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Phantasmagorical RGB LED Strip Driver This small module drives up to six RGB (red/green/blue) flexible LED strips to produce a rainbow of colours in multiple eyecatching patterns. Use it to decorate a Christmas tree, a shop window or anywhere else you want a bright, pulsating and flashing light show with many colours. It runs off a battery or a DC supply. T HIS PROJECT WAS designed to be used on a float in a street parade. No, this was not an official SILICON CHIP presence . . . It came about because I was helping a friend who was helping a friend to decorate the float and they wanted multiple flexible strings of LEDs, all constantly changing colours. When I first heard about this, the plan was that they were going to build the electronics by hand, using through-hole components on Veroboard and point-to-point wiring – to drive around 30 RGB strips. I’ve built 22  Silicon Chip many prototypes this way and knew that it was a dull and laborious process and the resulting boards can be quite delicate. So in order to head off the inevitable frustration I offered to design a “proper” PCB. This was two weeks before the parade so the design and assembly was a pretty quick affair. The boards were designed to be fast to build – I actually had to do them all in one evening after work and managed to assemble the five boards in just four hours and deliver them to be programmed and wired up. I didn’t see them in action but apparently they worked quite well although by the end of the parade the batteries were pretty flat. We hadn’t had time to put in a low-battery cut-out feature, something which has been rectified in the final design. Obviously, this board is not limited to use on a float so after some tweaking, we are publishing it for general use. Design RGB LED strips can be purchased on 5m reels, made up of 100 joined siliconchip.com.au By NICHOLAS VINEN sections each 50mm long. They are also available in shorter lengths. Fig.1 shows a typical arrangement. These components are mounted on a long, thin flexible PCB with a plastic cover over the top and in our case, with an adhesive backing. Power consumption is around 7.5W/m (375mW per section) at 12V, with all LEDs at full brightness. We measured 920mA for blue, 1150mA for red and 1040mA for green on a 5m strip. Our reels were supplied with mating 4-pin plugs at either end (2.54mm pin spacing), so they can be combined into longer lengths if required. If you cut the strip up into shorter lengths, this exposes a set of four pads on either side of the cut, to which a similar cable can be soldered. We got ours from an internet seller but very similar products are available from Altronics, Cat. X3213 (indoor) and X3214 (outdoor use). Jaycar also have rigid RGB LED pluggable modules (Cat. ZD0456 and ZD0466) and 1m flexible waterproof RGB LED strip (Cat. ZD0478). siliconchip.com.au To control these strips to get any colour we want, we apply 12V to the anode terminal and then vary either the resistance or (in this case) PWM duty cycle between the cathodes and ground, to vary the red, green & blue component brightness. These colours combine so, for example, if all three are driven at a similar level, the resulting light looks (more or less) white. Or if red & blue are driven but green is not, the result is mauve. Now since we drove our strips off a battery, the supply voltage wasn’t constant (this will also be true if the power source is unregulated 12V DC from mains). In fact, the Li-Po batteries we used were 4-cell packs with a full charge voltage of 4 x 4.2V = 16.8V and a flat voltage of 4 x 3V = 12V. An unregulated mains-powered 12V DC supply would have a similar voltage range but regulated supplies are more common at the high currents required. A discarded PC power supply would be eminently suitable. If we simply ignored the varying battery voltage, the LED strips would dim over time as the batteries discharged and we would also risk burning the strips out when the battery is fully charged and the supply is significantly higher than the 12V that the strips are designed to be driven with. One way to avoid this would be to regulate the supply to a constant 12V but