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NOT 1 WHITE LED TORC TOR Both of these LED torches have considerably more light output than our original design in the December 2000 issue. They use more LEDs and they run from a single AA or D cell which will have a long life. They make very good torches for camping, walking at night or for emergency work on your car. Design by JOHN CLARKE T hese LED torches produce a beautiful even spread of white light which is quite different from that of conventional torches using Krypton bulbs. Conventional torches tend to produce a “hot spot” that can penetrate the darkness for some distance and they have a larger cone of much less intense illumination. Overall, they tend to have quite a narrow beam and you have to move the torch around a lot to adequately light up the area in front of you. By contrast, these LED torches have a much wider diffuse beam, giving a very even spread of light without a central hot spot. For most of the time, this more diffuse beam is much easier on the eyes and the colour of objects is much more natural. In fact, it is like carrying a source of daylight around. So these LED torches are ideal for bushwalking (at night!), even in very 58  Silicon Chip rough terrain, for illumination inside a tent or over a picnic table and as noted above, for emergency work on your car if, perish the thought, you break down at night. Constant brightness Another big advantage of these LED torches is their constant brightness, regardless of battery voltage. Conventional torches start out with high brightness when the batteries are fresh but they soon dull down as the cells discharge. By the time the cells are down to 1V, the light output is woeful. These LED torches have the same light output even if the cell voltage goes below 1V. And they can also run with Nicad and NiMH cells which give a nominal 1.2V. Conventional torches are hopeless with 1.2V cells, unless they have been specifically designed to run from rechargeables. Not only that, torch bulbs have a notoriously short life and they can fail at the most inopportune moments. In fact, any time a torch bulb fails is inconvenient, by definition. After all, if a torch bulb failed when it was convenient, you probably don’t need it. Once you change over to a LED torch, you will never need to change a LED – they last a life-time (well, practically). Two versions We are describing two versions of this LED torch, both of which use the same basic circuit. One version uses three white LEDs and runs from a single AA cell in a 2-cell torch. The second version uses six white LEDs and runs from one or two D cells and can fit in a 2-cell or 3-cell torch. These torches use far less current than a conventional Krypton bulb torch. A twin D-cell torch bulb nor- BUT 2 CHES TO BUILD! RCHES Features       mally pulls about 0.8A at 3V, dropping to around 0.7A at 2.4V. In power terms, this is 2.4W at 3V, dropping to 1.68W at 2.4V – this is why conventional torches are so dull when the cells aren’t fresh. By comparison, our D cell 6-LED version of the torch pulls only 480mA at 1.5V, rising to 650mA at 1V. This is less than one third of the power drain of the conventional torch. Again, in a conventional twin AA cell torch, the Krypton bulb pulls about 0.47A at 2.2V or just over 1W. Super soft white light Constant brightness over cell life Indefinite lamp life Extended cell life Ideal for use with Nicad & NiMH cells D cell version has brightness control Our single AA cell 3-LED torch pulls 210mA at 1.5V, rising to about 360mA at 1V. Again this is one third of the power drain of the equivalent conventional torch. Circuit details As with our original white LED torch described in the December 2000 issue, both these torches are based on a DC-DC converter. The DC-DC converter for the AA-cell torch is about the same size as an AA cell, while the converter for the D-cell torch is about the same size as a D cell. The larger D-cell converter includes a brightness control and can drive six white LEDs instead of three. Fig.1 shows the D-cell torch while Fig.2 shows the AA-cell version. Both use a Maxim MAX1676 high efficiency step-up DC-DC converter and an inductor to provide the power conversion. The Maxim MAX1676 was originally intended for use in mobile phones, as a single cell voltage booster, so it is ideal for this torch application. An “exploded” view of the “D” torch which has a DC-DC converter capable of driving six ultrabright white LEDs from a single C or D cell. The white cylinder insulates the PC board assembly from any metal fittings in the torch. MAY 2001  59 Top trace is the inductor waveform at pin 9 of IC1 for the A-cell version. Its frequency is 176kHz. The period when the voltage is low charges the inductor and the high level is when the charge is transferred to the output. The lower trace is the output voltage. It is 3.96V and has a 160mV of ripple. The block diagram of Fig.3 shows the internal schematic of the MAX1676 and the external components needed, including the key component – L1, a 22µH inductor. The internal Mosfets, Q1 & Q2, do all the