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Special offer: 500 PC boards to give away A high intensity LED flasher for bicycles Have you joined the 2-wheel enthusiasts, cycling for fitness and enjoyment? If so, you'll want this high-intensity LED flasher to make sure you can be seen after dark. This one uses four high brightness LEDs and draws very little current. By DARREN YATES LED flashers for bicycles are a great idea. Because they flash and are so bright, they are much more visible than ordinary incandescent tail lamps. They also consume far less current than an incandescent lamp and so the batteries last a great deal longer maybe up to 100 times longer. Of course, a low-power high-brightness flasher such as this one need not be confined to use as a bicycle tail light. We're sure that our readers will come up with other applications. We are presenting this project as a cheap and brighter alternative to-units you can buy ready-made. With this unit, you have the option for going to super bright LEDs and bigger batteries. The circuit As you can see from Fig, 1, the circuit uses only one 8-pin IC, plus four high-brightness LEDs and a handful of other components. The IC is a National Semiconductor LM3909 LED flasher/oscillator which, as its name suggests, was specifically designed for The PC board fitted neatly inside a Basta No. 660 bicycle tail light case, which we obtained from K-Mart. We simply removed the standard 3V incandescent lamp & its reflector, & installed the flasher PC board instead. 40 SILICON CHIP the purpose of flashing LEDs. Normally, the LM3909 is used with a single 1.5V battery but our circuit runs off a 3V supply to provide extra light as well as longer battery life. The 2200µF and 470µF electrolytic capacitors are used to provide a charge reservoir and to provide the flash timing. The result is that the circuit only consumes an average of around 6mA yet the light from the four 2-candela (2000mCd) LEDs is bright enough to be clearly seen at night from a distance of several hundred metres. The four 18Q resistors provide current limiting and ensure that each LED only takes its share (one quarter) of the current. The circuit runs from two C cells, with alkaline types giving about 1000 hours of continuous operation. Nicad cells are not recommended for this application, as these should not be discharged below a certain voltage. Construction Before you begin soldering any components to the PC board, it's always a Fig.1 : the circuit uses an LM3909 flasher IC to drive four high brightness LEDs. 180ll 2200 + 16VW _ IC1 LM3909 3V ..%. KALEX The UV People ETCH TANKS • Bubble Etch • Circulating LIGHT BOXES • Portuvee 4 • Portuvee 6 • Dual Level 820ll TRIMMER BICYCLE TA IL FLASHER e ldeal PCB DRILL • Toyo HiSpeed MATERIALS • PC Board: Riston, Dynachem e 3M Label/Panel Stock • Dynamark: Metal, Plastic AUSTRALIA'S N0.1 STOCKIST * * 11111921 Fig.3: the PC board is coded 11112921 KALEX [Z & measures 47 x 33mm. good idea to check the copper side for any shorts or breaks in the tracks. You can do this by comparing your etched board with the published pattern. If you find any errors on the board, use a sharp utility knife to scrape away any shorts , or use your soldering iron and a small blob of solder to join any broken tracks together. Fig.2 shows the parts layout on the PC board. Begin the assembly by installing the resistors , then install the 470µF capacitor and the four LEDs. Note that the capacitor and LEDs are polarised devices, so take care with their orientation. It's easy to identify the LED leads, since the anode lead is always the longer of the two. Initially, the LEDs should be soldered at full lead length, since they will need to be adjusted later on. 40 Wallis Ave, East Ivanhoe 3079. Phone (03) 497 3422, Fax (03) 499 2381 Old Repr"nted Electrical Books The 2200µF capacitor is mounted proud of the board so that is can be bent over to clear the diffuser lens. The IC can n ow be installed on th e board, again taking care to ensure that it is correctly oriented . Don't take too long when soldering each pin , other- 500 Free PC Boards From RCS Radio In conjunction with our 5th birthday celebrations, RCS Radio Pty Ltd has kindly made available 500 free PC boards for this project. To obtain your free LED flasher PC board, send a stamped self-addressed envelope to RCS Radio Pty Ltd, 651 Forest Road , Bexley, NSW 2207. Only 500 boards are available free, so get your order in as soon as possible. Don't forget to put a 45 cent stamp on the self-addressed envelope. 