You need to prepare modules as small as possible to wear the watch on your wrist. There may be tons of variations, but I chose parts that are commonly used and inexpensive.

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Hardware parts for smart watch

Arduino micro-controller

I chose the smallest Arduino, Pro mini. Arduino Pro mini is a light version of UNO R3. It doesn’t have a USB interface chip in order to reduce the price and the size. There are two version depending on the operating voltage(3.3v/5v). And I used a 3.3v version, because the bluetooth and display that are connected with the board support 3.3v and the board goes well with the 3.7v LiPo battery. It runs at 8MHz and a 5v version at 16MHz, but 8MHz is enough. Overall, all you need to prepare are Arduino Pro Mini 3.3v and USB to UART module.

The main chip of the board, ATmega328, has only 2KBytes RAM, but ATmega128 has just 1KBytes, which is very limited to run my system. Most boards have ATmega328, but you have to make sure.

Bluetooth

One of most common bluetooth modules that you can get are HC-06 main module and the one with interface base board. The latter one has a reset button, the status LED, and it supports both operation voltage(3,3v/5v), so this one is more convenient but the size is rather big, the LED, which is not quite necessary drains the battery and a little more expensive. So I used a HC-06 without the interface board.

Display

To make a SMART WATCH, it would be necessary to find a small, low-power display. I’d given lots of thoughts, then I chose -.96’’ OLED Display. There are various sizes of the displays(0.96’’, 1.3’’, etc), it works on low-power, English font and the image out available, and it supports I2C, SPI, which makes easy to connect with Arduino.

<strong>Be careful at selecting a display!!</strong> I used an 128×64 OLED which is using I2C and SSD1306 driver chip. If you are using different one, you may need to use different graphic libraries and modify arduino source code.

Updated (2015.01.12) : RetroWatch supports u8glib. Now you can use various kind of OLED(or else). Check supported devices at this link.





Battery

I use LiPo(Lithum-Polymer) battery in this project. 1-cell LiPo battery flows out current in 3.7v, which works perfectly with Arduino Pro mini, and there are many kind of batteries in terms of the size and the capacity. The most ideal battery depends on what size of watch you want to make. I recommend the battery with protection circuit(overcharging, over-discharging safe), and it’s better if it has a removable socket. It might helpful for you if you get a female socket and USB or DC adaptor recharger.

Etc.

You need wires, soldering iron, a switch and 10K ohm resistance(for a button), and a batter jack. It would be helpful for you to prepare the assembly manual.





Preparing for Android

Android v.4.3 supports the service that is used to get notification information from an app. So RetroWatch app is based on Android v.4.3 to enable notification service. For users who don’t use Android v.4.3 yet, the app without this function is released also. You can download the sources at GitHub, or download the app from Google Play Store. (Search with “RetroWatch” or “RetroWatch LE“)