a much easier method is to figure out how the brightness of each colour varies as the supply goes above 12V, then monitor the supply voltage and reduce the duty cycle to compensate, giving constant brightness. This is a very efficient way to do it as very little power is lost and it also minimises the component count. Since we only need to switch the LED cathodes, this makes the circuit design easy. For each colour of each strip we just need one low-side switch and an N-channel Mosfet does the job. These are available in dual SMD packages which are quite compact and easy to solder, with suitable voltage and current ratings and an on-resistance figure of around 10mΩ. So for each FET handling 1A, the dissipation is only 10mW. To further simply the circuitry, the Mosfet gates can be driven directly from the outputs of a microcontroller and this is much easier for low-side switching than high-side switching. But we do have to be a little careful A G R B λ λ λ λ λ λ λ λ λ 150Ω 330Ω 150Ω A G R B Fig.1: the circuit diagram of a section of typical RGB LED strip. This is repeated every 50mm, with the connectors at top and bottom joined end-to-end. The strip can be cut into any number of whole sections (up to the maximum of 100 supplied on the reel) and can be driven from either end. The more sections you drive, the more current it draws – see text for details. since microcontroller outputs can provide relatively little current (typically ~40mA DC and 100mA peak) and we also need to make sure we don’t exceed the micro’s ratings. The switching time of a micro output driving the small capacitance of the type of Mosfet we’re using is quite fast at around 100ns so that isn’t really an issue. But when driving 6 x 3 = 18 Mosfets from a single micro, the instantaneous current is a concern should they all switch simultaneously. The micro we’re using has an absolute maximum rating of ±40mA (DC) per output pin and 400mA for the whole device. Examination of the I/O pin source/ sink current vs output voltage graphs suggests that the output transistors have an on-resistance of around 100Ω. So if eight outputs are switched simultaneously (the maximum possible with an 8-bit micro) to discharge Mosfet gates at 5V, the total current at that instant would be (5V ÷ 100Ω) x 8 = 400mA. That’s just equal to the rating but it’s also only for a brief period; as the gates discharge, the sink current May 2014  23 12-16V DC INPUT + – D Q1a* CON1 S G * Q1b D G * ONLY REQUIRED IF LOADS ARE POLARITY SENSITIVE 100k* K 100nF* S A BAT54C D1 100nF GND G 18 AVcc PD0 PD2 UP 8 S2 PD3 PB7 PD4 DOWN PD5 IC1 ATmega48-20AI +5V 2 4 6 8 10 6 Vcc PD1 S1 1 3 5 7 9 ICSP 15 PD6 PD7 MOSI/PB3 PC0 PC1 29 RST/PC6 17 SCK/PB5 16 MISO/PB4 PC2 PC3 PC4 +5V PC5 VR1 10k CON8 1 2 3 19 20 BRIGHTNESS PB0 ADC6 PB1 AREF PB2 GND 100nF 3 G S Q2a D G S BZX84-B15 D1: BAT54C K A1 (NC) Q4a D GND AGND 5 PB6 K S G S Q3b D G S Q6a D Q5b D 9 G 10 S G S Q5a D G S RGB LED CONTROLLER + 12 V 23 24 Q7b D Q7a D Q6b D 26 27 G 28 S G S G S + 12 V 13 14 Q9a D Q8b D Q8a D 21 S G S G S + 12 V Db Db Da Da Gb Sb SaGa Q10b D Q10a D G NC GN ND Vin D Q9b D Q2a G N G C G ND VouN D S G S G S TO RGB LED STRIP 5 CON7 A G R B Q1-10 : Si4944DY A2 TO RGB LED STRIP 4 CON6 A G R B 12 7 TO RGB LED STRIP 3 CON5 A G R B 11 25 TO RGB LED STRIP 2 CON4 A G R B 2 78L05M SC + 12 V 32 1 TO RGB LED STRIP 1 CON3 A G R B 31 G A 20 1 4 + 12 V 30 FB1 FERRITE BEAD Q2b D 100nF 4 22 Vcc ADC7 CON10 1 2 3 4 S Q4b D 2x 100nF 100nF 100k CON9 G OUT IN K Q3a D 10Ω REG1 78 L05 M A2 CON2 A G R B 22µF ZD1* BZX84 -B1 5 +5V A1 33k + 12 V F1 15A FAST TO RGB LED STRIP 6 t Fig.2: the complete circuit diagram of our 6-strip RGB LED driver. It’s a simple affair with microcontroller IC1 driving the gates of 18 Mosfets directly to control the cathodes for three strings of LEDs in each of six connected strips. REG1 derives power for the micro from the nominal 12V supply while S1 & S2 allow the pattern to be changed and VR1 varies the overall LED brightness. rapidly drops. So we don’t see any problems with this arrangement. Battery protection We also need to consider the health of the battery. A lead-acid battery could be used and these can be discharged to about 11.5V before being damaged, but