Fullscreen HTML5Med Res (??MB)HTML4

The prototype LED lighting system being tested on a Greenspeed recumbent trike. The headlamp uses the Luxeon LED Spotlight described next month and the rear lights comprise four 1W red Luxeons, two equipped with narrow beam collimators and two with wide-angle collimators. The headlamp has a range of well over 50 metres (and will light large reflective signs at 400 metres), while the rear lights are visible from over 500 metres. Note that two Universal High Energy LED Lighting Systems are needed to run this many LEDs! Universal High-Energy LED Lighting System This incredibly versatile LED lighting system uses a rechargeable battery pack and is suitable for nearly any application that needs powerful LED lighting. From a camping light to bicycle lighting to emergency blackout lighting, this project does it all! PART 1: By JOHN CLARKE & JULIAN EDGAR M ANY ELECTRONIC PROJECTS have been designed to run highpower LEDs – but that’s all they do! This project is very different – not only can it run multiple Luxeon LEDs but it also uses intelligent control to allow easy dimming, flashing and automatic switch-on facilities. In addition, the control system monitors the level of the high-capacity internal rechargeable battery pack and supervises battery charging. It also uses various schemes to automatically cut the light output when the battery voltage drops below a certain level, to 70  Silicon Chip give the maximum possible hours of light. The battery can be charged from a mains plugpack, a car, a solar cell or even a human-powered generator. Different light modes The key to the versatility of the “Universal High-Energy LED Lighting System” is the ability to select different operating modes. For example, the system can be used as a normal (always on) lantern or as a flashing lantern. It can also be used as a roadwork-style warning flasher that automatically switches on as it gets dark, or as emergency lighting that automatically activates when mains power is lost. In fact, no less than 11 different light operating modes are available! The desired operating mode is selected by rotating the BCD switch on the PC board. Note that in most cases, once the mode is selected, the switch will be left permanently in that position. The system is then controlled via an external pushbutton switch. Let’s take a closer look at the various modes as set by the BCD switch: (0). ALWAYS OFF – this can be used for siliconchip.com.au Suggested Uses • • • • • • • • Auto-on garden lighting Auto-on blackout emergency lighting Intelligent multi-mode handheld torch or lantern Caving light Fishing light Camping light Intelligent bike headlight or tail-light Industrial warning lights transporting the Universal High Energy LED Lighting System. (1). STANDARD LIGHT – a quick doublepress of the pushbutton switches the LED on and a single press turns it off. Three quick presses from off activates a fast attention-getting flash. (2). MULTIMODE LIGHT – a quick doublepress switches the LED on and a single press turns it off. Holding the pushbutton cycles between full brightness, a dimmed level and a slow flash. Three quick presses from an off state activates a fast attention-getting flash. The dimmed level is stored and reactivated at switch on. (3). CAMPING LIGHT – a quick doublepress switches the LED on, while a single press turns it off. Holding the pushbutton down decreases the brightness before returning to full brightness. Three quick presses from off activate a fast attention-getting flash. The selected dimmed level is reactivated at switch on. (4). WARNING FLASHER – a quick doublepress starts the LED slowly flashing and a single press turns it off. Three quick presses from off activate a fast attention-getting flash. (5). AUTOMATIC TORCH – a quick doublepress switches the LED on, with the LED intensity automatically increasing with falling light. Three quick presses from off activate a fast attention-getting flash and a single press switches the LED off. Unlike Mode 6, this mode provides LED indication of battery condition, even when the Luxeon LED is switched off due to high ambient light levels. (6). PLUGPACK-CHARGED GARDEN LIGHTS – a quick double-press switches the system on but the Luxeon (and battery monitoring LED) stay off until the siliconchip.com.au The High-Energy LED Lighting System is built into a rugged diecast aluminium box. It uses high-capacity C-size 4500mAh nickel metal hydride (NiMH) cells and can drive up to 6W of Luxeon LEDs. In addition, it has intelligent charge and discharge control and user-adjustable modes that allow it be adapted to nearly any Luxeon LED lighting use. ambient light level falls. The Luxeon LED then