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A Beam-Break Flash Trigger By JIM ROWE Here’s an easy-to-build accessory for the Time Delay Photoflash Trigger described in our February 2009 issue. It triggers the delay unit and your photoflash in response to an object interrupting an invisible beam of infrared (IR) light. Alternatively, it can be used on its own to directly trigger a photoflash. A FEW MONTHS ago (in February 2009), we described a “Time Delay Photoflash Trigger”. This unit was triggered by a sudden sound picked up by an electret microphone. It then immediately opened the camera’s shutter and then fired the photoflash shortly after, depending on the delay period programmed into the unit. Using sound pick-up in this manner is a popular and effective method of triggering a flash for “stop motion” and other kinds of special effects photography. However, in addition to the electret mic input, we also gave the delay unit a second “contact closure” input, so that it could be triggered using other techniques. Which was just 62  Silicon Chip as well, because as soon as the delay unit was published we started getting requests for a light beam trigger. This simple “Beam Break Trigger Unit” is the result of those requests. It’s mainly intended as an alternative triggering front-end for the Time Delay Photoflash Trigger and is connected to the latter’s “contacts” input. However, it can also be used to trigger a photoflash unit directly if you don’t need the programmable time delay capabilities. Note, however, that using the unit to directly trigger the flash has one important limitation. Unlike the Time Delay Photoflash Trigger, it doesn’t also trigger the shutter. This means that you have to open the shutter manually before the infrared beam is interrupted (eg, at night or in a darkened studio). The new project is in two parts: (1) an IR Source unit which produces the IR beam and (2) a Detector unit which monitors the IR beam and closes its output trigger contacts briefly if the beam is interrupted. These two units are linked with an interconnecting cable which supplies the Source unit with power. By the way, if you’re already wondering how you accurately line up the Source and Detector units when the IR light beam is invisible to the human eye, wonder no more. That problem has been solved by providing the detector unit with a visible green siliconchip.com.au S1 CON2 CON1 LINE-UP GUIDE  LED4 A 22k 100nF K 220k IC1: LM358 1k 820 6 A LED1 5  K 8 3 2 IC1a 10nF 1 +1.0V A 1k 470k 10k B C Q1 BC338 D Q2 2N7000 G S E  TRIGGER OUT 100 PD1   K A LED3 22k 4 A K LED2 7 IC1b 9V BATTERY (6 x AA CELLS) 470 F 16V 220 F CON3 10k 2.7k K OBJECT BREAKING BEAM IR LEDS SC  2009 'BEAM BREAK' TRIGGER UNIT PD1 (ZD-1948) ACTIVE AREA BC338 LED4 2N7000 B K A A K K A E C D G S Fig.1: the infrared beam is generated by LEDs 1-3 and picked up by photodetector diode PD1. Op amp IC1b functions as a current-to-voltage converter while IC1a is wired as a non-inverting amplifier. The latter drives transistor Q1 & Mosfet Q2 to briefly switch the trigger output when the IR beam is interrupted. LED which lights when the IR beam is being received. This makes the liningup process easy. Both parts of the project run from a 9V battery fitted inside the Detector unit’s box. The total current drain is about 15mA which means that the battery should be either a set of six AA (1.5V) alkaline cells or a single high-energy 9V lithium battery. A standard 9V zinc-carbon or alkaline battery is not up to the job, as its life would be too short. Circuit details Take a look now at Fig.1 for the circuit details. There’s really not a great deal in either part of the circuit. In fact, the IR Source unit is nothing more than three IR LEDs connected in series and with an 820Ω series resistor. This resistor limits the current from the 9V supply (and thus the current through the IR LEDs) to about 7.5mA. Power is derived from the battery in the Detector unit via a cable fitted with a 3.5mm jack plug (CON1). This mates with CON2 on the detector unit. In the Detector unit, the IR beam from the Source unit normally falls on PD1, an IR photodetector diode. This photodetector is connected between ground and the inverting input (pin 6) of op amp IC1b (an LM358). siliconchip.com.au Op amp IC1b is connected as a current-to-voltage converter. Its pin 7 output sits somewhere between +1.7V and +4.0V when the IR beam is present but rests close to +1.0V when no IR light is falling on PD1. This “dark” output voltage of +1.0V is basically set by the voltage divider formed by the 22kΩ and 2.7kΩ resistors, with the 220µF capacitor providing filtering. This