by then the battery will be well and truly flat and the LED strips will be noticeably dimmer. Li-Po batteries should not be dis24  Silicon Chip charged below about 3V per cell, ie, 12V for a 4-cell pack, or else they can be destroyed. So to be safe, the unit should stop drawing current once the battery voltage drops much below 12V. We’re already monitoring the supply to provide LED PWM duty cycle compensation, so it’s simply a matter of programming the micro to turn off all the outputs and go to sleep if the battery voltage drops too low. It can then periodically wake up to check the voltage and if it recovers sufficiently (eg, the battery is under charge), it can then go back to normal operation. In sleep mode, the only part of the circuit drawing any significant current is the 78L05M regulator at about 3mA. With the large battery required for this project, that will give you several days to disconnect the unit and recharge the battery before it goes totally flat. This time could be extended dramatically by replacing the regulator with a lower siliconchip.com.au Parts List 1 double-sided PCB coded 16105141, 82 x 55mm 1-6 RGB LEDs or LED strips 1 12V DC power supply or 12V battery 13 2-way PCB-mount terminal blocks, 5.08mm spacing, rated at 15A+ (CON1-CON7) (eg, Dinkle EK [Altronics P2032A], Weidmuller PM [Jaycar HM3130]) 1 15A SMD fuse, 3216 or 6432 size (1206/2512 imperial) (F1) (element14 2135886, Digi-Key 507-1059-1-ND)** 1 mini horizontal 10kΩ trimpot (VR1) (optional) OR 1 3-pin header (CON8) plus external pot & wiring (optional) 1 5 x 2 pin header (CON9) (not required with pre-programmed microcontroller) 2 PCB-mount tactile buttons (S1,S2) OR 1 4-way pin header (CON10) plus external buttons & wiring 1 SMD ferrite bead, 3216 size (1206 imperial) (element14, RS, Digi-Key) quiescent current type but in most cases this should not be necessary (the micro draws <1µA in sleep mode). The 12V supply is monitored using a 100kΩ/33kΩ resistive divider from that rail to ADC input 7 (pin 22). This 4:1 divider gives a voltage at pin 22 of 2.875-4.25V (11.5-17V supply) which is measured relative to the 5V rail. A 100nF capacitor from the AREF pin (pin 20) to ground filters switching noise from the reference voltage which is derived from AVCC. The microcontroller can be programmed via a standard 10-pin Atmel AVR in-circuit serial programming (ICSP) header (CON9). However, we can supply pre-programmed micros in which case CON9 can be omitted. The original design had a fixed LED display pattern but we decided to revise it to give multiple patterns, hence the addition of pushbutton switches S1 and S2. These are connected to input pins PB3 and PB7 of IC1 which have internal pull-ups enabled. S2 shares a line with the programming header, which is fine as long as you don’t press it during programming. CON10 allows off-board buttons to be used instead of S1/S2 if desired. Trimpot VR1 gives overall LED brightness control or an off-board pot can be wired to CON8 which is fitted in place of VR1. You can also simply solder a wire link between pins 1 and 2 of CON8 so that the LEDs run at full brightness all the time. The ground connection for switch- Circuit description Fig.2 shows the full circuit. The LED strips are wired to 4-way terminal blocks CON2-CON7 and Mosfets Q2a-Q10b switch the cathodes, with the anodes all connected together to the (nominal) 12V supply. This supply comes via input connector CON1 and passes through a 15A PCB-mount SMD fuse, which we put in as last-ditch protection against a serious fault such as a shorted output (Li-Po batteries don’t like to be shorted out). A 22µF capacitor smooths this supply and reduces its impedance. The micro we’ve used is an ATmega48 in a 44-pin SMD package. We chose this because it’s easy to program and as described above, has good output drive capability for switching the Mosfet gates. Its 5V supply is derived from the fused 12V rail via reverse polarity protection Schottky diode D1 and REG1. D1’s two internal diodes are paralleled for lower losses and higher current capability. The micro has a 100nF bypass capacitor for each of its VCC/AVCC (analog supply) inputs. AV CC is smoothed by a low-pass filter formed by a 