high speed switching work. Circuit operation is as follows: current flows through inductor L1 and Mosfet Q1. When the current builds up to 1A, Q1 turns off and Q2 turns on. The charge in inductor L1 is then transferred via Q2 to capacitor C1 and the load. The voltage at Vout is fed back to the MAX1676 via a resistive divider comprising R5 & R6. The internal control circuit derives its power from the Vout terminal and so when power is first applied to the circuit, current flows through L1 and Q2 to power the control circuit. Q2 is a P-channel Mosfet which requires at least 1V at the power source in order to be closed and pass the voltage back to the control circuit. For lower voltages it is necessary to include an external Schottky diode in parallel with Q2 to allow current to flow to the control circuit. Once the circuit starts up, it is powered from the Vout supply and Q2 then performs its task of switching the charge from L1 to the load with minimal voltage loss and the diode is effectively out of circuit. 60  Silicon Chip As shown in Fig.1 & Fig.2, the two circuits are very similar. Let’s have a look at Fig.2, the AA cell version. It has a fixed resistive divider for the voltage feedback at pin 1. Inductor L1 must have very low DC resistance to ensure high efficiency of the circuit. As mentioned above, the inductor is charged until the current through it reaches 1A. The inductor must not saturate at 1A and also it must have a low enough resistance to ensure that the current actually rises to 1A. The step-up circuit will not operate if the 1A limit is not reached. Thus we have used an inductor which has a DC resistance of 0.2Ω. Standard commercially wound inductors with wire resistances of more than 0.5Ω will not let the circuit operate. The output supply rail is close to 4V, as set by the 100kΩ and 47kΩ divider resistors and it is bypassed with a 47µF tantalum capacitor. Each LED is powered separately using a 27Ω current limiting resistor to ensure equal current sharing. The nominal LED forward voltage is about 3.5V and so the calculated current through each LED is (4V 3.5V)/27Ω = 18.5mA. In practice, the LED current is slightly higher than this. By the way, the AA-cell version could be powered with a C cell, if built into a C-cell torch. D-cell version The D-cell version uses an inductor which has a lower resistance again than in the AA-cell version and it uses a larger core. The value of inductance is the same at 22µH but the lower resistance ensures higher efficiency for step-up conversion. This circuit can Top trace is the inductor waveform at pin 9 of IC1 for the D-cell version. The glitches are a reset that automatically occurs within the IC to ensure operation at low loads. Frequency of operation is 133kHz and the low output is when the inductor is charging. The energy is transferred to the load when the waveform is high. Lower trace is the output voltage at 4.07V with a 350mV ripple. Fig.1 (above left) and Fig.2 (above right) show the “D” cell and “AA” cell variants respectively. Both are based on the MAX1676 IC high-efficiency DC-DC converter, a chip originally designed for use in mobile phones. MAY 2001  61 Fig.3: inside the MAX1676 DC-DC converter. Its operation is fully described in the text. drive up to six white LEDs. There is also a trimpot, VR1, to adjust the output voltage so that the LED brightness can be varied from almost zero to maximum brilliance. A 200Ω resistor at pin 7, in conjunction with internal Mosfet Q3, provides damping for the inductor when it is released from charging. This damps oscillations and ringing which can otherwise cause electromagnetic interference (EMI). The Schottky diode D1 is not required if the circuit is powered with two D cells. Both circuits include reverse polarity protection, by virtue of diode D2, which conducts if the battery is inserted incorrectly. Diode D2 provides only short-term protection since the current flow will be high. You should check the battery polar- ity immediately if the torch is found not to work. Construction Construction of these LED torches will require patience, good eyesight, a magnifying glass and some experience with soldering. Why? Because we are using a surface-mount IC for IC1. The IC is soldered onto a u10MAX carrier PC board for the D-cell version (Fig.4). but solders directly to the PC board of the AA-cell version (Fig.5). Regardless of which version you build, soldering this IC in place will require a modified soldering bit which has been filed to a narrow screwdriver shape. The idea is to solder all five pins on each side of the IC at the one time. Before soldering in the IC, check the PC boards for any shorts or breaks in the tracks. Any problems in the surface mount area probably cannot be fixed unless there is only a small short between tracks which can be cleared with a sharp knife. The PC boards must be tinned (solder-plated) before use so that the IC can be soldered in without damaging the fine tracks. This should have already been done by the PC board manufacturer. One method of soldering in the IC by hand is to initially cover the underside of the IC pins with solder by wiping over them with a standard chisel-shaped soldering bit which is lightly coated with solder. Make sure that the solder does not bridge between the IC pins. If it does, clean the soldering tip and wipe the excess solder off the IC pins with the now cleaned tip. Check the IC with a magnifying Fig.4: the component overlay of the “D” cell version. Note the position of the “daughter board” containing the MAX1676 SMD (surface mount device) IC. These devices can be a little tricky to solder – the text of this article should help! The photo at right shows the complete board but it is rotated through 180° compared to the component overlay. 