Fritzing

Using 3 PCF8574 I/O Portextentions, 16 LEDs and 3 pushbuttons it is possible to create a cool and simple binaryclock

For building the clock you only need an Arduino Board for controlling the 12C bus, 3 PCF8574 I/O Portextentions (eachone controlls one part of the clock), 6x yellow and red, 4x green 5V LEDs, 2x 4,7kOhm pull up resistors for the SDA and SCL wire and 3 pushbuttons for setting the time and resetting the board/clock.

The red LEDs are showing the current seconds,

the yellow ones the minutes and the green ones the hours.