Spacewrench over at Dorkbotpdx writes:
This is a rebuild of the TeensyPrime project I built a while ago, using a separate breadboard that’s almost too small (I had to use magnet wire to fit some of the connections) and a microcontroller that’s almost too small. The ATTiny13A is a neat chip: AVR with 1K of flash, 64 bytes of RAM and 64 bytes of EEPROM. I programmed it using a Teensy-2.0-based waldo running Ward Cunningham’s TXTZYME.
The programming for this is actually kind of interesting. Every time you push the button, the AVR retrieves the currently-displayed number (which is stored in EEPROM), and then increments it, clicks the counter, and tests for primality. If the number isn’t prime, it increments and clicks again. When a prime number is reached, it stops and waits for another button press.
TinyPrime project based on ATTiny13A - [Link]
Try making a double led dice with 14 leds driven only by 4 available pins of an Atmel Attiny13a. I did it, and it worked:
14 leds can be driven by a technique called charlieplexing when not many microcontroller pins are available. This technique works from the fact that leds are diodes and that those diodes have a little voltage drop. In the network of leds, you can make one led turn on by applying a voltage smaller than twice the voltage drop of a led. In this way, only one led lights up. The other leds do not turn on because the voltage is not high enough.
To make all the leds light up you must cycle through all the leds very fast. Only one led can be turned on at a time. If you cycle through all leds very fast they all seem to be on at the same time to the human eye.
Charlieplexed double led dice - [Link]