GB2150709A

GB2150709A - Apparatus for triggering a dedicated slave electronic flash device

Info

Publication number: GB2150709A
Application number: GB08429834A
Authority: GB
Inventors: William Alan Bodvel Evans
Original assignee: KENT SCIENT IND PROJECTS
Current assignee: KENT SCIENT IND PROJECTS
Priority date: 1983-11-26
Filing date: 1984-11-26
Publication date: 1985-07-03
Also published as: GB8331648D0; GB8429834D0; GB2150709B

Abstract

An apparatus (1) for triggering a dedicated slave electronic flash device (2) has a photo detector (12) arranged to detect the duration of a light pulse from a master flash light source (3). The photo detector may be directed either toward the master flash light source (3) or an area of the subject (14) illuminated by the master flash light source and provides a signal to trigger the slave device (2) on when light output is detected from the master flash light source (3). The photo detector (12) is arranged in an electrical circuit such that upon detecting the extinction of the master flash light source it provides a triggering quenching signal for the slave flash device (2). The apparatus (1) may have its own internal power supply but preferably utilises the battery power source of the dedicated slave electronic flash device (2). The polarity of the quenching signal may be varied in accordance with the requirements of the slave flash device utilised. <IMAGE>

Description

SPECIFICATION Apparatus for triggering a dedicated slave electronic flash device This invention relates to an apparatus for triggering a dedicated slave electronic flash device for use in photography.

It is well known to use an electronic flash light source in photography in which the flash light source is electrically connected to the camera either by mounting in a "hot shoe" connected to the camera body or in which the flash light source is used remotely from the camera and attached to the camera by an electrically conducting cable, in both situations the flash is fired in unison with the shutter mechanism opening. It is also known for flash light sources to contain a photodetector to extinguish the flash when the photodetector determines that a correct amount of light for a particular camera aperture and speed has illuminated a subject.Moreover many modern single lens reflex cameras monitor light received at the flim plane through the camera lens (known as TTL metering) and such camera are utilised with a so-called dedicated flash light source to quench the flash when a preset exposure has been reached.

In many situations a photographer may use a number of electronic flash light sources situated at different locations around the subject. In such instances the flash light sources remote from the camera may be arranged to be triggered to fire upon detection of light from the electronic flash light source electrically connected to the camera and such remote flash light sources are herein referred to as slave electronic flash devices and the flash light source electrically connected to the camera is herein referred to as a master flash light source.With the known arrangement of slave electronic flash devices and a master flash light source, unless means are taken to prevent it, the slave electronic flash devices continue to illuminate a subject after the master flash light source is quenched either by its own integral photo electric detector or if a dedicated flash light source is used, by the TTL metering mechanism of the camera. This problem, which usually leads to over exposure of the subject on the film has hitherto been overcome by using electrical connecting leads extending from the camera to the slave electronic flash devices but such a solution is inconvenient and dangerous since it requires long connected leads trailing around a studio.

Moreover, for location work, such as in animal photography, trailing leads are often dangerous and difficult to locate.

The present invention seeks to provide an apparatus for triggering a dedicated slave electronic flash device in which the aforementioned disadvantage of utilising trailing connecting leads is overcome.

According to this invention there is provided apparatus for triggering a dedicated slave electronic flash device for use in photography, said apparatus comprising a photodetector arranged to detect light from a remote master flash light source, trigger means connected to the photo detector and arranged to trigger the dedicated slave flash device upon a light flash being detected from the remote master flash light source and, also connected to be triggered by said photo detector, means for quenching said dedicated slave flash device in dependence upon said photo detector detecting quenching of said remote master flash light source whereby the period during which said dedicated slave flash device is on is substantially the same as the on period of the remote master flash light source.

Preferabiy said photo detector is supported on a flexible arm such that said photo detector may be directed toward the remote master source or an area illuminated by said remote master source.

Advantageously the trigger means is a first active switch element connected to receive a signal from the photo detector, said element being connected across a source of electrical potential. Conveniently the means for quenching comprises a second active switch element connected to receive signals from said photo detector and arranged to provide a quenching signal for said slave flash device upon detection of the remote master flash light source being quenched, said second switch element being connected across said source of electrical potential.

In a preferred embodiment a field effect transistor (F.E.T.) is connected between the input of the first and second active switch elements for rendering the biassing of the elements substantially independent of variation in potential of said source of electrical potential. Advantageously the first and second active switch elements each comprise a cascaded pair of PNP and NPN transistors and the F.E.T. is connected between the control electrodes of the first transistor of each pair.