10Ω resistor in combination with its 100nF bypass capacitor. siliconchip.com.au Semiconductors 1 Atmel ATmega48-20AI or -20AU 8-bit 4KB microcontroller pro­ grammed with 1610514A.HEX (IC1) (element14 Cat 9171312, Digi-Key ATMEGA48-20AU-ND) 1 78L05M SMD 5V 100mA regulator (REG1) (Jaycar ZV1540)* 9 Si4944DY SMD dual N-channel Mosfets or equivalent (Q2-Q10) (Jaycar ZK8821)* 1 BAT54C dual common-cathode Schottky diode (D1)* Capacitors 1 22µF 25V SMD ceramic, 3216 size (1206 imperial) (element14 2354129, Digi-Key 1276-30471-ND) 7 100nF 50V SMD ceramic, X7R, 1608 or 2012 size (0603/0805 imperial) (element14 1301790/ 1301894, Digi-Key 1276-11801-ND/311-1344-1-ND) Resistors (all SMD 1608 or 2012 size [0603/0805 imperial]) 1 100kΩ* 1%    1 10Ω* 1 33kΩ* 1% * These parts are available from element14, RS, Digi-Key and Mouser and can be found by part code or parameter search ** Spare SMD fuses wouldn’t go astray ing Mosfets Q2-Q10 is kept separate from the ground for the rest of the circuit, hence the use of two different symbols. These two grounds are joined at a single point by ferrite bead FB1, which reduces the coupling of switching noise into the microcontroller’s ground, thus reducing errors in its ADC readings. FB1 is shown in the botton lefthand corner of the circuit, connecting the Mosfet ground to the input supply ground. Finally, note that we show components to protect the load from reversed polarity on the input connector. These are Q1, ZD1 and a 100nF capacitor and 100kΩ resistor. However, the LED strips are unlikely to be damaged by reverse polarity so they probably do not need to be installed; a track on the board (shown dashed) connects the ground return directly to CON1 and must be cut if Q1 is to be fitted. We’ve left provision for these components on the PCB, in case a different type of load is connected which is polarity sensitive. Note that fuse F1 is a surfacemounting component and if it blows you will have to de-solder it and solder another in its place. However, with some care in wiring the unit up and ensuring that it’s used within its ratings, there’s no reason for it to blow. If you aren’t planning to use the full May 2014  25 Up CON10 R B 100k FB1 100nF 12-16V DC CON1 ICSP 1 Q10 B + Q6 100nF 100nF 100nF REG1 100nF 10 Ω Q9 G Q8 R + BAT54C 33k D1 IC1 ATmega48 -20AI Q7 22 µF CON6 CON7 R 1 G Q5 CON9 Down G R CON4 Q4 100nF + B CON3 Q3 S2 F1 15A + G 2014 C 16105141 RGB LED Strip Driver VR1 10k + R CON2 Q2 S1 + G 78L05M B D U CON5 B + G R B Fig.3: the PCB is quite compact and is fitted mostly with surface-mounting components, the exceptions being the connectors, pushbuttons S1 & S2 and trimpot VR1. S1, S2 & VR1 can also be mounted off-board to give external controls or left out entirely if their functions are not needed (VR1 must be linked out in this case). current capabilities of the device, eg, your load will never exceed 10A, it’s a good idea to fit a fuse with a lower rating (but higher than the expected maximum load current). You could also use an inline fuse from the battery which would be easier to replace. Software Since this chip only has a handful of PWM channels, we have to use the outputs as general purpose I/Os and arrange the software to provide PWM by constantly updating these output states. They have been arranged to make it simple for the software by wiring up the Mosfet gates to sequentially numbered pins. The micro runs at 8MHz with one of its internal timers configured to divide-by-128 to give 62.5kHz. It then divides this by 256 brightness levels to get 244Hz PWM operation. The main loop continuously calculates the next state of each output as an RGB value from 0-255 (ie, from off to maximum brightness) and then computes the timing for switching the Mosfets off and on to achieve this. The timer interrupt is then set to trigger a subroutine at the right times to turn the outputs on and off to achieve this pattern. This repeats indefinitely. It periodically stops to check the position of VR1 and whether S1 and/or S2 have been pressed. If so, it switches patterns. PCB assembly The PCB assembly is relatively 26  Silicon Chip Above: this photo shows a completed prototype PCB assembly. Note that the final version shown in Fig.3 