62  Silicon Chip glass to be sure the IC pins are all tinned, without any shorts between the pins. Then place the IC onto the PC board and align the pin 1 indicator on the IC (a small dot on the body) with the pin 1 pad on the PC board. Straighten up the IC so it sits correctly on the IC pads. Now heat up the modified soldering iron tip (sharp screwdriver shape) which is untinned or cleaned of solder with a wet sponge. Apply the tip to the leads on one side of the IC to solder it in place. Check that it is still aligned onto the IC pads correctly. If not reheat the pins and align correctly. When one side has been soldered in place heat the remaining pins on the other side of the IC to the PC board. Now you will need to carefully inspect the IC soldering using a magnifying glass. Check for lifted pins on the IC and shorts between pins. Finally, use a multimeter to check that each pin is indeed connected to its respective track on the PC board. D-Cell version The D-cell version of the LED Torch can be assembled as shown in Fig.4. Insert the PC stakes with the long end going down into the PC board to give a similar pin height above and below the PC board. Install the u10MAX PC board onto the main board using short lengths of tinned copper wire passing through each PC board. Make sure that the u10MAX board is oriented correctly, with pin 1 lined up on both boards. Insert and solder all the resistors and capacitors, taking care with the tantalum and electrolytic types which must be oriented with the polarity shown. Now solder in the diodes and trim- Here’s the 6-LED array for the “D” cell version. The five 27Ω LED current limiting resistors all solder to a spacer. Parts List – D Cell Version 1 2 x D-Cell torch (Eveready E250K or similar) or a 3 x D-cell torch 1 PC board coded 11105011, 59 x 33mm (46 holes) 1 micro-DIP x 10-pin PC board coded u10MAX, 13 x 12mm (10 holes) (must be solder plated) 1 ferrite toroid, 19 x 10 x 5mm (L1) (Jaycar LO-1230) 1 200mm length of 1mm enamelled copper wire 1 60mm length of 0.8mm tinned copper wire 1 50mm length of red hookup wire 1 50mm length of green hookup wire 1 12mm OD steel or brass washer 1 16mm OD x 10mm ID steel or brass washer 2 3mm x 70mm steel or brass threaded rod 1 M3 tapped metal spacer 1 M3 crimp solder lug 1 M3 x 10mm screw 1 M3 star washer 1 100mm long cable tie 9 PC stakes 1 plastic translucent diffuser (cylinder 23mm ID x 17mm long) (ours was cut from a cover cap supplied with a “FRUITY FLAVORITS” 250mm drink container) 1 72 x 115mm piece of thin cardboard Semiconductors 6 5mm 5600mcd white LEDs (LED1-6) 1 MAX1676EUB step-up DC-DC converter (IC1) 1 BYV10-20 Schottky diode (D1) 1 IN5404 3A diode (D2) Capacitors 2 47µF tantalum capacitors 1 1µF PC electrolytic capacitor 3 0.1µF monolithic ceramic capacitors (code 104 or 100n) Resistors 1 100kΩ (brown black black orange brown or brown black yellow brown) 1 43kΩ (yellow orange black red brown or yellow orange orange brown) 1 200Ω (red black black black brown or red black brown brown) 6 27Ω (red violet black gold brown or red violet black brown) 1 50kΩ horizontal trimpot (VR1) pot VR1. Inductor L1 is wound using 4 turns of 1mm enamelled copper wire around the ferrite toroidal core. Bare the ends of the wire with some fine emery paper or a sharp knife, to remove the enamel insulation before soldering to the PC stakes. Inductor L1 is secured to the PC board using short lengths of tinned copper wire which wrap over the toroid in the two positions shown. Solder a 12mm washer to the PC stakes at the positive end of the PC board (lefthand side of Fig.4). A crimp-type solder lug is attached to the other end of the PC board. You need to pry open the crimp end with pliers and flatten it and then solder the flattened section to the PC pins on the top side of the board; the circular lug section then hangs beneath the PC board. Solder a short length of hookup wire between the “A” PC board pin and one of the eyelet PC stakes. LED Array All of the steps for assembling the LED array for the D-cell torch are shown in Fig.5. First, we have to make the LED array. The 6-LED array for this torch is made using a 16mm OD (outside dia-meter) washer which has five 1mm holes drilled evenly around it. Insert the K (cathode) lead, which is the shorter lead, of each LED into a hole and solder in place. Do this for five LEDs and each should have about 4mm lead length above the washer. Also the anode lead should be orientMAY 2001  63 Fig.5: step-by-step assembly of the D-cell version of the torch. Naturally, this assumes you have already completed the PC board! 