automatically increases in intensity with falling light. Three quick presses from off activate a fast attention-getting flash, with this mode operating only when the Luxeon LED is already on; ie, below the low ambient light threshold. The system is normally left on but can be switched off with a single pushbutton press, with the battery condition LED also then switched off. (7). SOLAR GARDEN LIGHTS – this is very similar to the above mode except that after the Luxeon LED automatically activates, it stays on for six hours or until the ambient light level rises. (8). ROADWORK WARNING – a quick doublepress switches the system on but the Luxeon LED (and battery monitoring LED) stay off until the ambient light level falls. The Luxeon LED then starts slowly flashing. Three quick presses from off activate a faster flash. April 2006  71 Main Features • • • • • Runs nearly any combination of Luxeon LEDs from 1W to 6W total power • • • • Flashing, dimming and auto switch-off modes Self-contained high-capacity NiMH battery pack Rechargeable from any 8.7–18.6V voltage source Automatic control of charge rate User-selectable modes include auto switch-on as it gets dark or when plugpack power is lost Automatic light output conservation strategies as battery charge drops Rugged diecast aluminium housing Battery level/charge monitor multi-function LED The system can normally be left switched on (there’s very low current drain when the Luxeon LED is off, as the battery status LED is also off). However, if required, the system can be switched off by pressing the pushbutton switch. (9). BICYCLE HEADLIGHT – a quick doublepress switches the Luxeon LED on. The LED is on when ambient light levels are low but switches to flashing when light levels increase. If the Luxeon LED is on and the pushbutton is held down for about three seconds, the unit changes to a “parking flasher” mode. Three quick presses from off activate a fast attention-getting flash and a single press turns the system off (for more on bike lights, see the “Bicycle Lighting System” panel). (10). (A) BICYCLE TAIL-LIGHT – a quick double-press switches the Luxeon LED on. A low-duty cycle flash occurs when light levels are low, the duty cycle increasing as light levels increase. If the Luxeon is on and the pushbutton is held down for about three seconds, the unit changes to a “parking flasher” mode. Three quick presses from off activate a fast attention-getting flash and a single press turns the system off. (11). (B) BLACKOUT EMERGENCY LIGHTING – a quick double-press switches the system on but the Luxeon LED activates only when the light level drops below a preset threshold and charger power is lost. A single press switches the system off. (12). (C) EXIT LIGHT – a quick double-press switches the system on but the Luxeon LED activates only when charger power is lost. A single press switches the system off. 72  Silicon Chip (13). (D) MICROCONTROLLER RESET – used if the battery is discharged to the extent that IC1 behaves erratically. (14). (E) LUXEON DRIVE FREQUENCY – alters the drive frequency to the Luxeon LED. (15). (F) TEST – for setting the reference to 2.49V, testing the LDR and thermistor, and setting the charging current. In all but the Reset, Test and Drive Frequency modes, a quick double press is used to turn the system on while a single press switches it off. In most modes, a fast attention-getting flash is also available and is activated by three quick presses of the pushbutton from off. This fast attention-getting flash could be a lifesaver if something goes wrong when bushwalking or camping, etc. Of course, you aren’t limited to the uses described in our mode descriptions. The Camping Light mode could also be used for a dimmable torch or a reading light, for example. Luxeon LEDs The Universal High Energy LED Lighting System is designed to work with Luxeon LEDs with a total rating of up to 6W. You can use 1W, 3W or 5W units but where multiple LEDs are used, they must all have the same rating (the lowest wattage LED is the one that determines the LED current). In practice, this means that you can use up to six 1W LEDs, one or two 3W LEDs, or a single 5W LED. For example, a garden lighting system might use six 1W LEDs, while a bike headlight might use two 3W LEDs. The only combination not permitted is five 1W LEDs, as it’s not practical to drive five of these in parallel (six 1W LEDs are wired as three parallel groups of two in series). Before building this unit, you first need to decide on the number of Luxeon LEDs to be used and their power rating. That’s because the number of turns wound on the transformer, the value of a resistor and the adjustment of a trimpot all depend on the LEDs