is used to directly bias pin 5 of IC1b and to bias pin 2 of IC1a via a 1kΩ resistor. The output at pin 7 of IC1b is fed to the non-inverting input (pin 3) of IC1a, which is configured as a non-inverting amplifier with a voltage gain of 471. Because of this very high gain, IC1a acts very much like a comparator. Its pin 1 output sits at over +8V when the IR beam is present but falls to 0V when there is no IR light falling on PD1 (ie, the IR beam is interrupted). IC1a’s output in turn drives the base of transistor Q1 via a 10kΩ resistor. As a result, Q1 is turned on or off depending on whether the IR beam is present or not. When the IR beam is present, Q1 is on and when the beam is interrupted, Q1 turns off. LED4 and its series 1kΩ resistor form the collector load of Q1. This means that LED4 lights when Q1 is on and turns off when Q1 is off. This allows LED4 to be used as a guide when lining-up the Source’s IR beam with PD1, as described previously. Switching the trigger output Because Q1 is switched on when the IR beam falls on PD1, its collector voltage is normally held down to about 0.4V. However, if the beam is interrupted, Q1 turns off and its collector voltage rises to nearly +9V. This sudden voltage change is used to switch on Q2, a 2N7000 MOSFET which is used as an output switch across triggering output CON3. As shown, a 10nF coupling capacitor and Q2’s 10kΩ gate resistor form a simple differentiating circuit. This results in Q2 being switched on only briefly when Q1’s collector voltage rises when the beam is interrupted. The 100Ω resistor in series with the coupling capacitor is there to suppress any possible oscillation during switch-on or switch-off. That’s about it, apart from power switch S1 and the 470µF and 100nF capacitors which decouple the supply rail voltage to keep it constant. The current drain of the detector circuit varies between about 7.5mA when the IR beam is present and 1.5mA when it is interrupted, so the total battery drain for both sections varies between June 2009  63 Parts List IR Source Unit 1 PC board, code 13106092, 57 x 26mm 1 UB5 jiffy box, 82 x 53 x 31mm 4 6mm long untapped spacers 4 M3 x 12mm screws, countersink head 4 M3 hex nuts 1 Nylon cable tie, 75mm long 1 2m length of light-duty figure-8 cable 1 3.5mm mono jack plug, cable type (CON1) 3 5mm IR LEDs (LEDs1-3) 1 820Ω resistor Detector Unit 1 PC board, code 13106091, 122 x 58mm 1 UB3 jiffy box, 129 x 68 x 44mm 1 SPDT mini toggle switch (S1) 1 PC-mount 3.5mm stereo jack (CON2) 1 PC-mount 2.5mm concentric plug (CON3) 4 M3 x 15mm tapped spacers 8 M3 x 6mm machine screws, pan head 2 1mm PC board terminal pins 1 9V battery clip lead 1 8-pin DIL IC socket 1 30mm length of 12-15mm diameter black PVC conduit or brass tubing 1 piece of IR-transparent red film, approx. 16mm square 1 9V battery snap connector OR 1 x 4-way AA cell holder plus 1 x 2-way AA cell holder – see text Semiconductors 1 LM358 dual op amp (IC1) 1 BC338 NPN transistor (Q1) 1 2N7000 N-channel MOSFET (Q2) 1 IR photodetector (PD1) (Jaycar ZD-1948 or similar) 1 5mm green LED (LED4) Capacitors 1 470µF 16V RB electrolytic 1 220µF 16V RB electrolytic 1 100nF metallised polyester 1 10nF metallised polyester Resistors (0.25W 1%) 1 470kΩ 1 2.7kΩ 1 220kΩ 2 1kΩ 2 22kΩ 1 100Ω 2 10kΩ 64  Silicon Chip The IR Source board carries the three infrared LEDs (LEDs1-3) plus an 820Ω current-limiting resistor. It’s mounted inside a UB5 case on 6mm untapped spacers and derives its power from the Detector unit. 15mA (beam present) and 9mA (beam interrupted). Construction As shown by the photos, the two units which make up the Beam Break Trigger are each housed in a small jiffy box. The IR Source circuit is built on a small PC board coded 13106092 (57 x 26mm), while the Detector parts are installed on a larger PC board coded 13106091 (122 x 58mm). Start the assembly by building the IR Source board – see Fig.2. This should take you just a few minutes since there are only four components to install – the three infrared LEDs and the 820Ω current-limiting resistor. Be sure to orientate the three IR LEDs correctly as shown in Fig.2. In addition, these three LEDs must be fitted with their leads bent down by 90°, so they face out of the end of the box when the board is mounted inside. In particular, note that the centre LED (LED2) is fitted with its body relatively low down near the board, while the two outer LEDs are fitted higher and with their leads bent inwards towards LED2. This is done so that they form a triangular group, to provide a relatively compact beam source (see photo). Once these parts are in, install the power cable by soldering its leads to the +9V and 0V and pads. The cable is then anchored using a small Nylon cable tie which passes through the two 3mm holes on either side. Having completed the board, it can be mounted inside its UB5 jiffy box on four 6mm long untapped spacers and secured using four M3 x 12mm countersunk head screws and nuts. As shown in the photos, the IR LEDs face outwards through a 10mm hole in one end of the box, while the power cable exits via a small notch filed in the top at the opposite end. Fig.3 shows where to drill the holes in both boxes. Finally, complete the IR Source unit by attaching the front panel label to Table 1: Resistor Colour Codes No.   1   1   2   2   1   2   1   1 Value 470kΩ 220kΩ 22kΩ 10kΩ 2.7kΩ 1kΩ 820Ω 100Ω 4-Band Code (1%) yellow violet yellow brown red red yellow brown red red orange brown brown black orange brown red violet red brown brown black red brown grey red brown brown brown black brown brown 5-Band Code (1%) yellow violet black orange brown red red black orange brown red red black red brown brown black black red brown red violet black brown brown brown black black brown brown grey red black black brown brown black black black brown siliconchip.com.au CABLE TO CON1 (MATES WITH CON2 BELOW) CABLE TIE 29060131 9002 © +9V V9+ 820 V0V 0 INFRARED LEDS K LED3 LED2 LED1A An infrared transparent filter is fitted to the inside of the case at the receiving (PD1) end of the UB3 box, while a 30mm x 12mm-diameter “lighthood” (eg, brass or plastic tubing) is attached to the outside of the case. MOVING OBJECT PD1 ZD-1948 K A 220k 100nF IC1 LM358 220 F 2.7k 22k 470k 10k 22k 1k siliconchip.com.au 1k LED4 470 F A There are more components on the Detector board but its construction is still straightforward – see Fig.2. Install the resistors first, taking care to use the correct value at each location. Table 1 shows the resistor colour codes but it’s also a good idea to check each one using a digital multimeter before soldering it in place. Follow these parts with the metallised polyester capacitors, then fit the two electrolytic capacitors. The latter are polarised, so be sure to orientate them as shown. The two PC board terminal pins used to make the battery connections can then be fitted. Note that both pins are fitted on the copper side of the board, to make it easier to 10nF K Detector board assembly solder the battery clip leads to them. Switch S1 and connectors CON2 & CON3 are next on the list, followed by an 8-pin socket for IC1. Be sure to orientate the socket with its notched end towards the adjacent 100nF capacitor, to guide you when plugging in IC1 itself later on. Transistor Q1, photodetector PD1, MOSFET Q2 and LED4 can now all go in, again taking care to orientate them correctly. Note that PD1 is mounted vertically with its curved side facing outwards and with the centre of its body about 5mm above the PC board. LED4 should also be mounted vertically, with the bottom of its body about 12mm above the board (this ensures that it will protrude slightly from its matching hole in the box lid after assembly). The Detector board can now be completed by plugging IC1 into its 9002 © 19060131 the lid. A full-size artwork is shown in Fig.3 and is also available for download from the SILICON CHIP website. Q1 BC338 RE G GIRT KAER B MAE B Follow this photo and the parts layout diagram (Fig.2) at right to build the Detector PC board. 100 10k Q2 2N7000 S1 9V BATTERY POWER CON3 + – CON2 R S TRIGGER OUT TO FLASH, ETC T (TO EMITTERS) Fig.2: install the parts on the two PC boards as shown on this layout diagram. LED4 (green) on the Detector board is mounted vertically but be sure to bend the leads of IR LEDs1-3 through 90° before installing them on the IR Source board – see text & photo. June 2009  65 LID OF UB3 BOX IR SOURCE POWER TRIGGER OUT CL POWER RECEIVING END OF UB3 BOX BEAM FOUND TOWARDS BEAM B BEAM BREAK FLASH TRIGGER UNIT SILICON CHIP 10 47 IR BEAM OUT 9V DC INPUT 24.5 BEAM BREAK FLASH TRIGGER UNIT IR BEAM SOURCE 24.5 SILICON CHIP A A CL 4.75 B ALL DIMENSIONS IN MILLIMETRES HOLES A: HOLES B: HOLE C: HOLES D: HOLES E: 30.5 47 CL C 8 A E 3mm DIAMETER 5mm DIAMETER 6.5mm DIAMETER 10mm DIAMETER 3mm DIA, COUNTERSUNK E A CL IR LED END OF UB5 BOX 14 50.5 CL 8 9.5 D 9.5 11.75 D D 10 E 5 5 10 TRIGGER OUTPUT END OF UB3 BOX E BOTTOM OF UB5 BOX POWER CABLE END OF UB5 BOX Fig.3: these drilling diagrams for the UB3 & UB5 boxes can be either be copied and used directly as templates or you can mark the holes out manually using the measurements indicated. Also shown are the two front-panel artworks. They can either be copied and used direct or downloaded from the SILICON CHIP website and printed