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LED PARTY STROBE MK2 By ROSS TESTER Back in January 2014, we published a LED Party Strobe – something which we had been assured couldn’t be done! It not only worked, it was very popular. Now we have an even simpler – and much cheaper – approach using a standard 230VAC 30W LED floodlight which can be purchased quite cheaply online. It’s a bit bigger, too! T here is a common misconception that high power LEDs cannot be strobed – turned on and off quickly – for a very similar reason that an incandescent bulb cannot be strobed. Incandescent lamps have a high thermal inertia – their lamp filaments don’t have a chance to cool down enough after each “flash”. So even though the current through a filament may be switched on and off rapidly, the filament temperature responds much more slowly; very slowly, in fact. LEDs don’t have the inertia of filaments. White LEDs are typically based on blue LEDs with an accompanying phosphor which produces the white light. And that is where the misconception arises. 84  Silicon Chip Phosphors in fluorescent tubes do have inertia – certainly quite a lot of it. So when you switch a fluorescent light off, it takes a significant amount of time before the phosphor stops emitting light. However, the phosphor in white LEDs does not emit light by phosphorescence; it works by a process called scintillation. That means that there is no light persistence after each flash. In fact, our tests demonstrate that these very bright LED floodlights can be flashed very rapidly indeed, up to 10kHz or more is easily done. However, for a party strobe-light the flashing is not rapid at all, up to only about 18 flashes per second. The previous Party Strobe was based on a LED floodlight which had no internal power supply and so it did have one major disadvantage – a separate box containing a beefy mains transformer. It was intended to power siliconchip.com.au A (RED) 230VAC ~+30VDC LED CURRENT DRIVER (SUPPLIED WITH FITTING) D1 1N4004 5 (BLACK) N E NC 4 CON2 2 K A 3 0V 1k 100nF + A 22k  LED2 K 1 15V DC K ZD1 15V 100F 25V A 7 6 8 5 1 K A D5 100nF A COMPONENT NUMBERS REFER TO THOSE PRINTED ON ORIGINAL LED STROBE (16101141) PCB SC FLASH RATE D6 E B OUTPUT Q1 BC337 E D 10 G S K Q2 BC327 ZD2 15V C K 220k 2015 B 3 IC1 7555 2 330nF C 4     K CON2 100nF  A 3.3k 0.5W FLASH EARTH TO CASE  Q3 IRF540N 10–100W WHITE LED ARRAY SEE PHOTO FOR LED ARRAY CONNECTIONS LED2 (RED) K A A D5,D6:1N4148 27k A VR1 1M LIN LED ARRAY PARTY STROBE MK2 B E G C D D S K D1:1N4004 IRF540N BC327, BC337 A K ZD1,ZD2 A K Fig.1: the circuit is a simplified version of that used in our earlier strobe – mainly because the mains power supply is already supplied fitted to the floodlight. We’ve also done away with provision for controlling a hot wire cutter. LED floodlights ranging in power from 10W to 100W. Our new Party Strobe is based on a 230VAC LED floodlight, sold in large numbers via the internet with ratings of 10W, 20W, 30W, 50W or 100W. The mains powered LED Driver is supplied already fitted and wired inside the case. All are quite similar in presentation with their overall size increasing according to their rating. The one featured in this article is rated at 30W but you could use any rating from 10W to 100W. However, we think 30W is probably enough. 230VAC LED current driver Not only is the construction of all these LED floodlights similar, their internal circuitry is much the same, comprising a switchmode current driver module rated for inputs of between 160 to 265VAC, or thereabouts. The current driver module is housed in a rectangular metal housing with plastic end pieces and it is typically held in place in the rear casing of the LED floodlight with silicone sealant. The output voltage of these current driver modules is typically around 30 -36V DC and in the example we are using here, the current rating is 900mA. And how are we flashing it? We are using the same switching module with only slight changes to the original LED Strobe circuit and this can be seen in the diagram of Fig.1. The LED driver output of about 30V DC is fed via diode D1 is fed directly to the anodes of the 10W to 100W white LED array and also via a 3.3kΩ 0.5W Above is the LED driver mounted inside the case (held in place by silicone sealant [which is quite OK]). The mains input lead is secured with a captive gland