Aurai - retrofit dumb air conditioners

As I was putting together the breadboard for the receiver module, I made a horrible realization: I don't have enough IO pins. I forgot about the CE lines for the SPI chips. I'm sure I could do some trickery to make this work, but in the interest of time, I'm switching both MCUs to be ATMega328s.

This can be revisited later down the track. Maybe an ATtiny84 would work - with 14 pins, available in DIP, and half as much per piece at a 328 ($1.41 vs $3.23).

The two components communicate wirelessly. There were several options I considered for this link.

# Ethernet on the controller, wifi on the receiver

* Pros: good library code available, entire stack on the chip, high level protocol (http?)

* Cons: expensive, hard for the end user to set up

# XBees

* Pros: reliable, lots of library code, stack

* Cons: moderately expensive, no hardware UART on the ATTiny85

# Nordic Semi NRF24L01

* Pros: inexpensive ($3/pair), compact, respectable range - even indoors

* Cons: not a lot of library code out there, lower level protocol.

In the end, I decided to go for the NRF24L01 and purchased some sample units to use in this process.

I was deliberating which components to use for the two modules. In the end, I decided to use a ATMega328 for the controller/bridge component, and an ATTiny85 for the IR transmitter.

The ATMega328 was chosen for the controller because of the breadth of library code already available. The ATTiny85 was chosen because of its small size and low power consumption, whilst still providing SPI (via the USI) and comprehensive timers (needed for the IR carrier wave).