out. 66  Silicon Chip siliconchip.com.au socket (take care with the orientation). The detector board is then ready to be mounted behind the lid of the UB3 box. The first step is to drill and ream out the various holes in the base and lid, as shown in Fig.3. That done, fit the front panel label and cut out the holes using a sharp hobby knife, then secure the board to the lid using four M3 x 15mm tapped spacers and eight M3 x 6mm machine screws. Note that you’ll need to remove the upper nut from the ferrule of switch S1 before doing this, so the ferrule can pass up through its matching hole in the lid. Once the board is in place, the nut can be replaced and threaded down against the top of the lid. The lower nut and lockwasher can then be threaded up against the underside of the lid, using a small spanner. The next step is to fit a small square of red “IR transparent” film inside the box behind the single 5mm hole at the PD1 end. It can be held in place using a couple of narrow strips of transparent tape, one on either side. A short “light hood” is now be attached to the photodetector (PD1) end of the box. This must cover the 5mm hole and be as close as possible to concentric with it. The hood itself can be fashioned from a 30mm length of 12mm dia­meter brass tubing (see photos) or from a similar length of opaque (preferably black) PVC conduit. Whichever you use, it’s simply glued to the end of the box using 5-minute epoxy cement. Now for the final assembly. First, connect the battery snap lead to the terminal pins on the underside of the board, then place the battery in the bottom of the box and fasten it in place using either a small aluminium “U” bracket or a strip of gaffer tape. Finally, lower the lid and PC board assembly into the box before fitting the screws to hold everything together. Trying it out No adjustments are required, so you can try it out simply by plugging the power cable from the IR Source into CON2 on the Detector unit and turning on power switch S1. If the Detector’s light hood is now aligned with the output from the IR Source (or any other source of IR radiation), LED4 should immediately begin glowing. If it does, block the end of the hood with your thumb or a small piece siliconchip.com.au Above: the Detector board is secured to the lid of the UB3 case using four M3 x 15mm tapped spacers and eight M3 x 6mm machine screws Left: a “light hood” is fitted to the end of the Detector unit to prevent interference from stray IR light sources. of opaque material and check that the LED immediately switches off. The same thing should happen if you turn the IR Source away from the Detector or if you simply block the beam with your hand or some other small opaque object. If this happens, then your Beam Break Trigger Unit is probably working correctly and is ready for use. If you’re going to be using it in conjunction with the Time Delay Photoflash Trigger unit, all that remains is to make up a suitable cable to connect the two together. This simply involves connecting the Detector’s trigger output to the “external trigger contacts” input (CON4) of the delay unit. By the way, the Beam Break Trigger Unit should give reliable triggering with the IR Source unit placed up to a metre or so from the Detector box in normal room lighting. This “beam length” range can be extended considerably in dark (eg, night-time) conditions but in bright sunlight it will be shortened due to the relatively high SC level of IR in the ambient light. Direct Flash Triggering: Making The Cable A s mentioned in the article, the Beam Break Trigger can also be used to trigger an electronic flash directly, rather than via the Time Delay Photoflash Trigger. To do this, trigger output CON3 is simply connected to the photoflash via a suitable cable. However, when you’re making up this cable, make sure that the positive side lead from the flash input is connected to the centre contact of the plug that goes to CON3. If the polarity is reversed, MOSFET Q3 in the Beam Break Trigger Unit could be damaged. The procedure is to first use your DMM to check the polarity of the voltage at the end of the cable that’s plugged into the flash unit (ie, with the flash unit powered up and ready for triggering). Once that’s done, you’ll then know which way around to connect the cable to the plug that goes to CON3 on the Detector unit. While you’re checking the polarity of the cable leads, make a note of the actual voltage itself. If it is below 60V, that won’t be a problem. Conversely, if it’s higher than 60V, you’ll need to replace the 2N7000 MOSFET with one having a higher voltage rating – such as a IRF540N. June 2009  67