has a few changes, including the addition of trimpot VR1, pushbutton switches S1 & S2 and SMD fuse F1. straightforward with no particularly difficult-to-solder parts but some care does need to be taken to ensure the SMD solder joints are properly formed and there are no bridges. Start with the SMD ICs and Mosfets, then follow with the passive SMDs and finish up with the through-hole parts. IC1 is probably the best one to do first. This is installed by positioning it on the board with the correct orientation, placing some solder on one of its pads and heating that pad while sliding the IC into place. You should then check its alignment. Make sure all the pins are properly centred on the pads and then solder the diagonally opposite pin. Make a final check that the orientation is correct, then solder the rest of the pins. It’s possible to solder each of IC1’s pins individually with a fine-tipped soldering iron but it is not necessary to do so. You can place the tip of the iron between a pair of pins and flow solder onto both, then clean it up later using solder wick. You could also use a mini-wave/hoof tip or one of various other methods such as hot-air or oven reflow. It’s a good idea to use flux paste, both to aid the initial soldering and in combination with solder wick if cleaning up any bridges is necessary. When finished, clean off any flux residue with a good solvent (we mentioned some in our article on soldering last month), then inspect the joints carefully under a magnifying glass with good illumination. Check that they have all formed good fillets between the IC pins and the PCB. Next, you can then proceed with fitting Mosfets Q2-Q10 and regulator REG1. Pay close attention to the pin 1 marking which may be a dot or bevelled edge and make sure the 78L05M goes in the right place. The pin spacing on these parts is larger than IC1 so it’s realistic to solder the pins individually although the techniques mentioned above remain valid. As with IC1, a careful inspection of the joints is most important. Now fit D1 using a similar approach; you certainly can solder its pins individually. Then follow with the passives (resistors & capacitors) but remember to wait a few seconds after sliding the part into position before soldering the opposite side so that the first joint has had time to cool. One way to check whether these components have been soldered properly is to heat one end and apply gentle pressure on the part with the soldering iron; if the opposite joint is bad, it will slide out of position and you will have to remove it and re-solder it. Assuming that the joint is OK, let it cool and then check the other using a similar method. However, after doing this you should inspect the joints and re-flow them if they look crystalline or lumpy. Solder fuse F1 in place, then move on to the through-hole parts, starting with S1 & S2 or alternatively CON10 which is wired up to external buttons later. Or you could leave these parts siliconchip.com.au off altogether and the unit will then be permanently set to pattern cycle mode. Before fitting the terminal blocks, gang them up into two sets of six, using the integral slots and tabs. That done, make sure they are pushed down fully onto the PCB with their wire entry holes facing outwards before soldering all the pins. Then fit either VR1, a 3-pin header in its place or a wire link between the two lower pads. Finish off by soldering CON9 but note that it isn’t necessary if you’re using a preprogrammed microcontroller. Features & Specifications Outputs: 6 x 3-channel 12V RGB LED strip drivers (common anode), up to 5A each strip (15A total maximum). Input: 12-17V DC at up to 15A from battery (lead-acid, Li-Ion, Li-Po) or mains supply. Patterns: 10 different patterns plus auto-cycle mode which changes pattern periodically. Protection: fuse, reverse polarity protection, battery over-discharge protection. Other features: constant brightness, optional brightness control. Battery cut-out: ~11.5V with 0.5V hysteresis. PWM frequency: ~250Hz. Programming If using a blank micro, now is a good time to program it. First, connect a 12V supply (current-limited, if possible) to CON1 and check that there is 5V between pins 2 & 