but the earth connection is not up to par. We will refit this to the proper standard. At right is the rear view. As mentioned in the text, we should have fitted the pot much further forward so it didn’t foul the mounting bracket. Speaking of which, we had previously cut this to suit our location – it’s normally a single “U” shape. siliconchip.com.au August 2015  85 Commercial Strobes We are aware that you can purchase ready-built LED strobes online, for not much more than the cost of a LED floodlight. But we’re not convinced that the commercial models are as good as ours! You can also buy a variety of LED arrays, either fitted to a floodlight housing (as ours was here) or loose. “Naked” LED arrays are commonly available in 10W, 20W, 50W, 70W and 100W ratings at quite attractive prices (100W LED arrays, for example, are less than $10.00 each!). Incidentally, Xenon-tube strobes are still available. But we’ve seen some pretty extravagant claims of Xenon strobes power – for example, 1500W in one case! Now when you consider that the majority of Xenon tubes we used to use in DIY strobes were usually rated at 5W (actually 5 joules, or 5 watt-seconds) 1500W or even 1000W would seem to be a bit over the top. It’s not dissimilar to a 5W RMS audio amplifier being advertised as 1000W PIMPO. In those immortal words from “The Castle” . . . they’re dreamin! resistor to a 15V zener diode, ZD1. This provides a 15V DC supply rail to a 7555 timer (IC1) and transistors Q1 & Q2. IC1 is connected so that it repeated charges and discharges the 330nF capacitor at pins 2 & 6 via diodes D5 & D6 from its pin 3 output. The two diodes provide different charge and discharge times because of the different series resistances. For example, the charging path is via D6, and the 27kΩ resistor while the discharge path is via D5, the 220kΩ resistor and the 1MΩ potentiometer which is wired as a variable resistor (rheostat). If you unsolder the LED array, or want to replace it, identifying the anode and cathode can be rather difficult. Look for a “+” sign moulded into the plastic (indicated by the red circle above). Contrarily, this is closest to the – terminal or cathode (see red and black wires). Never operate the LED array without it being secured to a heatsink. 86  Silicon Chip Fig.2, the PCB component overlay, which matches the same-size photo at right. This is the same PCB as used in the January 2014 Party Strobe but the circuit it simpler, as evidenced by the number of empty holes. Note also the four links required. Only the red and orange wires to the pot are actually required but we had a length of 3-wire cut from a ribbon cable so used it! The result is a fixed flash duration of 8ms (milliseconds) and a flash rate which can be varied from about three flashes per second up to about 15 fps. The flash rate is a compromise between apparent brightness and the “freeze motion” effect which is the whole point of a strobe. The variable pulse train from pin 3 of IC1 is fed to the complementary transistors Q1 & Q2 and these buffer the output to provide cleaner switching of the following N-channel Mosfet Q3, an IRF540N and this is connected to the cathodes of the LED array to provide the rapid switching. Since the average current is only around 1A maximum for a 100W LED array, no heatsink is required is required for the Mosfet. A 1kΩ resistor is connected across the LED array to damp high frequency artefacts from the LED current driver. At same time, red LED fed by a 22kΩ resistor provide a visible indication that the circuit is working if the LED array is not actually connected or cannot be seen. Obviously, this LED will not be visible once the switching PCB is housed inside the casing of the LED floodlight. Note that the LED driver does not appear to be “troubled” by having its current output interrupted by this switching process. In fact, its output voltage will tend to rise to about 40V at low flash rates but then it simply shuts off intermittently to limit the output voltage to a safe level. Construction Like the earlier party strobe, this one is constructed inside a LED floodlight fitting. Unlike the earlier model, though, it is wholly self-contained because the floodlight is designed to operate of AC power (160-265V) through its tiny switch-mode supply module already fitted inside. Believe it or not, mains-powered LED floodlights are not