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BUILD TIHS SLAVE FLASH TRIGGER When your camera's flashgun fires, this simple device automatically detects the flash and triggers a second (slave) flashgun. It's easy to build and can be put together for less than $20. By JOHN CLARKE & GREG SWAIN Commercial slave flash triggers are expensive so you can save money by building one yourself. As you'll see, only a handful of parts is required and the unit will only take an hour or so to put together. But why would you want one? Let's find out. While a single flashgun may be adequate in some situations, it often causes harsh shadows because the light is only coming from one direction. Alternatively, the photograph can appear to lack contrast and depth, particularly if 01 FPT100 B A + HOT SHOE ADAPTOR 470k SLA~~Jh~SH VIE~:ea:oM ~ ~ Fig.t: the circuit uses a phototransistor (Qt), an SCR and a few resistors. When a high-intensity flash occurs, Qt conducts and turns on the SCR which triggers the slave flashgun via the hot shoe adapter terminals. all the light is coming directly from the front. The best way around these problems is to employ a second flashgun. By correctly positioning and aiming this second flash, you can fill in any shadows that would otherwise be created and greatly improve the "depth" of your photographs. You can also use a second (or even a third or fourth) flash to light the background behind a subject. Of course, for this scheme to work, the multiple flash units must be made to fire at the same time. This can be done by simply connecting them in parallel to the camera's flash socket via a multiway adaptor cable. Although this technique generally works OK, cable-connected flashguns do have their limitations. For example, you may be prevented from positioning a flashgun exactly where you want it because the cable isn't long enough. Cables are also a nuisance - they're easy to trip over, they get in the way, and they're often unreliable. A far better method is to use an electronic slave flash trigger such as the unit to be described. This device automatically triggers a APRIL 1988 47 PARTS LIST 1 plastic case, 83 x 54 x 28mm 1 PCB, code SC13-1-588, 44 x 26mm; or 1 piece of Veroboard, 44 x 26mm 1 Scotchcal label (optional) 1 SPOT toggle switch 1 9V battery 1 battery clip to suit 1 LED bezel 4 rubber feet 1 hot shoe adaptor (available from photographic stores) 1 C106D SCR 1 FPT1 00 phototransistor 1 470k0 0.25W resistor 2 1 kO 0 .25W resistors Miscellaneous All the parts fit easily inside a small plastic jiffy case. The PCB is held in place by the phototransistor which is clipped into a LED mounting bezel. Fig.2: wiring diagram for the PCB version. The SCR is mounted flat against the PCB while the phototransistor should be stood off the board by about 10mm. slave flashgun whenever it detects high-intensity light from the primary flashgun. This eliminates trailing cords which means that you can place the slave flashgun anywhere you want. The performance of our "homebrew" unit is on a par with expensive commercial units. It has excellent sensitivity, will not false trigger, and uses readily available parts. How it works Take a look now at Fig.1. The circuit is really very simple. It uses a phototransistor (Ql), an SCR (C106D1), three resistors and a 9V battery. The SCR takes the place of the camera contacts and is wired 48 S/IJCON CHIP 0 O , CA O o o c \.cl 0 000 00 0 000 Solder, screw for hot shoe adaptor, shielded cable, hookup wire. of this resistor (ie, by tying the base more firmly to ground). The lkO resistor between the gate and cathode of the SCR prevents the SCR from false triggering if high voltages are applied between the anode and cathode. Power for the Slave Flash Trigger is derived from a 9V battery. On/off switch Sl disconnects power when the Slave Flash Trigger is not in use. Assembly Fig.3: parts layout for the alternative Veroboard version. across the trigger circuit of the flashgun. Normally, the SCR is off and so the flashgun is able to charge to its trigger voltage. Phototransistor Ql is used to monitor the light level. When a high-intensity flash occurs, Ql briefly conducts and supplies gate current to the SCR. This causes the SCR to turn on which then triggers the slave flashgun via the hot shoe adapter terminals. Once the flashgun has triggered, the SCR quickly turns off again. The reason for this is that the current in the flashgun circuit quickly falls below the SCR's holding current. The 470k0 resistor at the base of Ql sets the sensitivity of the circuit. If you wish, you can reduce the sensitivity simply by reducing the value We made up two versions of the Slave Flash Trigger - one on a small PCB (printed circuit board) and the other on Veroboard. Fig.2 shows the wiring diagram for the PCB version while Fig.3 shows the Veroboard version. The PCB for this project is coded SC13-1-588 and measures 44 x 26mm. Install the parts as shown in Fig.2, with phototransistor Ql mounted about 10mm proud of the PCB. The body of the SCR should be mounted flat against the PCB, and its leads