that will be driven. In addition, the choice of LEDs determines whether they are wired in series, in parallel or in a series/parallel combination. The higher the total power rating of the LEDs, the greater the current drain and so the shorter the battery life. However, there are major practical advantages in specifying high-wattage LEDs and then dimming or flashing them. Let’s take a look at a typical use to see why this is the case. As an example, you might be running two 3W LEDs (6W total) in a camping lantern. At full brightness, the battery pack will last something in the order of two hours – but that’s at full brightness. If you have the system set to Camping Light mode, you can use the pushbutton to dim the LEDs substantially and in many applications, one-quarter of the available power will be quite sufficient. At this power level, the battery pack will last well over four times as long – ie, eight hours with ease. And the reason we specify 3W LEDs rather than 1-watters? Well, that’s for when you hear some rustling in the bushes and immediately want lots of light. A few pushes of the button and you’ll be illuminating the whole site! The same idea applies when you’ve picked one of the flashing modes. In many cases, the duty cycle of the flash (ie, the proportion of time the LEDs are on for) will be only 3%. The current drain on the battery will then be about 97% less than it would if you were running the LEDs at constant full brightness. In this case, you can take advantage of the attention-drawing capabilities of the very powerful flash while still retaining excellent battery life. In fact, in “Roadwork Warning” mode – where the flasher turns itself on at night and stays off in daylight – the battery life will be weeks! Finally, in many applications it makes more sense to use multiple siliconchip.com.au The LED Lighting Controller is designed for use with Luxeon LEDs or with similar generic units such as those shown at top left. LEDs rather than a single high-power unit. That’s because using multiple LEDs allows you to aim them in different directions and/or use different optics with each LED. For example, emergency blackout lighting usually uses two broad beam lights aimed widely, while a bicycle headlight might use a narrow beam aimed higher than a second broad lower beam. Note that although we’ve referred to Luxeon LEDs throughout this article, any equivalent high-power, high-brightness LEDs (rated at 1W or more) can be used. However, all the prototypes used Luxeon LEDs and optics (eg, collimators) designed for those LEDs. Note that the circuit is not designed for driving conventional 5mm or 3mm high-brightness LEDs. Batteries and charging Four C-size 4500mAh nickel metal hydride cells are used to power the Universal High Energy LED Lighting System. These provide the best compromise between volume, capacity and cost. Battery charging is automatically supervised by the microcontroller. In its default mode, all you need do is provide an 8.7V-18.6V DC input voltage from a source capable of supplying 700mA. This means that the batteries can be charged directly from a 1A 12V plugpack or a car cigarette lighter socket. Note, however, that a power source with greater or less current capability siliconchip.com.au than 700mA can also be used – see the Adjustable Charging Current panel next month. If the charging voltage is outside the required range, the system automatically switches off the charge. In operation, the unit automatically selects one of three battery charging modes. These are (1) Fast Charge, (2) Top-Up and (3) Maintenance. Unless the user has requested a non-standard battery charge rate, the Fast Charge mode (indicated by the battery monitor LED showing a 4Hz green flash) charges at 700mA. A timer prevents Fast Charge mode running longer than appropriate (to prevent over-charging), the actual time depending on the charge rate. For example, if Fast Charge is set to operate at 700mA, the timeout is typically nine hours. Table 1 shows the time-out periods for the other charge rates. Note that the charge rate referred to here is the current supplied by the charging source. As we shall see later, this is not necessarily the battery charge current. In addition to timing the duration of charge, the unit also monitors the battery temperature to detect an appropriate end of charge point. If the battery temperature rises by 20°C during charging, the charge mode switches from Fast Charge to Top-Up. Top-Up mode, indicated by a slower 2Hz green flash of the battery monitor LED, runs for one hour at half the fullcharge rate (unless the full-charge is only 100mA, in which case this rate of charge is maintained). Finally, in Maintenance