash means  17  connected to the output of the respective flash generator  16 , form the flash devices  18 . To activate the flash devices  18 , starting from the camera  12  via the lines  13  the input  10  of the circuit  11  has applied to it a flash activating signal which causes the control circuit  11  to generate flash activating signals for activating at least one flash device  18  at at least one of the outputs  14  of the control circuit  11 . 
         [0017]    The control inputs  15  of the flash generators  16  provide a DC voltage of between 5 and 15 V with respect to their earth potential via a high-resistance internal resistor which is usually greater than or equal to one megohm. Activation of the respective flash generator  16  therefore takes place by means of a flash activating signal by the fact that the control input  15  of the respective flash generator  16  is set to earth potential via the output  14  of the control circuit  11  which is connected to the control input  15  by lines  13 . 
         [0018]    The manner of activating the flash devices  18  in time can in this case be selected: for synchronous activation, with each flash activating signal at the input  10  of the circuit  11  all flash devices  18  are activated via simultaneously generated flash activating signals at all outputs  14  of the control circuit  11 , whereas for alternate activation, flash activating signals succeeding each other in time at the input  10  of the circuit  11  lead to the sequential generation of flash activating signals at one output  14 , each, of the control circuit  11  each, so that the flash devices  18  are alternately activated one after the other. 
         [0019]    In a further preferred embodiment, the number of outputs  14  of the control circuit  11  and flash devices  18  is not, as shown by way of example in  FIG. 1 , limited to two each time, but rather can be extended to any number N of outputs  14  for controlling a number N of flash devices  18 . According to the preferred embodiment of the control circuit  11  with N outputs  14  of the control circuit  11  and N flash devices  18 , in the case of synchronous activation of flash devices  18  any number N-x of flash devices  18  are activated by simultaneous generation of flash activating signals at n outputs  14  of the control circuit  11 . In the case of alternate activation of flash devices  18 , the N flash devices  18  are alternately activated one after the other, with a free choice of the sequence of activations. Also several groups of flash devices  18  can be activated one after the other, in which case the flash devices  18  belonging to a respective group are each activated simultaneously. 
         [0020]    A supply voltage  19  for the control circuit  11  is obtained via the outputs  14  of the control circuit  11  which are connected to the control inputs  15  of the flash generators  16 . The outputs  14  of the control circuit  11  therefore fulfil two functions. Firstly, via the outputs  14  of the control circuit  11  the corresponding flash devices  18  are activated via the control inputs  15  of the flash generators  16 , and secondly the control circuit  11  obtains the required supply voltage  19  for operating the control circuit  11  from the flash generators  16 . The control circuit  11  is for this purpose assigned correspondingly constructed and designed switching means  20  which are in each case connected to at least two outputs  14  of the control circuit  11 . Thus, apart from the synchronous activation of all flash devices  18 , it is ensured that the supply voltage  19  is obtained via the output  14  of the control circuit  11  to which a flash activating signal is not applied. 
         [0021]    In  FIG. 2  the construction of the control circuit  11  is shown schematically with the aid of a construction by way of example for controlling two flash devices  18 . The outputs  14  of the control circuit  11  are each connected to switching means  20  for obtaining the supply voltage  19  of the control circuit  11 . Each of the switching means  20  comprises a circuit consisting of a diode  21  and at least one current-limiting resistor  22  connected in series. The diodes  21  prevent feedback of the supply voltage  19  to the outputs  14  of the control circuit  11 , while the current-limiting resistors  22  prevent an unwanted voltage drop at the internal resistor in the control input  15 , so that it is ensured that the current consumption of the control circuit  11  via the outputs  14  of the control circuit  11  from the control inputs  15  of the flash generators  16  does not lead to erroneous activation of one of the flash devices  18  as a result of an unwanted drop in voltage at one of the outputs  14  of the control circuit or at one of the control inputs  15  of the flash generators  16 . To ensure this, the quantity of the current-limiting resistors  22  is at least as great as the quantity of the input impedance of the respective control input  15  of the flash generators  16 . 
