A simple example of a Christmas light project using store bought LED lights and components you probably have lying around the shop. Mark VandeWettering found a set of battery powered LED Christmas lights and decided to combine this this an ATtiny13 to make an MCU controlled lighting display. He explains:
I found a set of battery powered LED Christmas lights at CVS on sale for $.99 each. I took the lights out, and found that they consisted of two strands: one of 4 red and 4 yellow LEDs, and the other of 4 blue and 3 green LEDS. Each strand is wired in parallel and draws about 20ma at 3 volts. I have a few ATtiny13’s lying around, which can use between 2.7 and 5.5 volts supply. I think for this test, I’ll wire up a little 7805 regulator to provide 5V, and then control each strand with one of the two PWM outputs. If I want to drive the strings directly from the microcontroller, I’ll have to insert another 50 ohms of resistance to limit the current to around 20ma per pin. I could also add a 1K resistor and a small NPN transistor to drive each pin at the higher voltage. At 5V, the strand will draw about 120ma.
You can find the source code on Mark’s brainwagon webpage.