mode, the charg­ing rate is set to 100mA – indicated by the battery monitor LED flashing at a 1Hz rate. Note: when the LED is flashing green, it will go red as it switches off each time. This is normal. If the battery level falls to 1.15V/ cell while in the Top-Up or Maintenance charge modes, Fast Charge is automatically reinstated. If an overtemperature condition is detected, the system switches back to Maintenance charge mode. And if a cell over-voltage condition is detected (cell voltage greater than 1.95V), the charging system switches off until cell voltage drops below 1.95V, at which point Maintenance mode is activated. Finally, if the input power is removed during Fast Charge and then re-applied, charging will not restart unless the cell voltage is below 1.5V per cell. Also, if the leads to the thermistor are broken, charging cannot occur All that might sound complicated but in normal use, all charging is done completely automatically. All you need do is look at the indicator LED – the slower it is flashing, the greater the charge level in the battery pack. Flat battery strategy The indicator LED also shows the battery level when the system is not being charged (but the power is on). It uses the following logic: (1) >1.2V per cell – green (2) >1.15V – orange (3) >1.1V – orange flashing (4) >1.05V – red (5) >1V – flashing red (6) <1V – off The logic is easy to remember – green for good (more than 50% capacity left), orange for less than half battery capacity (not-so-good), and flashing red for bad. And if the battery LED is off, that’s very bad. However, the user has plenty of warning when the battery voltage is low. That’s because when cell voltage drops below 1.05V (and the battery LED starts flashing red), the Luxeon LED output automatically decreases to half power. Should the battery voltage fall even further, the Luxeon output switches to flashing at the “attention-getting” rate and the battery monitor LED is switched off. Note, however, that if the April 2006  73 Fast Charge Rate Setting Timeout Period 8.7-12.6V Input Timeout Period 12.6-15.6V Input Timeout Period 15.6-18.6V Input 100mA Indefinite Indefinite Indefinite 200mA 33h 22h 17h 300mA 22h 15h 11h 400mA 17h 12h 9h 500mA 14h 9h 7h 600mA 11h 7h 6h and Automatic Torch), the LDR can be mounted on the box containing the rest of the system so that it detects the ambient light level. However, in the Bicycle Tail-light mode, the LDR is primarily used to detect the headlights of cars approaching the bike from the rear. In this way, the duty cycle of the flashing tail-light increases as the cars draw nearer. To be effective in this application, the LDR needs to be remotely mounted in a tube facing rearwards. 700mA 9h 6h 5h How it works 800mA 8h 5h 4h 900mA 7h 4h 3h 1A 7h 4h 3h Refer now to Fig.1 for the circuit details. It’s based on a single microcontroller (IC1) and its custom software. As detailed above, it controls the lighting of the Luxeon(s) as well as supervising battery charging. In addition, the microcontroller also controls the Luxeon output based on the mode selected by the user. In short, IC1 forms the heart and soul of this project. The four C-size NiMH cells provide a nominal 4.8V supply to power the circuit. In addition, the supply for IC1 is regulated using a low drop-out 3-terminal regulator (REG1). This is needed to ensure that IC1’s supply voltage is maintained at 5V even when charging, when battery voltage can rise above 7V. Table 1: Charger Time-Out Periods When connected to a power source, the battery pack fast charges until a timeout period elapses or the battery temperature rises by more than 20°C. This table shows the time-out periods for the different user-selectable charging rates versus input voltage. The default is 700mA and for input voltages below 12.6V, the charger will change from “Fast Charge” mode to “Top-Up” after nine hours . system is set to one of the slow flashing modes, the flash rate doesn’t change as the battery drops to this level. The attention-getting flash rate uses a duty cycle of just 12.5% at a frequency of 2Hz, so the unit continues to provide lighting for a very long time, even after the battery is nominally flat. Note: a single LED is used to indicate both battery level and charging rate. Normally, it will be obvious whether charge or battery level is being shown by the LED. However, if the Universal High-Energy LED Lighting System is being charged by an intermittent (eg, human-powered) generator and at the same time is powering a Luxeon, the function might not be immediately clear. In this case, there’s an easy rule to remember: flashing green indicates charging is occurring – see Table 2. Flashing & dimming Because the unit can flash and dim in a number of different modes, let’s take a look at what actually occurs in each mode. First, as already stated, the attention-getting flashing uses a 12.5% duty cycle at 2Hz. This allows the Luxeon LED to be used to light your way and/ or to attract attention while using very little power. This feature is available with three quick button pushes in most modes (as well as occurring when the battery is nearly exhausted). 