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Ma • High in Featu • 50-h intensity am res duty c our battery ber LED life (A A hea • 1 0 0e- lls) vy h o ur ba alkalin tter y e cells l i fe ( • Com ) AA • Conspact size t a nt LED batter bright ness o • Battye life ver ry con dition indica tor Novel LED torch has low battery drain Using a highly efficient amber LED, this LED torch has the advantage of small size & prolonged battery life. In addi­tion, you will never need to replace the “lamp”. By JOHN CLARKE This torch is not as bright as, say, a conventional penlite torch but it has much better battery life. It’s great for finding keys in a handbag, lighting up a keyhole when opening your front door at night or any time you don’t want an ordinary torch which is really too bright for the job. There are many instances where ordinary torches are just too bright. After all, we do not always want to spot possums in the trees or ward off intruders. In fact, the lower light output from a LED torch is useful when 58  Silicon Chip checking on a sleeping child at night and for use by astronomers who don’t want to disturb their “dark-adapted” eyes. A big problem with ordinary torch­ es is that their batteries always seem to be on their last legs when you want to use them. With this LED torch, you can expect up to 15 times the battery life of a standard penlite (two AA cells) torch. These torches typically draw 300mA from the battery while this LED torch only draws about 25mA. The gain in battery life is partly due to the lower current drain and partly to better battery efficiency at lower currents. The idea of a using LEDs in a torch has been around for some time but until recently, suitable LEDs were not available. Grant­ed, high intensity red LEDs can be used but the red colouring is not pleasant. Since there is no such thing as a white LED, the new high-brightness amber LEDs are the go. Specifically, the new Hewlett Packard AlInGaP (Aluminium Indium Gallium Phosphide) LEDs are preferred for this job. The amber light corresponds to the more sensitive spectrum region of our eyes and it gives better render­ing of the colour of objects. We built the LED torch into a small plastic case. The LED is mounted on one end of the case while a slide switch on the top turns it on and off. In the base is a battery condition indicator. A OFF CELL1 220 16VW LED1  K S1 ON LAMP1 CELL2 A K LED TORCH Fig.1: the circuit consists of two AA cells which drive a high-brightness LED via a series 1.5V lamp. The lamp ensures constant LED current. It glows brightly when the battery is good but gradually dims over the life of the battery, eventually ceasing to glow when the battery is at the end of its life. Circuit details There is not much to the circuit although there is more than you might expect. In the simplest arrangement, we could have had two AA cells feeding the LED via a resistor selected to set the current at around 25 milliamps. This works but has the disadvan­tage that the LED will gradually dim as the batteries age. Hence, we have used a slightly more exotic circuit involving a series 1.5V incandescent lamp and a 220µF 16VW capacitor. The lamp ensures that the current through the LED will remain relatively constant over the life of the battery. It operates on the principle that the resistance of a light bulb increases with the filament temperature. So when the batteries are new, the light bulb will have almost 1.5V across its filament but when the batteries are old, there is almost no voltage lost across the filament. Just how well the current regulation works can be judged by comparing it with a LED driven directly via a resistor. As the batteries age, their total The PC board is dominated by the two AA cell holders. Make sure that the LED & the 220µF electrolytic capacitor are correctly oriented. voltage will range from 3.3V down to about 2.2V and this will result in a reduction of LED current of more than 70%. The LED/lamp system, by comparison, results in a current reduction of just over 40% for the same voltage range. But that is not the end of the story. With the LED/lamp system, the LED will continue to put out useful light when the battery voltage has diminished to 1.9V; ie, 0.95V per cell. The 220µF capacitor is included to prevent surge current through the LED when power is first applied. This would otherwise occur due to the low cold resistance of the lamp bulb. Note that the capacitor is shorted each time the slide switch is turned off to ensure that it is discharged before power is reapplied. Construction To make assembly easy, the parts are mounted on a PC board measuring 79 x 41mm and coded 08302941. You can begin construction by clipping off the corners of the PC board so that it will fit neatly into the case without fouling PARTS LIST 1 PC board, code 08302941, 79 x 41mm 1 plastic case, 24 x 50 x 90mm (Jaycar HB-6031) 2 adhesive labels, 25 x 8mm 1 SPDT slider switch (C&K 1101 or equivalent) 2 AA cell holders with flying leads or solder terminals (Altron­ics S-5026 or Tandy 270-401) 1 1.5V 25mA mini lamp with grommet (Tandy 272-1139) 1 5mm Hewlett Packard amber LED, HLMA-DL00 (VSI) 1 5mm LED bezel 1 220µF 16VW PC electrolytic capacitor 4 2.5 dia. x 5mm machine screws & nuts 4 3 dia. x 5mm machine screws 3 PC stakes 1 50mm length 0.8mm dia. tinned copper wire 220uF CELL 1 A LED1 LAMP1 K CELL 2 S1 Fig.2: install the parts on the PC board as shown here. Fig.3: the full-size etching pattern for the PC board. February 1994  59 The 1.5V lamp protrudes through a hole in the PC board & is shock-proofed by fitting it with a rubber grommet. the corner pillars. You may also need to drill out the hole for the light bulb grommet. Install the PC stakes, capacitor and single cell battery holders, making sure that the latter parts are correctly oriented. The battery holders are secured to the PC board with 2.5mm screws and nuts. Note that you cannot use AA battery holders which have clips at each end since they will be too long for the case. Install the grommet and wire the bulb in place. This done, temporarily mount the switch on the PC stakes by soldering to the centre pin only. This will allow easy adjustment later. Drill a hole in the centre of one of the case ends to fit the LED bezel and solder the LED in place on the board, as shown in the photograph. You can also drill a small hole in the base of the case for the lamp to shine through, for battery indication. Secure the PC board to the case using four 3mm screws. Now you will OFF ON LED TORCH Fig.4: here are the full size artworks for the two adhesive labels. need to drill and file out a rectangular hole in the lid of the case for the switch slider. Adjust the switch height and position so that it can be operated freely when the case is assembled. Finally, solder the remaining PC stakes to the switch leads. We made up a couple of small labels for the lid and base of the case. If you have these, roughen the surface of the case with emery paper to allow them to stick properly. Install the batteries and check that the torch operates. If not, check the polarity of the LED. Finally, assemble the case with the self-tapping screws. Note that the end pieces of the case fit properly only one way around even though they appear to be able to go in SC either way. High-Brightness LED Options While we recommend the HP HLMA-DL00 LED which has a 30° beam and 3001000mcd output at 20mA, there are two other Hewlett Packard amber LEDs which may be more suitable for your application. (1). HLMA-CL00 is also 5mm in diameter and similar to the DL00 except that it has a narrower beam of 7° and 1000-3000mcd at 20mA. It is more useful as an inspection light. (2). HLMA-BL00 is 13.3mm in diameter with a 4° viewing angle and with a higher intensity of 15 candelas at 20mA. This costs about $20 and has a powerful but very narrow beam. These LEDs are available from VSI Electronics Australia Pty Ltd in your capital city. Alternatively, the HLMA-DL00 and HLMA-CL00 are available from Farnell Electronic Components. Phone (02) 645 8888. 60  Silicon Chip