significantly more expensive than their 12V counterparts which we used last time. We obtained ours online for about $30.00 but prices do vary significantly so shop around! The PCB is also mounted inside the fitting – but where the supply module is mounted inside the “lid”, we secured the new PCB inside the body of the fitting, so that when the two halves are brought together, the new PCB clears the supply module. There is not a huge amount of space to spare – but there is enough. We’re getting a bit ahead of ourselves but we used double-sided thick foam pads to secure the PCB to the case. These have several advantages, not the least of which is that they don’t require any drilling and also act as insulators between the bottom side of the PCB and the metal case. Double-sided foam pads are commonly available at stationery stores, office suppliers and newsagents. Assembling the PCB This is quite straightforward using the PCB designed for the original strobe (16101141), with the obvious difference that there are several component positions left unfilled and some components are slightly different to the original. We have retained the component identification numbers from the silkscreen PCB overlay on the original siliconchip.com.au graphic on the screen overlay. Whether you use an IC socket is entirely up to you (but if you do, make sure its orientation is the same). Mounting the speed pot Due to its earlier multi-use format, the PCB had an end which could be cut off – shown above at the right end (but cut off in the overlay at left). It’s up to you. As mentioned in the text, the LED is redundant once the case is closed – but it saves your eyes from the really bright LED array while testing! PCB to avoid confusion; there will obviously be “gaps” in the component numbering (eg, there is a D1, D5 and D6 but no D2, D3 or D4). Simply follow the new component, noting which components are left out and which are replaced by a wire link. Start with the lowest-profile components first, ie the resistors and small capacitors. Follow these with the diodes and Zener diodes, taking care not to either mix them up nor get them around the wrong way. Next are the LED and transistors (again, the BC327 and BC337 appear identical so watch their position). Strictly speaking, the “flash” LED (LED 2; there is no LED 1) is not really required because when the unit is complete, it will be hidden inside the case. However, we left it in situ because, for the sake of a few cents, it meant we could confirm proper operation without having to connect the blindingly bright LED array until we had to! Next, bend the leads of the MOSFET down 90° in the appropriate place so it can mount flat on the PCB with its screw hole aligned with the hole in the PCB. Many people find it easiest to temporarily screw the MOSFET to the PCB, grip the leads in the right place with a pair of fine pliers, remove the screw while still holding the leads with pliers and then bend them down along the edge of the pliers. Now, snap together the two-way and three-way terminal blocks and solder them in position as a five-way, with the access facing towards the outer edge of the PCB. siliconchip.com.au Only four terminals are used; position number four is not connected. The last component to mount is the 7555 timer IC – make sure its notch goes in the same orientation as the Unlike the earlier strobe, the speed pot is mounted external to the board via a short length of ribbon cable. While only two wires are required (shown in red and orange on the overlay diagram) we wired all three terminals. Make the ribbon cable (and indeed the connections to the LED array and the switch-mode supply long enough to be able to open the two halves of the case to work on. We found that we needed to lengthen some of the cables with short lengths of the same colour hookup wire, with the soldered joints covered by heatshrink insulation. With 20/20 hindsight, we wouldn’t have mounted the pot in the middle of Parts list – LED Party Strobe Mk II 1 double-sided PCB, coded 16101141, 95 x 49.5mm (First used January 2014) 1 10 to 100W LED floodlight, mains operated (see text) 1 2-way terminal block (CON1)# 1 3-way terminal block (CON2)# (combine to make 1 x 5-way) 1 short length tinned copper wire 1 knob to suit VR1 1 3-pin mains plug 1 M3 6mm machine screw and nut 4 double-sided foam adhesive pads 1 100mm