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By JIM ROWE Want to use an external flash unit with your new hi-res digital or film camera but it doesn’t have a trigger socket or “hot shoe”? Cheer up, this new slave flash trigger will let you do it and it will cope with those cameras which only work in multiple-flash “red-eye reduction” mode. You can build it for a fraction of the cost of similar “smart” trigger units, too. M OST OF THE LATEST digital still and film cameras have a built-in electronic flash, which at first glance seems great. The trouble is that it’s almost impossible to take a good profession­al photo with only a single flash. They’re OK for “happy snaps” but that fixed flash, right next to the lens and pointing in the same direction is a big problem. It gives very “flat” lighting and very dark shadows. For much better modelling and control of shadows, you really need at least one additional source of light and/or a system of light diffusion. But 60  Silicon Chip neither of these options is easy with most digital cameras, not only because of their fixed for­ward-facing internal flash but because they generally don’t have a “hot shoe” or conventional flash contact socket to trigger an external flash. So the only way to trigger a second flash with these cam­eras is to use a slave flash trigger unit. This has an optical sensor which detects when the camera’s own flash operates, to trigger an external “slave” flash. But there is a further complication with many new digital cameras. Their internal flash often operates only in “red-eye reduction” mode, where the flash gives not just one single pulse of light but multiple flashes. There may be one, two or even a bunch of short pre-flashes shortly before the main flash. This is done so that when you’re taking portraits, the irises in your subjects’ eyes are made to “stop down” before the main flash. This reduces the reflection of light from their retinas (the cause of that annoying red-eye effect). It’s nice that the camera makers do provide this feature to minimise the red-eye effect. But if you can’t turn off red-eye reduction, it makes it impossible to use a conventional slave flash trigger. That’s because the first pre-flash will trigger the slave flash unit, long before the camera takes the actual shot! What’s needed is a “smart” slave flash trigger unit which can ignore the red-eye reduction pre-flashes and only trigger the external flash when the camera’s main flash occurs. That is exactly what this new trigger unit is designed to do. This compact, low-cost unit counts up the camera flash pulses and only siliconchip.com.au Fig.1: the camera flash is picked up by photodiode PD1 and this drives transistor Q1 which in turn clocks IC1. IC1 is wired as a programmable counter and the output of gate IC2c (pin 10) will go low only when the right number of pulses have been counted. IC2c then triggers SCR1 (via IC2b & Q2) to trigger the slave flash unit. triggers an external flash unit when the last flash is detected. It operates from a standard 9V battery and everything fits in one of the smallest jiffy boxes (UB5 size). How it works At first sight, the circuit of Fig.1 may look a little com­plex but there is not a lot to it. PD1 is the photodiode which senses the camera flashes. For PD1 we’re using either a BP104 or a Z-1956 (DSE) device. Actually these both have an inbuilt IR (infrared) filter but they still have more than adequate response to visible light to do the job here. PD1 is connected in series with a 47kΩ load resistor across the 9V supply, as a reverse-biased light detector. To make the sensor insensitive to ambient lighting levels, we AC-couple its output to the base of transistor Q1 via a 4.7nF capacitor. As the base is pulled to ground via a 10kΩ resistor, Q1 is normally off; it only conducts briefly when the photodiode detects a flash of light. But during that time Q1 is switched on fully, so that a negative-going pulse of very close to 9V siliconchip.com.au peak appears at its collector. In other words, the combination of PD1, Q1 and the asso­ciated surrounding components forms a sensitive light-to-voltage pulse converter. The pulses from Q1’s collector are fed directly to the clock input of IC1, a 4024 binary counter which is connected as a programmable counter. To make IC1 programmable, we’ve added logic circuitry involving DIL switches S4-S8, diodes D1-D5 and gates IC2c & IC2d. The two gates are part of IC2, a 4093 quad Schmitt NAND device. Programmable counter The programmable counter works as follows. The cathodes of diodes D1-D5 are each connected to one of the five counter out­puts O0-O4 via one of the DIL switches. The anodes of all five diodes are connected together and to +9V via a 10kΩ pull-up resistor. This diode arrangement functions as a five-input AND gate, because the output (the junction of the five diode anodes and the 10kΩ resistor) can only be pulled up to +9V (logic high) when all five diode cathodes are also at logic high. If any diode cathode is pulled low, it pulls the output low as well. So if we close switches S4 and S5, this means that the gate output can only go high when IC1 has counted three pulses (so that its outputs O0 and O1 both go high). We can therefore pro­gram the counter for any desired pulse count, simply by setting the DIL switches for the binary equivalent of that number. The switches can be set for a total pulse count between 1 and 31 – more than enough for our needs. The output of the diode AND gate is connected to pin 8 of IC2c, used here as an inverter. And IC2c’s output (pin 10) is connected to pin 12 of IC2d, which is again used as an inverter. Pin 11 of IC2d is connected to the master reset input (pin 2) of counter IC1 via a small RC delay circuit (series 10kΩ resistor and 10nF bypass capacitor). This means that shortly after the programmed count is reached, the counter is reset, ready for the next sequence of flashes. By the way, the 100kΩ resistor and 100nF capacitor connect­ ed to the second input of IC2d (pin 13) form a simple power-up reset circuit, to ensure that the counter is reset to zero July 2003  61 about 4mA from the 9V bat­tery, which should therefore give a very long service life. Construction As can be seen from the photos, all of the slave flash trigger’s circuitry fits on a small PC board which measures 76 x 45mm and is coded 13107031. The board has cutouts in each corner so it fits snugly inside a standard UB5size plastic jiffy box, with the battery underneath. Programming switches S4-S8 and power switch S1 are actually all part of an 8-way DIL switch, making it cheap and compact. This is mounted in the centre of the board. The leftmost switch is the power switch (S1), while the five nearest the righthand end are used for programming (S4-S8). The two remaining switches (S2 & S3) are not used. Photodiode PD1 is mounted at the top of the board. If a BP104 diode is used, a pair of PC board terminal pins are fitted in this position and the diode’s very short leads soldered to the pins so that the top surface of the diode is 6mm above the board. On the other hand, if you use a Z-1956 diode from Dick Smith Electronics, this has fairly long leads which can be sol­dered directly to the PC board pads. However in this case the leads also have to be bent by 90 degrees and cranked so that the diode’s sensitive side is facing upward (again 6mm above the board) and directly above the two connection pads. The complete PC board assembly is mounted behind the lid of the jiffy box, using four M3 tapped Nylon spacers 6.3mm long. The spacers are Fig.2: here’s how to install the parts on the PC board. Note that the 100µF capacitor must be mounted on its side, while transistors Q1-Q3 must all be bent over so that they sit close to the board surface (see text). The full-size etching pattern for the PC board is at right. when power is first turned on. Summarising the action so far, we now have a light pulse sensor and counter which can be programmed using the DIL switches so that the output of IC2c (pin 10) will go low only when the right number of pulses have been counted. It also goes low only briefly (about 75µs), because of the way the counter is then quickly reset via IC2d. This narrow pulse from IC2c is used to trigger the slave flash. It is inverted by IC2b which drives transistor Q2. The resulting narrow pulse at the emitter of Q2 is then used to switch on SCR1, which acts as the triggering “contacts” for our slave flash unit. SCR1 is a 400V-rated C106D silicon-controlled rectifier, which is connected to the slave flash trigger input via the bridge formed by diodes D6-D9. The bridge ensures that the vol­tage applied across SCR1 from the flash unit is always of the right polarity (ie, positive to the anode), regardless of the circuitry inside your flash unit. So that’s how the main part of the trigger circuitry works. The only part left to explain is the purpose of gate IC2a, tran­sistor Q3 and LED1. These provide a simple power-on indicator, as well as indicating that the counter circuit is reset and ready for the next flash pulse sequence. Gate IC2a is again connected as a simple inverter, so that when the counter is reset and waiting for pulses, output pin 3 is held low (because pins 10, 2, 1 and 12 are high). This turns on PNP transistor Q3, which allows a low current (about 3.5mA) to pass through LED1. The LED therefore glows weakly, showing both that the power is turned on and that the counter has been cor­rectly reset. The LED goes out for the duration of the slave flash trigger pulse but it comes back on again as soon as the counter resets. The complete circuit draws only Table 2: Capacitor Codes Value 100nF  10nF  4.7nF µF Code EIA Code IEC Code 0.1µF 100n 104 (.01µF)  10n 103 (.0047µF)  4n7 472 Table 1: Resistor Colour Codes o o o o o No. 1 1 6 2 62  Silicon Chip Value 100kΩ 47kΩ 