         [0022]    Each of the outputs is connected via a circuit consisting of a diode  23  and a switching means  25  connected in series, to the input  10  of the control circuit  11 , so that when the switching means  25  becomes conductive, flash activating signals of a camera  12  are conducted via the input  10  of the control circuit  11  to the respective outputs  14 . In the embodiment given by way of example in  FIG. 2 , the switching means  25  are designed as n-conducting field-effect transistors, as they can be controlled without power, so to speak, and therefore the current consumption of the control circuit  11  is reduced to a very great extent. However, the design of the switching means  25  is not limited to field-effect transistors, but instead a large number of components can be used as switching means  25 , for example IGBTs, MOSFETs, bipolar transistors or other non-semiconductor-based switching means  25 . In particular the use of transistors is not limited to n-conducting types. 
         [0023]    The control inputs  24  of the switching means  25  are connected to the inverting output  26  and the non-inverting output  27  of a flip-flop  28 , so that for alternate activation of the flash devices  18  one of the switching means  25  is in the conducting state, and the other switching means  25  is in the non-conducting state, and vice versa. As a result, one of the outputs  14  of the control circuit  11  is connected in conductive relationship to the input  10  of the control circuit via the respective switching means  25 . A flash activating signal at the input  10  of the control circuit  11  is therefore conducted via the corresponding output  14  of the control circuit  11  to the corresponding control input  15  of one of the flash generators  16  depending on the switching state of the switching means  25 , for activating one of the flash devices  18 . 
         [0024]    The construction of the flip-flop  28  by means of a connection of the clock input  29  of the flip-flop  28  to the inverting output  26  of the flip-flop  28  causes a change of logic level of the flip-flop outputs  26 ,  27  with each falling edge at the clock input  29  of the flip-flop  28 . The input  10  of the control circuit  11  is connected to the clock input  29  of the flip-flop  28 , so that flash activating signals starting from a camera  12  via the input  10  of the control circuit  11  cause the flip-flop  28  to be triggered. In other words, the logic level of the flip-flop outputs  26 ,  27  changes alternately with each flash activating signal at the input  10  of the control circuit  11 . Via a pull-up resistor  30 , the clock input  29  of the flip-flop  28  and hence also the input  10  of the control circuit  11  is connected to the potential of the supply voltage  19  for flash activating signals with respect to the earth potential terminal  31  of the control circuit. Therefore the input  10  of the control circuit  11  is firstly terminated in the event that no camera  12  is connected to the input  10  of the control circuit  11 , and secondly a corresponding voltage is made available in open collector technique to the output circuits which are usually used for flash activation in cameras  12 . In a further preferred embodiment of the invention, the pull-up resistor  30  is dimensioned higher in resistance than the smallest current-limiting resistor  22  to avoid collapse of the supply voltage  19  during application of a flash activating signal to the input  10  of the control circuit  11 . 
         [0025]    The input  10  and/or the outputs  14  of the control circuit  11  have parallel-connected voltage-limiting means  34  for protection against overvoltages. Zener diodes are in particular used here, but of course other means for limiting overvoltages can also be used such as for example varistors, suppressor diodes or the like. 
         [0026]    For the synchronous activation of at least two flash devices  18 , the outputs  14  of the control circuit  11  are each connected conductively to the input  10  of the control circuit  11  via a diode  32  connected for positive potential differences between corresponding output  14  and input  10  of the control circuit  11 , by a switch  33  arranged in series therewith. If the switch  33  is in the closed state, both outputs  14  of the control circuit  11  are connected via the diodes  32  to the input  10  of the control circuit  11 . A flash activating signal of a camera  12  at the input  10  of the control circuit  11  thus passes via both outputs  14  of the control circuit  11  to the control inputs  15  of the flash generators  16  for synchronous activation of the flash devices  18 .