Never heard of Electric Imp? It's pretty neat. "The Imp" is a tiny wifi-connected microcontroller packed up in the same form factor as an SD card. It's really easy to build imp-powered things, and once it's built you can connect your device to any wifi network and interact with it from anywhere in the world. Check out the imp if you haven't before before digging deeper: http://electricimp.com/
Devices that are powered by or enabled to work with an Electric Imp are called "impees" (just like one who is employed is an employee).
Step 1: Emma Schematic Overview
The pdf of the Emma design is attached to this step in addition to the screen shots shown. The full project source for this design is available from the Electric Imp website at http://devwiki.electricimp.com/doku.php?id=emma. Note that many other reference designs are also available at http://devwiki.electricimp.com/doku.php?id=referencedesigns.
Emma's design consists of four functional blocks: the power supply, the imp slot and ID chip, the LED driver and digit circuit (which is repeated 9 times in the design), and a digital ambient light sensor which was added just for some extra flair. The light sensor can be excluded without any ill effect (except, of course, that you won't be able to view the light level from anywhere in the world).
Emma's power supply uses the Texas Instruments AP1117E33G LDO to obtain a 3.3V supply from the board's 5V supply voltage - the imp, as well as the driver ICs for each digit, require a 3.3V supply. This LDO is inexpensive and easy to solder, and is a new addition in revision two of this design.
Note the Atmel ATSHA204-TSU-T part at U2, in the "Imp Slot and ID Chip" section. This part is an ID chip which provides the impee with a unique ID number. This number is used when the imp comes online and registers with the Electric Imp cloud; this is how the Imp Cloud knows what firmware to provide to the imp when it comes online. This part can be purchased and installed right off-the-shelf; no special initialization or configuration is needed. Just solder it down and you're good to go.
The Taos 856-TSL2561FN is a simple I2C sensor which can be used to detect the ambient light level around the impee. As shown on the schematic, this part has I2C slave address 0x29. Source code to read the sensor is included in this tutorial.
Next we'll take a quick look at the circuit which controls each display digit.