10kΩ 2.2kΩ 4-Band Code (1%) brown black yellow brown yellow violet orange brown brown black orange brown red red red brown 5-Band Code (1%) brown black black orange brown yellow violet black red brown brown black black red brown red red black brown brown siliconchip.com.au This is the fully-assembled PC board, ready for mounting inside the case. The DIP switch sets the number of flashes from the main flash unit before the slave is triggered (see text). attached to the lid using four 6mm x M3 machine screws with countersink heads, while the board is fitted to the spacers using four round head 6mm x M3 machine screws with lock washers. The lid has a central rectangular cutout to allow easy access to the switches and small circular holes top and bottom – one to allow light to reach PD1 and the other to allow LED1 to protrude through and be seen. The board mounting details should be fairly clear from Fig.3. By mounting the board assembly only 6.3mm behind the box lid, we provide just enough room inside the box to fit the 9V battery – plus a sheet of thin plastic to ensure that the battery case can’t short out any of the board wiring. Assembling the board The location of all of the parts on the PC board is shown in Fig.2. Note that because the board must be mounted only 6.3mm behind the case lid, some of the taller parts have to bent over so that they fit into this space. We suggest you begin assembling the board by fitting the PC board terminal pins. There are two on the left side of the board for battery connections and another two on the right for the flash trigger output lead connections. If you are using a BP104 for PD1, you’ll also need two more pins at the top centre. If the tops of all four/six pins are longer than 6.3mm, cut them so that they are only about 5mm long. Now you can fit the resistors, which all mount flat down against the board. This is also the case with the diodes, which all mount with their cathode ends towards the top the board. The capacitors can all be fitted next. Note that the 100µF electro mounts on its side as shown and make sure you get the polarity right. Next, fit the SCR. It mounts with its “metal insert” face down against the board. All three leads are bent down at 90° at a distance of 5mm from the body, so they pass through the board holes. The device itself is held down using a 6mm x M3 machine screw and nut. IC1 and IC2 can be fitted next, taking care to fit them the correct way around. Observe the usual precautions to avoid damage due to static charge, too – remember that both devices are CMOS types. Now fit the three transistors. These all have to be mounted leaning over so they will allow the board assembly to be fitted only 6.3mm behind the case lid. For the two PN100 devices, this is achieved by carefully bending their three leads so the centre base lead is about 3mm shorter than the other two when they are passed down through the board holes. In other words these transistors have their leads bent so they are mounted leaning back, with the short base lead underneath and the two longer leads bending down at about 60°. There isn’t space to mount the PN200 transistor Q3 in this way, Are Your Issues Getting Dog-Eared? REAL VALUE AT $14.95 PLUS P & P Are your SILICON CHIP copies getting damaged or dog-eared just lying around in a cupboard or on a shelf? Can you quickly find a particular issue that you need to refer to? Keep your copies of SILICON CHIP safe, secure and always available with these handy binders Available Aust, only. Price: $A14.95 plus $10 p&p per order (includes GST). Just fill in and mail the handy order form in this issue; or fax (02) 9939 2648; or call (02) 9939 3295 and quote your credit card number. siliconchip.com.au July 2003  63 Fig.3: the PC board is attached to the lid of the case on 6.3mm spacers and secured using machine screws, nuts and washers. Also shown here is the mounting detail for the Z-1956 photodiode (see text). 6mm above the board. Remember that the Z-1956 should be fitted so that its cathode lead is furthest from Q1. The way to identify this lead with the Z-1956 diode is by noting that it’s on the same side of the device as the small top bevel. All these component mounting details should be apparent from Fig.2 and Fig.3. With the photodiode fitted, only two steps remain to com­plete the PC board assembly. One is to fit the 3mm Ready LED, making sure that the longer anode lead is nearest to the 2.2kΩ series resistor and the “flat” side of the body is towards the 10kΩ resistor. Also take care that you solder the leads with the LED and its leads truly vertical, and with the bottom of the LED’s body just 5mm above the board. The final step is to connect the 9V battery clip lead, the wires of which connect to the PC board terminal pins over on the lefthand side. Note that the red wire connects to the lower pin (ie, the one nearer the two 100nF capacitors), while the black wire connects to the