In such an emebodiment the quenching signal is produced from the collector electrode of the second transistor of said second active switch element.

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows, in schematic form, an apparatus for triggering a dedicated slave electronic flash device in accordance with this invention in combination with a dedicated slave electronic flash device, a remote camera and associated master flash light source, and Figure 2 shows a circuit diagram of the apparatus in accordance with this invention.

The combination of integers shown in Fig.

1 has an apparatus 1 for triggering a dedi cated slave electronic flash device 2 on and off upon detecting the turn on and turn off of a remote master flash light source 3 electrically connected to the hot shoe of a metering automatic camera 4 such as a Nikon F3. The apparatus has a housing 5 for the electronic components schematically shown in Fig. 2 and on top of the housing is a "hot shoe" 6 into which a foot 7 of the flash device 2 is mounted. Alternatively, for footless flash devices, e.g. hammer head types, the apparatus 1 may be arranged to plug into a TTL cord socket 8 of the slave electronic flash device. The "hot shoe" 6 has a pair of earth potential electrical terminal contacts 9 which also serve to mount the foot 7, and disposed between contacts 9 is a positive terminal contact 10 and a quenching contact 11.Electrical power for the apparatus is derived from the battery supply of the device 2 but it may alternatively be derived from a battery internal of the housing 5. The apparatus 1 also includes a photo detector 1 2 which is connected to the housing 5 by a sheathed electrically conducting flexibled arm 1 3 which arm is arranged to be sufficiently stiff to support and present the photo detector at a required angular position and height. The photo detector may be provided with an optical system (not shown) to present a narrow acceptance angle so as to improve the selectivity of the detector. The remote master flash light source 3 is capable of being triggered on and off in dependence upon the amount of light reflected from a subject 14.As described above, the camera 4 is an automatic single lens reflex camera with through the lens flash metering and the master flash light source is a TTL dedicated flash light source arranged to have the flash duration thereof quenched upon the lens metering system of the camera detecting the desired amount of reflected light :rom a subject. Alternatively any camera could be used with a master flash light source having its own photo detector to quench the light output thereof when the subject illumination is determined to be correct.

The photo detector 1 2 may be pointed at the master flash light source 3 or, is used for example in macrophotography, where the photo detector would be extremely close to the master flash light source 3 so the photo detector 1 2 is directed at an area of the subject primarily illuminated by the light from the master flash light source 3.

In Fig. 2 the positive potential contact 10 is connected to a respective anode of diodes 21, 22 and to a collector electrode of an NPN transistor 23. The cathode of each diode 21 and 22 is connected to a respective capacitor 24, 25 and the combination of the diodes and capacitors are employed to control the voltage supply which in this embodiment is taken to be from a battery housed within the slave electronic flash device 2. The cathode of diode 21 is also connected to the emitter of a PNP transistor 26, one end of a resistor 27 and one end of a further resistor 28, the other ends of the resistors 27, 28 being interconnected by a capacitor 29, the base electrode of the transistor 26 being connected to junction of the resistor 27 and capacitor 29.The cathode electrode of the photo detector diode 1 2 is connected to the junction between the resistor 28 and capacitor 29 with the anode of the photo detector diode 1 2 being connected to the earth potential contacts 9. The cathode of the photo detector 1 2 is also connected to one side of a capacitor 30 the other side of which is connected to the gate electrode of an N-gate field effect transistor (F.E.T) 31, the source electrode of the F.E.T.

31 being connected via a resistor 32 to the junction between the gate electrode and the capacitor 30. The drain electrode of the F.E.T.

31 is connected to the base electrode of transistor 26. The gate electrode of the F.E.T.

31 is also connected to the earth potential contacts 9 through a resistor 33 and diode 37, and the gate electrode is furthermore connected to the base electrode of an NPN transistor 34 the emitter of which is connected to the earth potential contacts 9. The collector of the transistor 34 is connected via a resistor 35 to the cathode of diode 22 and directly to the base of a PNP transistor 36.

The emitter of the transistor 36 is also connected to the cathode of diode 22 and the collector of the transistor 36 is connected to provide a positive potential pulse to the quenching contact 11. The base of transistor 23 is connected to the collector of transistor 26 and the emitter of the transistor 23 is connected to the earth potential contacts 9.

Thus the transistors 26, 23 are connected in cascade as are the transistors 34, 35. The photo detector 1 2 is biassed by selecting the value of resistor 28 and is arranged such that the photo detector 1 2 will not saturate even at peak illumination from the remote master flash light source 3. The F.E.T. 31 and resistor 32 provide a supply voltage indpendent of bias on the bases of transistors 26 and 34.