74  Silicon Chip The modes that incorporate a specific flasher function (ie, Multimode Light, Warning Flasher and Roadwork Warning) and also the bicycle “parking light” use a 3.1% duty cycle at 0.5Hz. In other words, the Luxeon LED flashes once every 2s for 1/32nd of the available time. This mode draws only low average current and so battery life is excellent. The Bicycle Headlight mode flashes the headlight when the light level is relatively high. This uses a flash frequency of 4Hz and a duty cycle of 25%. By contrast, the Bicycle Tail-light mode flashes the light at the same frequency but has a duty cycle that varies between 12.5% at low light levels to 50% at high light levels. In both cases, this conserves power while providing excellent visibility and illumination in all ambient lighting conditions. Dimming is available in the Multimode Light mode (where current is reduced to 25%) and in the Camping Light and Automatic Torch modes (where the current is reduced from 100% to 2.5% in 2.5% steps). These dimming increments are so small that the light appears to dim steplessly. Ambient light measurement A number of modes require the use of an external light sensor. A Light Dependent Resistor (LDR) is used for this purpose. In some applications (eg, Blackout Emergency Lighting LED driver The Luxeon LED driver circuitry is based on Mosfet Q2, transformer T1 and current feedback resistor R1. In operation, a pulse width modulation (PWM) output from pin 9 of IC1 drives Q2 on and off at a duty cycle that can be varied to set the LED current. Q2 is a logic level Mosfet that can be fully switched on with logic level (5V) signals at its gate. Standard Mosfets usually require at least 10V at the gate in order for the device to fully switch on, so a logic level Mosfet is best suited to this circuit since we have only a low voltage drive from IC1 to Q2. The circuit works as follows: when Q2 is switched on, current flows through T1’s primary winding. Then, when Q2 is subsequently switched off, the current through this winding (and thus its associated magnetic field) collapses and induces a voltage across T1’s secondary winding. This voltage is then rectified using diodes D3-D6 and filtered by a 470mF capacitor. The resulting DC supply drives the siliconchip.com.au siliconchip.com.au April 2006  75 Fig.1: microcontroller IC1 controls both the charging current (via MOSFET Q1 and a filter circuit consisting of inductor L1 and diode D2) and the Luxeon LED current (via Q2). It also monitors switches S1 and S2 (Mode), the battery temperature (via a thermistor) and the ambient light level (via an LDR). Bicycle Lighting System This unit can be configured to produce very effective bicycle lights – both front and rear. Let’s look at the headlight first. Bike headlights perform two functions: (1) they illuminate the road ahead for the rider (obviously); and (2) they alert motorists to the rider’s presence. The best way of alerting motorists is to flash the headlight rapidly, while the best way of showing the road ahead is to light the headlight continuously. So a headlight that automatically changes from flashing (in high ambient light) to constantly on (in low ambient light) provides the best of both worlds. The Bicycle Headlight mode gives just this function. The designers of tail-lights also face a dilemma. A tail-light that flashes with a long duty cycle is more attention-getting than one that flashes with a short duty cycle. However, a short duty cycle means less total current draw from the battery and less heat build-up in the LED. This means it’s best to use a short duty cycle when the cyclist is alone on the road. This problem is easily overcome by selecting Bicycle Tail-light mode. This normally flashes the tail-light with a short duty cycle but automatically increases the duty cycle when the headlights of an approaching car are detected from behind. Luxeon LED (or LEDs), the current also passing through feedback resistor R1. The voltage developed across R1 is then sampled using a voltage divider consisting of a 1kW resistor, trimpot VR4 and a 2.2kW resistor to ground. VR4 adjusts the voltage “seen” by IC1 at its AN0 input (pin 17). In operation, IC1 maintains the LED current set by trimpot VR4 at a constant value. It