bent at right angles to mate with the appropriate holes. Assembly of the Veroboard version is similar to the PCB version. You can make cuts in the copper tracks using an oversize drill bit. Be careful when using Veroboard though - it's very easy to make a mistake. We mounted the completed board assembly in a small plastic ;t Above are actual size artworks for the PC pattern and the front panel. The board should only take a few minutes to assemble, no matter which version you choose to build. The phototransistor must be clipped into a LED bezel at one end of the case so that it is exposed to the ambient light. case measuring 83 x 54 x 28mm. This case is used upside down, with the lid becoming the base. The hot shoe adaptor is secured to the top of the case using a screw, while the on/off switch is mounted at one end. A third hole is drilled at the other end of the case to accept a mounting bezel for the phototransistor. The board is then supported ver- Mailbag On examination however, the article in question was found to suffer from several inaccuracies which detracted from the quality of the information provided. The following aspects are of particular concern: continued from page 3 cold.climates, weldm esh will quickly ice up and then present very high windage. Similarly, solid sheet bow-ties have much higher windage and work no better than the vestigial bow-ties we have specified. As far as our dimensioned diagrams are concerned, we try to include all the relevant dim ensions without making t'he diagrams too crowded and difficult to follow. Hazards of the MEN system I refer to an article entitled, "Your House Wiring Could Kill You" which was recently brought to the attention of this Department. The article appeared in the November 1987 issue of SILICON CHIP. As you may be aware, the Department of Energy has a responsibility to promote correct electrical installation practices and the safe end use of electricity in New South Wales. Magazine articles which assist in the promotion of electrical safety are always welcomed by this Department and your interest in this important issue is commended. (1). The incorrect statement that the resistance of the return path through the ground is consistently lower than the resistance of the neutral return path in the MEN system, resulting in substantial currents flowing in the earthing conductors of the installation under normal operating conditions. (2). The implication that the loss of the earth connection and neutral connection due to corrosion can be expected as a normal course of events. (3). The suggested method of calibration for the makeshift clamp-on meter requiring contact with live single insulated conductors. (4). The photograph (page 80) depicting the makeshift clamp-on meter utilising the 'old relay' is misleading. It does not show the live conductor passing unbroken through the relay; it could be misconstrued that the live conductor is connected to the relay frame or coil. tically in the case when the phototransistor is clipped into the bezel (see photograph}. Figs.2 and 3 show how to connect the board to the on/off switch, battery and hot shoe adaptor. Note that some flashguns do not include a hot shoe plate. In that case, just delete the hot shoe adaptor and connect a cable and plug (to suit the flashgun trigger socket} instead. Just remember - the centre terminal of the plug is positive (ie, it goes to the anode of the SCR}. To test the unit, simply connect it to a flashgun, switch on, and check that the flashgun fires whenever the primary flash connected to the camera fires. You should find that the unit will trigger reliably at distances of up to 15 metres or more. Finally, here's a rather unusual application for your slave flash trigger. If you have a motor drive, you will probably find that the slave flash trigger will trigger that as well (depends on the motor drive}. This means that you can set a motor driven camera up some distance away and trigger it by setting off a flash. 1b (5). The advice that the consumer may repair the main earth connection is not supported. Such repairs are within the scope of electrical wiring as defined by legislation; consequently the involvement of a licensed electrical mechanic is mandatory. The above criticism is offered in the interest of the consuming public. You would no doubt agree that accuracy is essential if the article is to expect credibility from the technically informed reader and avoid panic in the technically uninformed. It is recommended that in future any information of this nature be thoroughly checked by a suitable qualified person prior to publication. N.C.Watson NSW Department of Energy Comment: we are not happy with this statement from the NSW Department of Energy. It is more concerned with criticising the detail of the article rather than addressing the real problem, a dangerous j]aw in the M.E.N. system. We have written to the Secretary to ask if the Department has a policy on this topic and if not, whether such a policy will now be formulated. We will publish details as they come to hand. APRIL 1988 49