DIY Smartwatch: Screen Current Draw

To give a better idea of the power budgeting I'll be working with, I wrote a small test program to cycle through various percentages of the screen set to white with varying brightness levels.  Since black pixels on OLED displays are completely turned off, they consume no power and so a power saving method is to use darker tones.  That is why I'm testing the screen at various "ON" percentages.  As for the brightness, this display uses a 4 bit value to determine how bright the display will be set to with the brightness being in increments of 1/16.  Below is a table relating current draw to percentage of the screen on and the brightness for a 5V supply.

	0/15	1/15	2/15	3/15	4/15	5/15	6/15	7/15	8/15	9/15	10/15	11/15	12/15	13/15	14/15	15/15
0%	6.1 mA	6.4 mA	6.5 mA	6.7 mA	6.8 mA	6.7 mA	6.8 mA	6.8 mA	7.0 mA	7.1 mA	7.1 mA	7.1 mA	7.2 mA	7.4 mA	7.4 mA	7.4 mA
25%	17.1 mA	20.8 mA	24.9 mA	29.1 mA	32.8 mA	36.2 mA	39.9 mA	42.4 mA	44.3 mA	46.2 mA	47.9 mA	49.0 mA	49.7 mA	52.1 mA	52.6 mA	53.4 mA
50%	28.1 mA	35.5 mA	43.5 mA	51.4 mA	58.8 mA	65.7 mA	73.5 mA	79.7 mA	81.9 mA	85.3 mA	89.2 mA	91.2 mA	92.6 mA	97.1 mA	97.8 mA	99.6 mA
75%	39.6 mA	50.7 mA	62.6 mA	74.2 mA	85 mA	95.6 mA	107.5 mA	118.8 mA	121.3 mA	125.4 mA	131.2 mA	134.4 mA	141.5 mA	143.2 mA	144.4 mA	146.7 mA
100%	51.1 mA	66.2 mA	82.2 mA	97.1 mA	111.5 mA	125.5 mA	139.2 mA	152.6 mA	160.8 mA	167.3 mA	174.3 mA	177.7 mA	186.8 mA	189.2 mA	191.5 mA	195.2 mA

That's quite a lot more than what I originally thought the display would draw.  It looks like I'm going to have to do some serious power budgeting to make up for this.

DIY Smartwatch: Test Screen

Here's just a quick video that provides a basic outline of what I want the watch GUI to look like.  Feel free to provide comments and criticism!

DIY Smartwatch: Code Hosting

While I was waiting for my board to come in, I started playing with the screen, learning about Adafruit's library as well as designing the watch interface and visuals.  I've created a Github repository to host all of the test programs, image files, and final code.  I'm currently using a modified Adafruit GFX library and the SdFat library, but will probably strip out the code not required for my application and start using a custom display library soon.

DIY Smartwatch: Board Layout

After the circuit design comes the hard part, the board layout.  I went through several iterations of trying to fit all of the different components on the board but essentially based the design around the Bluetooth module, which I knew I wanted on top, the screen header, which I wanted on the bottom, and the buttons which I wanted on the right side of the watch.  Below is the fruit of many hours of labor.  The board design is actually the stage which decided the final components I would use.  During the circuit design phase I had included an RTC and crystal oscillator, but as you can see below, there wasn't really any room for it.  I also figured that the internal oscillator on the AVR is reasonably accurate and the time could be included in a data packet sent from the phone.

The top layer of the board is a 3.3V pour and the bottom is a ground pour, but after adding all of the components to the board I noticed that the power would have to take a rather circuitous route to reach some of the components on the right side.  To remedy this, I decided to add some last minute capacitors and shift some components around to provide better connectivity.  However, in my rush I ended up making a few minor mistakes.  Namely I placed several capacitors and components directly under the Bluetooth board.

And here are the fabbed boards!  I ordered them from Fusion PCB since they are the cheapest option I could find and provided a quantity of ten boards in the event I made a lot of soldering mistakes.  I only glanced at a few of the boards but am very satisfied with the quality.  The only downside of their service is the turnaround time.  It took twenty days from day of order to day of delivery, which is reasonable since they're based in China, but can be frustrating if you need to make multiple iterations and improvements.

DIY Smartwatch: Circuit Design

This post is meant to be an explanation of the final schematic for my Smartwatch board, version 1 with a bit of reasoning behind each component and my reasoning behind why I did what I did.

Total Schematic Overview

This is just the reset button, connected to reset the AVR and BLE module.  I also originally tied the screen reset to it as well until I realized the Adafruit screen library required it to be connected to a GPIO.

This is the vibration motor circuit.  Since the motor uses up to 85mA, about twice what the AVR can supply, it's hooked up to an N-MOSFET to amplify the current.  I also hooked up the gate of the MOSFET to a PWM pin on the AVR in case I don't need the full vibration.  There is also a diode and capacitor attached to the leads of the motor to help reduce noise and prevent large voltage spikes.

Next are the switches.  The four-pin box above is actually a navigation switch with each pin being a connection for up, down, and press actions.  Since the AVR has built in internal pull-up resistors, the switches are all tied to ground.  I also quickly ran out of interrupt pins (there are four or five on this AVR but they also doubled as communication pins that I needed for other things) so am using PCINT pins instead.  I have never used these before and they aren't built into the Arduino toolchain so we'll see how using them goes.

Here is the BLE module in all of its low power glory.  I grabbed the eagle part over from the same place I got the breakout board and used his design as a reference.  There are debugging LEDs on the TX, RX, and Connect lines to show the status of the BLE module, but odds are I won't be keeping these since LEDs take several mA and I won't have power to spare.

Above is the power regulation circuit.  It takes the battery (3.4V - 3.7V) and regulates down to 3.3V.  I also added a power switch at the last minute in case I want to solder the battery leads directly onto the PCB.

This is the charging circuit.  It takes 5V power from the USB port and feeds it to a chip that handles the charging for me.

Next is the cheap 3-axis accelerometer I'm hoping to use for a good deal of the interactivity of the watch.  However, it is a secondary feature so if I messed up it's not too big of a deal.

Here's a simple brightness-detection circuit.  I want to automatically control the brightness of the screen as a means of saving power so I've hooked up a photoresistor in a voltage divider circuit with the output being sent to an analog pin on the AVR.

This is just the header for the screen.

Lastly is the AVR itself.  In addition to the connections I've already covered there's also a voltage divider connected directly to the battery as a simple means of detecting the battery voltage as well as the USB data pins in the hopes that I can load the Arduino bootloader and program the watch directly from the computer.