These days you can buy various types of LED garlands at a reasonable price. With these, you can illuminate various objects indoors or outdoors with interesting lighting effects. When you use a garland with RGB LEDs you can even create a range of colors or continuously cycle through the color spectrum.
In this case, the author wanted to enhance several kitchen cabinets with a novel lighting system. To this end a waterproof LED garland with builtin resistors was chosen. This type of garland can be obtained from e.g. . The strip is available in lengths up to 5 m (approx. 16 ft) and it comes with double-sided sticky tape so it can be mounted onto any clean, flat surface. The supply voltage required by the LEDs is 12 V and the power requirements for the strip used by the author is 7.2 watts per meter.
Various types of a controller can be bought to drive such RGB LED garlands, but as an electronics hobbyist you ‘just’ design one yourself of course, so that it does exactly what you want it to do. The design for this circuit turns out to be very simple: an ATtiny2313 microcontroller surrounded by a 5 V voltage regulator and three power transistors. The latter is driven via base resistors from port pins PD0 to PD2 of the microcontroller. In the circuit, you’ll also see a 6-way ISP connector for programming the microcontroller. The program in the ATtiny varies the brightness of the R, G and B LEDs using an internal pulse-width controller. The program was written such that the color of the LED garland changes continuously. At the start of the code are several items that can be modified to suit your preference before programming the microcontroller. For example, the speed and the number of color changes can be modified, or a self-test can be activated. If this is selected then just after the circuit is switched on, it will show the colors red, green and blue sequentially, followed by white. This way it is easy to verify that all LEDs in the strip are still working. The current consumption of the LED garland is in practice very close to that stated by the manufacturer. Each LED color takes a current of just over 200 mA/m.
When the transistors specified in the circuit are used (BD139, ICmax 1.5 A) you could, in theory, drive a strip with a length of 1.5/0.2 = 7.5 m. In practice, it’s best to limit this to about 5 m. For higher currents, you can change the BD139 to a TIP122. This can handle 5 A so that it should be able to cope with LED garlands up to a length of 20 m. Remember that in that case, you’ll need a power supply that is rated at least at 12 A at 12 V. If you decide to use the TIP122 there is no need to modify the printed circuit board. Although the pinout is different, it is the mirror image of that on the BD139. The TIP122 can simply be mounted the other way round, with the heatsink tab facing the outside, which makes it possible to mount the transistors on a common heatsink next to the PCB. Note that the PCB tracks are not designed to cope with currents of 10 A or higher. It can happen that the R, G, and B labeling is incorrect on some LED garlands, so it’s best if you check that they correspond to the correct LEDs before connecting the LED garland to the PCB. The source and hex code for the program can be downloaded from the Elektor website .
Internet Links www.ledlightdepot.co.uk/9-led-waterproof-flex-strip  www.elektor.com/120217