Fun project and easy to solder kit.

Only 2 inch squared, small enough to carry around in your pocket.

Based on the attiny 12 microcontroller.

Features:

• 2 dice (14 leds charliplex).
• Uses one CR2032 Coin Cell
• Small footprint (2 inches squared)
• Stores last dice rolled
• Real random number
• Roll Animation

3D case is not included but can be download from thingverse.

Theory of Operation:

When you press the roll button, the first thing that happens is that the ATTINY wakes up and starts counting all 36 possible dice combinations continuously until you release the button. The last counted number is the number that will be shown. The ATTINY is running at about 1Mhz (most instruction run in one cycle) so it counts really fast. This makes a pretty good random number generator.

Once the dice numbers have been generated, a rolling simulation will be displayed.

After that, the resulting dice numbers will flash several times.

And finally the ATTINY will power down and go to sleep to save battery.

About the Leds: ATTINY has only 6 ports and we use one port for the roll button, that leaves us with only 5 ports left and we need to drive 14 leds. To accomplish that, we arrange the leds in a configuration commonly called “Charlieplexing”. This is a technique used for multiplexing leds. The problem with Charlieplexing is that only one led can be on at a time. So we scan the 14 leds really fast to appear as if they are on at the same time.

The 33Ω resistors are used to limit the current to the leds. Two resistors in series are used to light any given led. So the maximum current thru a led is less than 2mA.