4 of CON9. You can then connect an AVR ICSP tool and upload the HEX file, which can be downloaded from the SILICON CHIP website (free of charge for subscribers). You will also need to set the ‘fuse bits’. An unfortunate aspect of programming AVRs is that these are not included in the HEX file and there is no consistent way of referring to them. There are two bytes to set. Set the fuse high byte to ‘DC’ hex and the fuse low byte to ‘C2’ hex. Depending on your programmer, you may not be able to set these as hex values so instead, for the high byte, set BODLEVEL to ‘100’ (4.3V) and leave the rest of the settings at their defaults, ie, RSTDISBL = 1 (off), DWEN = 1 (off), SPIEN = 0 (on), WDTON = 1 (off) and EESAVE = 1 (off). For the low byte, set CKSEL = 0010 (Calibrated Internal Oscillator), with CKDIV8 = 1 (off) and SUT = 00 (fast rising power). Leave CKOUT at its default value, ie, CKOUT = 1 (off). This sets the chip to operate at 8MHz, as expected by the software. Testing There isn’t much to test; check that the 5V supply is correct as described above and that the current draw is reasonable (<30mA), then connect a proper 12V supply and a LED strip to one of the outputs and power it back up. You should see the LEDs light up and the colour change over time. If so, you can then switch off and connect strips to the remaining outputs, switch back on and check that they are all operating and displaying the full range of colours. Press S1 & S2 to see that the pattern changes and if VR1 is fitted, adjust it and check that it controls the brightness. Note that if you are using an off-board pot, this will need to be wired up for testing or else the results will be unpredictable (but no damage should occur). Using it Pressing S1 cycles to the next pattern and pressing S2 switches to the previous pattern. Initially, the unit starts with pattern 1, then after a minute or so switches to pattern 2 and eventually after pattern 10, it goes back to the first one. This cycle repeats ‘forever’ but it is cancelled by pressing either S1 or S2 after which it will remain on that same pattern. To switch back to auto-cycling mode, press S1 & S2 simultaneously. VR1 adjusts the maximum duty cycle but note that the duty cycle is also automatically reduced as the supply voltage rises above 12V to give even brightness regardless of battery voltage (down to a minimum 12V). Note also that should the battery voltage drop below about 11.5V (including wiring drops), the unit will shut down until SC it rises above 12V or so. Radio, Television & Hobbies: the COMPLETE archive on DVD YES! A MORE THAN URY NT QUARTER CE ICS ON OF ELECTR HISTORY! This remarkable collection of PDFs covers every issue of R & H, as it was known from the beginning (April 1939 – price sixpence!) right through to the final edition of R, TV & H in March 1965, before it disappeared forever with the change of name to EA. For the first time ever, complete and in one handy DVD, every article and every issue is covered. If you’re an old timer (or even young timer!) into vintage radio, it doesn’t get much more vintage than this. If you’re a student of history, this archive gives an extraordinary insight into the amazing breakthroughs made in radio and electronics technology following the war years. And speaking of the war years, R & H had some of the best propaganda imaginable! ONLY Even if you’re just an electronics dabbler, there’s something here to interest you. Please note: this archive is in PDF format on DVD for PC. Your computer will need a DVD-ROM or DVD-recorder (not a CD!) and Acrobat Reader 6 or above (free download) to enable you to view this archive. This DVD is NOT playable through a standard A/V-type DVD player. Exclusive to: SILICON CHIP siliconchip.com.au 62 $ 00 +$10.00 P&P HERE’S HOW TO ORDER YOUR COPY: BY PHONE:* (02) 9939 3295 9-4 Mon-Fri BY FAX:# (02) 9939 2648 24 Hours 7 Days <at> BY EMAIL:# silchip<at>siliconchip.com.au 24 Hours 7 Days BY MAIL:# PO Box 139, Collaroy NSW 2097 * Please have your credit card handy! # Don’t forget to include your name, address, phone no and credit card details. BY INTERNET:^ siliconchip.com.au 24 Hours 7 Days ^ You will be prompted for required information May 2014  27