Ever since I saw this How-To: Shrinkify Your Arduino Projects I wanted to do a project using Arduino Attiny85s. And because I want to bring my son into the world of Arduino and he was eager to learn how to solder I was thinking of an easy project. With Christmas being around the corner I wanted to do some kind of a blinking Christmas tree.
The Attiny85 has 5 pins to use so I thought it would be cool to hook up a shift register to blink some LEDs. First thing I did was buying Attiny85s and 8-Bit Shift Register (SN74HC595N) on ebay. 10 pcs of Attiny85 cost me 12 USD and 10 shift registers were 2.50 USD.
The idea was to control 8 LEDs with the shift register. That would use up 3 pins on the Attiny85… so this left me with 2 more pins. I decided to use them kind of doing multiplexing LEDs. So I could hook up 2 LEDs per shift register output but they would not share the same connection to ground. This was controlled by two transistors that are connected to the two remaining pins on the arduino.