I saw, on Hack-a-Day, a guy named Randy had brilliant idea for a 2011 555 timer contest when he made electronic dominos that can pass a message across many of them using LEDs and photo transistors. That was just about the coolest thing I had ever seen. Anyways, I thought it could use a few extra features so I've replaced the 555 with a PIC12F683 (leftovers from an old project) and the compatible and much more efficient PIC12LF1822. Both chips come down to $1 in 25+ volume anywhere and don't really require any extra circuitry to operate.This project aims to build a flexible platform for small electronic dominoes with programmable logic. The board itself has to be a simple single-sided PCB to make it easy for anyone to reproduce.
The PIC12 allows for much greater control over the devices than a 555 while using even less power and fewer external components. This all comes at a cost of an extra $0.75 minus a bunch of passives. It has 256B EEPROM, an ADC, comparator (with basic DAC for reference), and the ability to generate PWM easily. PWM can be used to generate simple sounds or simply control the brightness of an LED. The PIC12LF1822 has the bonus feature of being able to switch pins for the PWM output, allowing for a simpler board design to support both. The ADC can be used to sample a potentiometer to change the delay or length of a pulse, pitch of a note played, the brightness of the LED, or it could be some more complex control interface. Imagine being able to compose (crappy) music with a bunch of these! The comparator can be used to more efficiently use a photocell rather than a photo transistor.
Anyways, here's what I've got so far: The PCB fits in 0.6"x0.8" on a single side with easy-to-produce features and even a debug header. The cost looks to come out to ~$1.50 each, mostly because the only parts to consider are the MCU, photocell / photo transistor, LED, and battery. A CR2032 costs about $0.20 on eBay (in reasonable quantities) but you can use any 3V flat cell.