It will be realised that an important parameter of the circuit shown in Fig. 2 is the time delay between firing or quenching of the master flash light source 3 and the subsequent operation of the dedicated slave electronic flash device 2 and this parameter is controlled by the time constants of integrators formed by resistor 27, capacitor 29 and resistor 33/diode 37 combination and capacitor 30 respectively. These time constants are made as short as possible and are typically in the order of microseconds.

A further important design consideration is to minimise the quiescent consumption of the apparatus so as to reduce the drain upon the battery of the power supplying battery and in the circuit of Fig. 2 this drain has been reduced to microamperes by choosing the F.E.T. 31 to have a low pinch-off voltage.

In operation, when the photo detector 9 detects the commencement of a light flash from the master flash light source 3 so the photo detector 9 conducts bringing the junction of capacitors 29 and 30 down toward earth potential thereby causing transistor 26 to turn on which in turn turns transistor 23 on. The collector-emitter junction of transistor 23 thus substantially short circuits to trigger and fire the dedicated slave electronic flash device 2. When the potential at the junction of the interconnected capacitor plates of capacitors 29 and 30 decreases so the other side of the capacitor plates tends to drop below earth potential which if no steps were taken to prevent it would cause a not inconsiderable delay in enabling the circuit to recover and provide a quenching signal.For this reason diode 37 is provided to raise the potential on the transistor 34 base side of capacitor 30 so that even if a very short duration flash is detected from the remote master flash light source by the photo detector 9 so the transistors 34 and 35 will conduct to provide a positive quenching pulse to the quenching contact 8 in determination of the photo detector 1 2 detecting quenching of the master flash light source 3.

It will now be realised that the present invention provides an apparatus for triggering a dedicated slave electronic flash device (a) which is capable of being used remotely from a master flash light source (b) which is triggered on and off in dependence upon the duration of light from the master flash source without the necessity of utilising trailing leads and (c) in which correct illumination of a subject may be determined by either the through the lens metering system of the camera or a photo detector associated with the master flash light source. It will also of course be realised that a plurality of apparatus' each in accordance with the invention may be used to illuminate a subject and that the polarity of the components can be changed to suit the requirements of the polarity required for the quenching contact 11.

Claims

1. Apparatus for triggering a slave electronic flash device for use in photography, said apparatus comprising a photodetector arranged to detect light from a remote flash light source, trigger means connected to the photo detector and arranged to trigger the dedicated slave flash device upon a light flash being detected from the remote master flash light source and, also connected to be triggered by said photo detector, means for quenching said dedicated slave flash device in dependence upon said photo detector detecting quenching of said remote master flash light source whereby the period during which said dedicated slave flash device is on is substantially the same as the on period of the remote master flash light source.

2. Apparatus as claimed in claim 1 wherein said photo detector is supported on a flexible arm such that said photo detector may be directed toward the remote master source or an area illuminated by said remote master source.

3. Apparatus as claimed in claim 1 or 2 wherein the trigger means is a first active switch element connected to receive a signal from the photo detector, said element being connected across a source of electrical potential.

4. Apparatus as claimed in claim 3 wherein the means for quenching comprises a second active switch element connected to receive signals from said photo detector and arranged to provide a quenching signal for said slave flash device upon detection of the remote master flash light source being quenched, said second switch element being connected across said source of electrical potential.

5. Apparatus as claimed in claim 4 wherein a field effect transistor (F.E.T.) is connected between the input of the first and second active switch elements for rendering the biassing of the elements substantially independent of variation in potential of said source of electrical potential.

6. Apparatus as claimed in claim 5 wherein the first and second active switch elements each comprise a cascaded pair of PNP and NPN transistors and the F.E.T. is connected between the control electrodes of the first transistor of each pair.

7. Apparatus as claimed in claim 6 wherein the quenching signal is produced from the collector electrode of the second transistor of said second active switch element.

8. Apparatus as claimed in any preceding claim in combination with a dedicated slave electronic flash device.

9. Apparatus as claimed in claim 8 when appended to claim 3 or 4 wherein the slave electronic flash device is in electrical communication with said apparatus and the source of electrical potential is housed in the dedicated slave electronic flash device.

10. Apparatus as claimed in claims 8 or 9 in combination with a master flash light source.

11. Apparatus for triggering a slave electronic flash device substantially as herein described with reference to and as shown in the accompanying drawings.