does this by adjusting the duty cycle of the PWM switching signal applied to Q2’s gate. This duty cycle can be very finely controlled in 1024 steps between fully off and fully on to control the LED brightness. The PWM signal is normally 7.8kHz but 13kHz can be used instead. This higher frequency reduces the faint but audible squeal produced by the transformer but the dimming control is not as precise. Note that the PWM output at pin 9 of IC1 drives Q2 via a 1mF capacitor. This AC coupling is included as a safety measure, in case IC1 locks up and sets pin 9 permanently at 5V. If this occurs, Q2’s gate is held low via a 10kW resistor, thus preventing a short circuit with T1’s primary permanently connected across the battery. Note: IC1 could “lock-up” if the batteries were allowed to discharge to below 3V, at which point IC1’s operation cannot be guaranteed. Zener diode ZD2 protects the 470mF Table 2: LED Status vs Battery Condition Charging Discharging LED Status Condition Fast green flash Fast Charging Medium green flash Top-up charging Slow green flash Maintenance charging Steady green Battery high level Steady orange Battery medium level Orange flashing Charge needed Steady red Charge urgently needed Flashing red Luxeon output halved Off Luxeon output flashing This table shows the tri-colour LED indications for the battery condition. Note that at the threshold voltages between the various conditions, the LED flash rate and/or colour may alternate until the battery voltage rises or falls sufficiently. 76  Silicon Chip cap­acitor from instantaneous excess voltage if the Luxeon load is disconnected while being driven. In addition, the software shuts down the drive circuit and switches the unit off if the connection goes open circuit. LED drive strategy As already noted, the Luxeon LEDs are supplied with current from a nominal 4.8V battery via transformer T1, which is switched on and off using Q2. This type of driver is far more efficient than using a series limiting resistor to set the LED current and also allows us to maintain the LED current as the battery voltage falls. In addition, this arrangement allows us to provide drive for a wide range of LED combinations that would otherwise be impossible to power from a 4.8V battery. For example, a 5W Luxeon LED internally incorporates two LEDs in series, so the voltage drop across it is similar to two 3W Luxeons connected in series. This voltage drop amounts to about 6.8V. This means that a series dropping resistor between a 5W Luxeon and a 4.8V battery would not drive the LED to anywhere near full brightness. However, with transformer T1 and Q2, the switching can be arranged to fully drive a 5W Luxeon. In this case, T1’s windings need to be wound to step-up the voltage, since the 4.8V battery voltage is lower than the total LED voltage of about 6.8V. When driven at its maximum current of 350mA, a single 1W Luxeon LED will have approximately 3.4V across it. In this case, T1 is wound to step down the voltage – ie, less turns on the secondary winding than on the primary – because the supply voltage is greater than the voltage required across the LED. As previously mentioned, when driving more than one Luxeon LED, they are connected in series/parallel combinations. For example, two 1W Luxeons are connected in series and we need twice the voltage used for a single LED – ie, about 6.8V. The current through each LED is still set at 350mA, however. Ideally, when driving more than one LED, it’s best to connect them in series so that they all receive the same current. However, for more than three LEDs, this becomes impractical as the drive voltage needs to be increased to a relatively high value and there isn’t siliconchip.com.au Par t s Lis t– LED Lighting System 1 PC board, code 11004061, 104 x 79mm 1 diecast IP65 box, 115 x 90 x 55mm 1 selection of Luxeon LEDs to suit application 4 C-size 4500mAh NiMH cells with solder tabs 1 12VDC 1A plugpack (or similar) 2 FX2240 or equivalent pot core and bobbin assemblies (L1, T2) 1 IP65 sealed single-pole pushbutton switch (Farnell 312-0880, Omron B3WN-6002) (S1) 1 binary coded DIL rotary switch (0-F) (S2) 1 LDR with light resistance of 50kW (Jaycar RD-3480 or equivalent) (LDR1) 1 NTC thermistor with 47kW resistance at 25°C 1 30A in-line blade fuse holder 2 M205 PC-mount fuse clips 1 M205 2A fast blow fuse (F1) 1 5A blade fuse (F2) 1 DIP18 IC socket 1 3-6.5mm IP68 waterproof cable gland 1 2-pin DIN panel socket 1 2-pin DIN line plug 1 red neon bezel for LDR window (Jaycar SL-2630 or equivalent) 2 TO-220 silicone insulating washers 2 M3 x 9mm tapped Nylon spacers (cut to 4 x 4mm) 8 M3 x 12mm Nylon screws 8 M3 Nylon nuts 1 4m length 0.63mm enamelled copper