upper pin (nearer the 100µF electro). Preparing the case The close-up view shows the completed assembly, just before it is fitted to the case. The flash trigger lead emerges through a small semicircular notch near the top centre of one side of the jiffy box. because it’s quite close to one of the Nylon mounting spac­ers. So Q3 has all three leads bent at 90° towards the emitter side, so it can be mounted “side on” with its body bet­ween IC2 and the 100kΩ resistor. The flat side of the body is towards the 100kΩ resistor, with the emitter lead lowest and the collector lead uppermost. The 8-way DIL switch is fitted next, taking care to fit it with the ‘ON’ side of the switches towards IC1. Also make sure when you’re soldering its pins to the board pads that you don’t accidentally link the pads with fine solder bridges. Now fit photodiode PD1. If you’re using a BP104 device, you need the extra two PC board pins, as noted above. Cut off both pins at a point 3mm above the board. Then very carefully bend the leads of the BP104 down at right angles about 1mm from the body 64  Silicon Chip and solder them to the PC board pins. The flat top of the diode should be horizontal and just 6mm above the top of the board. Make sure you solder the diode’s cathode lead (the one with the small side tag) to the pin furthest from transistor Q1. The procedure is a bit different if you are using the Z-1956 photodiode from DSE. This doesn’t need the PC board pins, but it does need both of its leads first bent down at 90° (ie, away from the sensitive front face), at about 2mm from the body. Then they are bent inwards by a further 90°, at a point only about 2mm behind the diode’s rear face, and finally outwards again at a point 3mm from the top of the body. This allows the diode to be mounted with its leads passing down through the two inner holes on the board, with its sensitive front face uppermost and horizontal, and again Your board assembly should now be complete, and you can put it aside while you prepare the box lid. If you’re building the project from scratch, this will involve drilling and cutting the required holes using the drilling template of Fig.4 as a guide. Note that the four 3mm holes for the board mounting spacer screws are countersunk at the top, so that the tops of the screws will be flush with the lid’s upper surface. This allows them to be hidden beneath a stick-on front panel if one is used. Once the lid is prepared, you can attach the four 6.3mm tapped Nylon spacers to it using four 6mm x M3 countersink-head machine screws plus four M3 flat washers (see Fig.3). Then you should be able to mount the PC board assembly on the four spacers in turn, using four 6mm x M3 roundhead screws and lockwashers. There’s only one remaining step before you can test the trigger unit and finish its assembly. This is to fit a suitable output lead, to connect to the external flash unit it will be triggering. The main requirement here is that this lead will need to be fitted at the far end with a connector to suit the trigger input of the flash unit. siliconchip.com.au Parts List 1 PC board, code 13107031, 45 x 76mm 1 Jiffy box, UB5 size (83 x 54 x 28mm) 1 8-way DIL switch (S1, S4-S8) 1 9V alkaline battery, 916/PP3 type 1 battery clip lead to suit 6 PC board terminal pins 4 6.3mm M3 tapped spacers (Nylon) 4 6mm x M3 screws, countersink head 4 6mm x M3 screws, round head 1 6mm x M3 machine screw & M3 nut 4 M3 flat washers 1 flash trigger lead with connector Semiconductors 1 4024 binary counter (IC1) 1 4093 quad Schmitt NAND (IC2) 1 C106D 400V SCR (SCR1) 2 PN100 NPN transistors (Q1,Q2) 1 PN200 PNP transistor (Q3) 1 BP104 or Z-1956 photodiode (PD1) 1 3mm green LED (LED1) 5 1N4148 diodes (D1-D5) 4 1N4004 diodes (D6-D9) The 9V battery sits in the bottom of the case and is wedged in position using pieces of foam. A sheet of plastic is then fitted over the top of the battery, to prevent it shorting against the bottom of the PC board. If the flash unit has a conventional 3mm concentric connec­tor, your best approach is probably to buy a short flash exten­ sion lead from a photographic store and cut off the unwanted connector so the wires at the free end can be soldered to the output pins on the trigger unit board. On the other hand, if your flash unit is only fitted with a “hot foot” connector, you will have to either salvage a matching “hot shoe” connector from a junked camera or make one yourself. This could be done with some pieces of blank PC board laminate or some 1mm sheet brass and a piece of insulating material. That done, the hot shoe connections can be wired to the trigger unit’s output pins with a length of shielded audio cable. Checkout time Ready to roll? Make sure that all the siliconchip.com.au DIP switches are set to Off (down) and connect a 9V battery to the clip lead. That done, switch on S1, set timing switch S4 to On (leave S5-S8 Off) and check that the green Ready LED lights. Now