64  Silicon Chip These three photos give a good idea of the mounting “hardware” associated with the D-cell PC board. In particular, note the opened-out crimp eyelet in the shots above and the washer in the shot at right; also the soldered joint between the threaded rod and D2. ed toward the centre of the washer. The sixth LED is placed in the centre of the washer with its cathode lead bent over to be soldered to the washer. Each anode lead is cut to about 5mm long and a 27Ω resistor soldered to it. The other ends of the resistors are soldered to a tapped spacer so that there is 25mm between the end of the spacer and the lower lip of the washer. The spacer should be mount-ed along the centre axis of the washer. The torch bulb holder is unscrewed from the reflector and the bulb, spring and contactor plate are removed. Drill a 3mm hole in the end for the screw. Remove the reflector cap and glass by squeezing the cap to an oval shape and then prising it off. Insert the LED assembly from the reflector end. Screw on the bulb holder and secure the LED assembly with an M3 x 10mm screw and star washer through the crimp lug on the PC board. Solder a wire from the GND PC terminal to the reflector switch flange. Two 70mm-long threaded rods are attached by soldering to the PC stakes on the positive end of the PC board and secured to the bulb holder with a plastic cable tie. This will provide a stiff mechanical assembly. Solder diode D2 between the GND PC stake under the PC board and the threaded rod as shown. The inside of the torch includes a spring as the negative contract for the cell. This spring is too stiff and may distort the PC board when it is assembled inside the torch. We recommend removing the spring and squashing it down so that the overall height is about half of its original. Squash the spring by bending the smaller diameter loops closer together with pliers. The PC board assembly will require a cardboard tube around it to prevent it from being caught within the torch as it is turned while the cap is screwed on. We made our tube with a piece of cardboard measuring 72 x 115mm. It was wrapped around to make a 30mm ID (inside diameter) cylinder x 72mm long. We glued the ends with PVA adhesive and used pegs to hold the joint in place while the glue dried. The LED array is surrounded with a cylinder of translucent plastic 23mm in diameter by 17mm long and it is retained between the reflector and front glass. This prevents star effects caused by the reflector focussing the light emitting from the sides of the LEDs. The plastic cylinder diffuses this light to substantially reduce the effect. Our cylinder was obtained from the cap cover of a “Fruity Flavorits” 250mm drink container. The whole assembly can now be inserted into the torch with the D cell inserted first, negative end down. Then place in the diffuser, the reflector glass and then press on the screw cap. Now screw the assembly in place.The torch should operate when switched on. You can remove the assembly to adjust VR1 for the brightness required. In most cases this would be at maximum (fully clockwise) but for some uses it may be helpful to turn it down. Testing If your torch does not work, firstly check that the cell has voltage across it. It should be at least 1.0V when measured with a multimeter. Clean the cell terminals to ensure good contact and check that the torch switch is operating correctly. Sometimes the switch contact is bent incorrectly so it does not make contact with the reflector switch flange. You can check that the washer for the LED array makes contact with the inside of the reflector. Other problems could be that the LEDs have been installed with reverse polarity or the components on the PC board have been incorrectly oriented or placed. Check that the leads on IC1 make contact with the PC board tracks. You can operate the torch using a power supply which produces about 1.2-1.5V, but make sure the polarity And here’s the final assembly, ready to be placed into the torch barrel – naked (left) and clothed (right)! MAY 2001  65 Fig.6 (top right) and the above photographs show the “AA” version PC board from both sides. Inset at right is an enlarged view of the MAX1676 IC – in this version it is soldered direct to the PC board. is correct. Check that the converter produces voltage at the “A” terminal. It should be adjustable from below 3V up to about 4.2V by varying VR1. AA-cell version First solder the surface-mount IC direct to the PC board (see “D” version for the method used). Next, insert the PC stakes with the long end going down into the PC board to give a similar pin height above and below the PC board. Insert and solder all the resistors. They are shown mounted vertically in the diagram but should sit parallel with the PC board. The capacitors go in next, taking care with the tantalum types which must be oriented with the correct polarity. Now solder in the two diodes and wire link. Inductor