wire 1 1m length of 5A figure-8 cable enough room to wind sufficient turns at the required wire thickness on T1 to achieve this. As a result, we run a maximum of two LEDs in series when driving four or six LEDs. These series-connected LEDs are then connected as two or three parallel pairs, with the current shared between them. Admittedly, the current sharing may not be perfect but it is better than just running all the LEDs in parallel. Charging Power to charge the batteries is provided by an external supply, with siliconchip.com.au 1 100mm length of red or brown 7.5A hookup wire 1 100mm length of black or blue hookup wire 1 50mm length of red 5A hookup wire 1 50mm length of green 5A hookup wire 1 100mm length of twisted pair light-duty hookup wire 1 150mm length of 0.8mm tinned copper wire 1 100mm length of 3mm heatshrink tubing 2 11mm-dia. x 0.5mm-thick PVC discs (as a gap for L1 and T1 cores) (from plastic book covers, roll-up cutting mat, etc) 3 100mm cable ties 2 200mm cable ties 16 PC stakes 1 small tube of neutral-cure silicone sealant Semiconductors 1 PIC16F88 microcontroller programmed with Luxeon.hex (IC1) 1 IRF9540 100V 23A P-channel Mosfet (Q1) 1 STP45NF06L 60V 38A N-channel logic level Mosfet (Q2) 2 2N7000 N-channel Mosfets (Q3,Q4) 1 BC337 NPN transistor (Q5) 1 LM336-2.5 voltage reference (REF1) 1 LP2950CZ-5.0 regulator (REG1) 1 tri-colour (green/red) 3-leaded LED (LED1) 2 FR302 fast 3A diodes (D1,D2) diode D1 providing reverse polarity protection. Fuse F1 protects against short circuits in the charger circuitry. In operation, the charge rate is controlled by rapidly switching Mosfet Q1 on and off. This sets the duty cycle and thus the charging current through the batteries. Mosfet Q1 is a P-channel type and is switched on when its gate voltage is pulled below its source voltage. It’s driven by transistor Q5 which in turn is controlled via the RA6 output of IC1. When RA6 goes high, Q5 turns on and pulls Q1’s gate low via a 47W resistor, thus turning Q1 on. Conversely, when 4 1N5822 3A Schottky diodes (D3-D6) 1 1N4148 signal diode (D7) 1 18V 1W zener diode (ZD1) 1 20V 1W zener diode (ZD2) Capacitors 1 4700mF 10V low-ESR capacitor 3 470mF 25V low-ESR capacitors 1 100mF 16V PC electrolytic 2 10mF 16V PC electrolytic 4 1mF 16V PC electrolytic 3 100nF MKT polyester 2 1nF MKT polyester Resistors (0.5W, 1%) 3 470kW 2 470W 1 220kW 1 330W 1W 2 56kW 1 47W 2 10kW 1 10W 2 2.2kW 1 1.2W 5W 2 1kW 1 10kW 7-resistor 8-pin array (Bournes 4608X-101) (Farnell 148-973) 1 0.5W 2W surface mount (Welwyn LR series 2512 case) (Farnell 361-0433) 1 0.2W 2W surface mount (Welwyn LR series 2512 case) (Farnell 361-0410) Trimpots 1 500kW horizontal trimpot (code 504) (VR1) 1 50kW horizontal trimpot (code 503) (VR2) 1 10kW multiturn top adjust trimpot (code 103) (VR3) 1 10kW horizontal trimpot (code 103) (VR4) Q5 turns off, Q1’s gate is pulled to the source voltage via a 330W resistor and so Q1 also turns off. ZD1 ensures that Q1’s gate-source voltage is limited to 18V if the external supply voltage is too high. Diode D2 and inductor L1 form a step-down filter circuit. It works like this: when Q1 is switched on, current flows from through L1 and charges the batteries. Then, when Q1 switches off, D2 becomes forward-biased and the energy stored in L1 continues to supply a charging current – rather like a flywheel effect. Altering Q1’s duty cycle varies the charging current, to give the variApril 2006  77 Battery Amp-Hour Ratings The NiMH cells used in this project have a 4500mAh capacity. This rating refers to the amount of current that can be drawn continuously from the cells over a 5-hour period. For 4500mAh cells, this means that we can draw 0.9A (4.5/5) for five hours before the battery is discharged. Typically, individual cells will be at 1.25V during discharge (provided they were fully charged in the first place) but their voltage drops to around 0.9V when discharged. In many cases, the Luxeon LED Lighting System will draw more than 0.9A from the cells. For example, this occurs when driving LEDs rated at 2W or more at their full power rating. In this case (ie, if the current drawn exceeds 0.9A), the usable cell capacity will be less than the specified 4500mAh. There are two reasons for this. First, when drawing higher currents, the cell voltage is lower and this means that the system stops driving the LEDs at full power before the cells are fully discharged. Second, the cells dissipate power as heat when delivering high currents and so there is a loss of efficiency. On the other hand, the capacity of the cells will be higher if the current drawn from the cells is intermittent rather than continuous. So the Universal High-Energy LED Lighting System can