length 3-way (or 2-way) rainbow cable Short lengths heatshrink tube Short lengths red and black hookup wire Semiconductors 1 7555 CMOS timer (IC1) 1 BC337 NPN transistor (Q1) 1 BC327 PNP transistor (Q2) 1 IRF540N Mosfet (Q3) 1 3mm red LED (LED2) 1 1N4004 1A diode (D1) 2 1N4148 signal diodes (D5,D6) 2 15V 1W Zener diodes (ZD1,ZD2) Capacitors 1 100µF 25V electrolytic 1 330nF MKT (code 334, 330n or 0.33µF) 3 100nF MKT (code 104, 100n or 0.1µF) Resistors (0.25W, 1% unless otherwise stated) 1 220kΩ (code red red yellow brown or red red black orange brown) 1 27kΩ (code red purple orange brown or red purple black red brown) 1 22kΩ (code red red orange brown or red red black red brown) 2 3.3kΩ 0.5W (code orange orange red brown or orange orange black brown brown) 1 1kΩ (code brown black red brown or brown black black brown brown) 1 10Ω (code brown black black brown or brown black black gold brown) 1 1MΩ linear 9mm potentiometer (VR1) August 2015  87 the case because it restricts the travel of the mounting bracket. Fortunately, this didn’t matter too much in our case but if you need to be able to swing that bracket over a wide arc to aim the strobe where you want it, mount the pot as far up the case as you can. Aaagh – no power plug! Probably because the floodlight is sold to all corners of the earth (it does have a 160-265V supply), there is no power plug fitted. But perhaps worse, the mains lead is only about 200mm long, so you’ll either need to fit a mains plug and use it with an extension cord, or fit a mains junction box to the short cable. Whatever you choose, ensure that your mains wiring is safe and triple checked before use. Testing it After checking your component placement and soldering, connect the assembled PCB to the switch-mode supply module (watch the polarity!) without yet connecting the LED array. Connect the mains plug and switch power on. You should find that the “flash” LED does just that – flash – with timing (ie, flash rate) adjustable via the potentiometer. If it doesn’t, you obviously have a component error or bad solder joint. Check the voltage across the power input terminals (3 and 5; remember 4 has no connection) where you should read somewhere around 30V DC. If this is OK, check the voltage across pins 1 and 8 of the IC – that should be very close to 15VDC. If this is OK, the only possibility is (again) a wrongly-placed component or a bad solder joint. Make sure you haven’t mixed up the Zener diodes and the signal diodes (D5/D6), or that you WATTS THE LED ARRAY POWER? There is no marking on the LED Arrays to tell you what the power is. But this one is a 30W array; the one overleaf is a 100W type. How do we know? Simple: count the number of vertical rows. Each row accounts for 10W; here there are 3 rows so it’s a 30 watter. The one overleaf has 10 rows, so it is a 100W type. As mentioned earlier, don’t be tempted to operate these without a heatsink (they don’t need insulation) and if you stare straight into them, well, your mother told you . . . 88  Silicon Chip Here’s the almost-completed Party Strobe, immediately before we fixed that dodgy earth termination and then joined the two halves of the case. You can see the four adhesive foam pads we used to secure the new PCB to the case, along with the lengths of heatshrink cable over wire joins to prevent shorts. Make sure you have the neoprene washer in situ before screwing together AND that none of your internal wiring is poking out! Incidentally, we had to fit the three-pin mains plug seen at the top of the photo above – the floodlight is supplied with only a very short mains cable. haven’t swapped the two transistors (Q1 and Q2). If you find that the LED flash rate is highest when the pot is at minimum, simply reverse the connections to the pot (the ones shown in red and orange). Connect the LED array If everything checks out, unplug it and wait for the Flash LED to stop flashing. Then connect the wires to the LED array (watch the polarity!). Turn the reflector away from your eyes and briefly plug the power in again. You should be rewarded with some very bright flashes – again, adjustable via the flash rate pot. Screw the case together, ensuring that the gasket is in place and that none of the internal wires have managed to spill outside the case. And that’s it: your Party Strobe is now complete. Finally, throw a party! SC siliconchip.com.au