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Payment can be made by cheque made out to Plough Book Sales, PO Box 14, Belmont, Vic. 3216. Phone (052) 66 1262. ]ANUA R Y 1993 41 Protect your valuable issues . Silicon Chip Binders wise the heat will destroy the insides of the chip. Finally, install the 2200µF capacitor with its base about 10mm proud of the board, as shown in the photos. This is done so that the capacitor can later •be bent across the PC board, so that it clears the diffuser lens of the recommended case. Testing These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a distinctive 2-tone green vinyl. They hold up to 14 issues & will look great on your bookshelf. · * Hold up to 14 issues * 80mm internal width * SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price: $A 11.95 plus $3 p&p each (NZ $6 p&p) . Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. l ----------Enclosed is my cheque/money order for $_ __ or please debit my O Bankcard O Visa O Mastercard Card No : Card Expiry Date _ _/__ Signature _ _ __ _ _ _ _ __ Name _ _ __ _ _ _ _ _ __ Address_ _ _ _ _ __ __ _ . ___________ . _ _ _ _ __ _ P/code_ 42 SILICON CHIP Capacitors 1 2200µF 16VW PC electrolytic 1 470µF 16VW PC electrolytic Resistors (0.25W, 1%) 1 820Q 1180Q 418Q Housing * High quality Use this handy form Before switching on, go over the PC board carefully and check for possible wiring errors. When you are satisfied that the assembly is correct, connect a 3V DC supply to the board and switch on. Within a second or so (this is while the 2200µF capacitor charges up), the LEDs should all begin flashing in unison at a 2Hz rate (ie , twice per second). If they don't flash as expected, disconnect the battery and check that the LEDs are correctly oriented and that the supply polarity is correct. PARTS LIST 1 PC board, code 11112921 , 47 x33mm 1 LM3909 flasher/oscillator IC (I C1) 4 high-brightness LEDs, 2000mCd or better (LED1 -4) 1 miniature toggle switch (S1) 1 Basta No. 660 tail light with batteries (from K-Mart stores) _ _ If you are going to use this flasher on your bike, you will really want a case that is designed for just that purpose and fortunately there is at least one available. We obtained ours from the local K-Mart store. The unit in question is a Basta No. 660 bicycle tail light and at the time of writing it was available for $11.95 (includes two C-size cells). As supplied, the unit comes with a 3V incandescent lamp and red diffuser lens. To disassemble the unit, first slide off the battery compartment lid and then unclip the red diffuser lens. This reveals the lamp reflector and the lamp. To remove the reflector, unscrew the lamp. To make room for the PC board, we removed the integral lamp socket and the brass contacts associated with the integral on/off slide switch. We then mounted a miniature toggle switch on the back panel of the unit, as shown in one of the photos. The board itself was held in place using a suitable adhesive. You will need to run short hook-up wires between the battery contacts, the toggle switch and the PC board. As noted above, you will need to position the 2200µF capacitor so that it lays flat over the board. You will find that the PC board sits at an angle in the Basta tail light hous- The on/off switch was mounted on the rear panel, behind the diffuser lens. Make sure that the switch does not intrude into the mounting bracket are of the case. ing. Adjust the LEDs so that when the red diffuser lens is in place, they sit just behind its rear surface. Make sure that the LEDs are perpendicular to the diffuser lens, to obtain the best light transmission. Just as a matter of interest, we measured the current consumption of the supplied 3V lamp. It pulled about 215mA. Compare that with the under 6mA current of the LED flasher circuit. At that rate, the batteries will possibly last 100 times longer when powering the flasher circuit. SC