L1 is wound on a Xenon trigger transformer former. The original windings are removed from the trigger transformer; unwind the primary winding and then cut the Parts List – AA Cell Version 1 2-AA cell torch (Dorcy FrostBrite or equivalent) 1 PC board coded 11205011, 49 x 13mm (must be solder plated) 1 Xenon tube trigger transformer (L1) 1 900mm length of 0.4mm enamelled copper wire 1 10mm OD steel or brass washer 1 15mm OD x 10mm ID Neoprene “O” ring 1 50mm length of red hookup wire 1 50mm length of green hookup wire 6 PC stakes Semiconductors 3 5mm white LEDs (LED1-3) 1 MAX1676EUB step-up DC-DC converter (IC1) 1 BYV10-20 Schottky diode (D1) 1 IN4002 1A diode (D2) Capacitors 2 47µF tantalum capacitors 3 0.1µF monolithic ceramic capacitors (code 104 or 100n) Resistors (0.25W, 1%) 1 100kΩ (brown black black orange brown or brown black yellow brown) 1 47kΩ (yellow violet black red brown or yellow violet orange brown) 3 27Ω (red violet black black brown or red violet black brown) 66  Silicon Chip finer secondary wires with a knife. Unsolder the wires from the end leads and attach one end of the 0.4mm enamel copper wire to one end of the former, making sure the end is stripped of insulation before soldering. Wind on 45 turns and terminate the wire to the other end of the former. The inductor is mounted from the underside of the PC board. Solder a 10mm washer to the PC stakes at the positive end of the PC board. Solder a short length of hookup wire between the “A” PC board pin and one of the end PC stakes. LED array The 3-LED array is made within a torch bulb socket. The details are shown in Fig.7. First, remove the glass and filament from inside it. Wear goggles when doing this; crack the glass with pliers and scrape out the inside with a screwdriver. The solder at the end can be removed with some solder braid or by using a solder sucker. Cut the LED anode leads to 5mm in length and solder each one of these leads close to the bodies of a 27Ω resistor. The other end of the resistor is passed through the solder hole at the end of the bulb. The K (cathode) leads need to be cut to 5mm in length and soldered to the rim of the The reflector must be slightly modified to fit the three LEDs through, as shown here. bulb. The metal switch flange is also tack-soldered to the bulb. Now solder the resistor leads to the solder end of the bulb and cut the lead ends flush. The reflector will need to have cutouts made so that the LED array can be inserted into the reflector area. You can do this with a small round file. The PC board pins at the end of the board solder directly to the brass end cap on the bulb holder. This must be done quickly to avoid melting the plastic. We found that the internal spring contact did not give a reliable connection so we drilled a small hole in the side of the bulb holder just at the base of the spring and passed a wire through this and soldered it directly to the solder end of the bulb. The other end of the wire connects to the “A” PC stake. Insert the LED assembly into the reflector and secure the bulb holder in place with the wire soldered to the end of the bulb. Now attach a ground wire to the switch flange. The positive end of the PC board requires a 15mm diameter locator so that it will be centrally positioned inside the torch. We used a 15mm outside diameter “O” ring which was secured with some hot glue adhesive. Fig.7: here's how to assemble the “AA” version of the LED torch. These details suit the Dorcy FrostBrite torch but should be adaptable to most similarly switched and similar size torches. This circuit can also be powered by a single “C” cell in installed in a C-cell torch. This close-up of the “AA” torch reflector assembly shows the three-LED array and the way it pokes through the reflector. In this case, the LEDs are soldered into the old (filament) globe base. The corners of this end of the PC board will require filing down a little so that the “O” ring is not distorted out of shape when attached to the end of the PC board. Insert an AA cell (negative end first) and place the PC board and reflector assembly into the torch body. Secure with the reflector cap. The torch should now work. If it does not work, check that the cell has voltage across it. Again, it should be at least 1.0V when measured with a multimeter. Clean the cell terminals to ensure good contact and check that the torch switch is operating correctly. Sometimes the switch contact may be bent incorrectly so it does not make contact with the switch flange on the reflector. Other problems could be that the LEDs have been installed with reverse polarity or the components on the PC board have been incorrectly oriented or placed. Check that the leads on IC1 make contact with the PC board tracks. You can operate the torch using a power supply which produces about 1.2-1.5V, making sure the polarity is correct. Check that the converter produces voltage at the “A” terminal. SC It should be about 3.9V. Fig.8: PC board patterns for the “AA” version (above) and the “D” version (far right), with its SMD IC daughter board at immediate right. MAY 2001  67