be used for a longer periods on the one charge if the LEDs are not driven continuously until the cells are discharged but rather used intermittently. Calculating the expected discharge time for each Luxeon LED load is rather difficult. That’s because the current drawn by the Luxeon LEDs automatically increases as the battery voltage drops over the discharge period. However, in a worse case scenario of driving 6W of LEDs continuously, full output power will be maintained for about two hours. ous charging modes – ie, Fast Charge, Top-Up and Maintenance. Note that this switching circuit also acts as a power converter – stepping down the input voltage to the battery voltage allows the charging current to be increased. The charging current is monitored using a 1.2W 5W resistor. In operation, the voltage across this resistor is proportional to the input current and this is monitored by the AN2 input of IC1 via Mosfets Q3 & Q4. It works like this: two voltage dividers consisting of 470kW and 56kW resistors sample the voltage at both ends of the 1.2W resistor. The resulting attenuated “input” and “output” voltages are then filtered using 1mF capacitors and fed to the drains of Q3 & Q4 respectively. By alternately switching these Mosfets on and off, IC1’s AN2 input can monitor first one voltage and then the other. In practice, Q3 and Q4 are turned on when the RB4 and RB5 outputs alternately go high respectively. Thus, when Q3 turns on, IC1 monitors the voltage on the input side of the 1.2W resistor. Similarly, when Q4 turns on, IC1 monitors the voltage on the 78  Silicon Chip output side. The microcontroller then calculates the charging current and adjusts the duty cycle output at RA6 to maintain the required rate. Switching the RB4 and RB5 outputs also simultaneously changes the function of the coloured LED – ie, from showing “battery level” to “charge”. As a result, each time the system switches on Q3 to measure the charger’s input voltage, the battery/charge LED briefly flashes. This allows you to find the control unit in the dark! Battery indication LED1 provides battery level indication. It can produce a green light, a red light or an orange light (both red and green LEDs lit) – see Table 2. IC1 monitors the battery voltage at its AN1 input (pin 18) via a 470kW/220kW attenuator. Voltage measurements The voltage measurements made by IC1 are all referenced to an accurate voltage source. This is provided by REF1 which is an LM336 2.490V precision voltage reference. In operation, current is supplied to REF1 via a 2.2kW resistor when RA7 goes high. Trimpot VR3 is used to set the reference voltage to 2.490V and this is then fed to IC1’s VREF+ input. Note that the RA7 output is only momentarily activated (taken high) at regular intervals when the Luxeon LEDs are off, whereas RA7 is always high when the Luxeons are on. This gives a worthwhile power saving when the LEDs are off. As well as driving REF1, IC1’s RA7 output is also connected to the common (C) terminal of binary-coded rotary switch S2. S2’s switched connections are normally pulled low using four 10kW resistors which are part of a 7-resistor SIL package. However, if a contact is closed, its corresponding pin will be pulled high each time the common terminal is pulled high by RA7 and this sets the mode. The LDR and the thermistor are both powered from the REF1 supply. IC1’s AN6 input monitors the LDR, while AN5 monitors the thermistor. VR1 and VR2 set the levels for the LDR and thermistor respectively. Power switch S1 is monitored by IC1’s RB0 input. This input is normally pulled low via a 10kW resistor but when S1 is pressed, RB0 is pulled to +5V and the circuit toggles on or off. Low power modes IC1’s internal oscillator operates at either 31.25kHz or 8MHz, depending on the mode of operation. When the circuit is ostensibly off, the oscillator operates at 31.25kHz to conserve power. In addition, RA7 is low and no current is supplied to REF1, the LDR or the thermistor. In this state, current is drawn only by REG1 and IC1, with switch S2 and the charger input voltage monitored once every few seconds. However, if the Luxeon LED is to be driven or if charging starts, the oscillator is reconfigured to operate at 8MHz. In addition, the Plugpack-Charged Garden Lights, Solar Garden Lights and Roadwork Warning modes are all special low-power modes. When the Luxeon LED is off, the standby current in these modes is decreased to just 400mA. To help achieve this very low current draw, the battery indicator LED is also switched off. That’s all we have space for this month. Next month, we’ll cover the construction and show you how to build some very effective LuxeonSC powered lights. siliconchip.com.au