connect your slave flash unit to the trigger unit’s output lead and turn on its own power switch so the flash capaci­tor becomes charged and ready for action. Also get your camera ready and set it for flash operation. To check out the trigger unit’s basic operation, set timing switch S4 only to the On position and then press the shutter release of the camera to produce a flash (or more than one, if it’s only capable of working in redeye reduction mode). You don’t need to aim the camera flash at the trigger unit’s sensor – aiming it at the ceiling should be fine. As soon as the camera’s flash (or first flash) occurs, you should also see Capacitors 1 100µF 16V electrolytic 3 100nF (0.1µF) MKT polyester 1 10nF (.01µF) MKT polyester 1 4.7nF (.0047µF) MKT polyester Resistors (0.25W, 1%) 1 100kΩ 6 10kΩ 1 47kΩ 2 2.2kΩ the slave flash fire. Assuming this is the case, your trigger unit is probably working correctly. If not, you may have made a wiring mistake somewhere. Per­haps you’ve connected a component the wrong way around or bridged a couple of tracks on the board with a whisker of solder. So turn off the flash unit and disconnect it from the trigger unit, then unclip the trigger unit’s 9V battery and look for the problem. Once the trigger unit is operating correctly, you can then set the DIL switches so that the trigger unit only July 2003  65 switch setting by one (ie, S4 off and S5 and S6 on, for 2 + 4 = 6) and try again. If the slave flash still operates, you did underestimate the number of camera flashes the first time – so increase the setting by one more and try again. Conversely, if the slave flash doesn’t fire this second time, your previous guess must have been correct. In this case, return the switches to their previous setting and your trigger unit is correctly set up. In short, the correct setting for the trigger unit’s flash count programming switches is the highest count that still re­sults in the slave flash being triggered for each flash shot - because it’s being triggered on the last and ‘main’ camera flash. Final assembly Fig.4: here are the full-size artworks for the front panel and the drilling template for the case lid. operates the slave flash in response to the camera’s main flash. Of course, if the camera is able to be operated in normal single-flash mode, there’s nothing further to be done. Setting the flash count You’ve already set the trig­ger unit to respond to the first camera flash, by turning on only DIP switch S4. As you’ve probably realised by now this is the correct setting for cameras that can operate in this mode. Even if your camera can only operate in multi-flash red-eye reduction mode, it’s still quite easy to find the correct switch setting. You don’t have to count exactly how many flashes the camera does produce for each shot. Just have a guess and set the trigger unit’s switches initially to that figure. For example, if you think it produces five flashes in all (four pre-flashes and the main flash), turn on switches S4 (1) and S6 (4). Then press the camera’s shutter release to take a ‘shot’, and see if the slave flash is triggered. If it does fire, you’ve either guessed the total number of camera flashes correctly or you have underestimated. To find out which, increase the This view shows how the 9V battery is wedged in position using polystyrene foam. Note the semicircular groove in the back of the case for the flash trigger lead. 66  Silicon Chip Once you’ve completed this checkout and setting up proce­dure, your trigger unit is ready for final assembly. Just before doing this, though, you’ll need to file a small semicircular notch near the top centre of one side of the jiffy box, to allow the output trigger lead to exit the box when it’s assembled. To work out exactly where the notch should be located, offer the lid and PC board assembly up to the top of the box, and mark the position where the lead will need to exit for minimum strain on the lead and the connections. Then file the notch with a jeweller’s rat-tail file, making it only just large enough for the lead – so that when the lid is screwed to the box, the lead will be securely clamped. Now place the 9V battery (still connected to the trigger board via the clip lead) in the centre of the box and cut four small pieces of expanded poly­ styrene foam to go around it and hold it in position. That done, cut a piece of thin sheet plastic (or presspahn insulating material) to the same size and shape as the trigger unit PC board, to provide an insulating layer above the battery. You can now fit the lid/board assembly to the box, wind­ing the battery lead carefully around so it doesn’t get caught between the edge of the lid and the box rim. The final step is to secure the lid using the four screws provided with the box, to hold everything together firmly. Your slave flash trigger unit is now complete and ready for some serious SC flash photography. siliconchip.com.au