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Hardware Block Diagram

A project log for ELPH skin

A flexible and low-power electrophoretic (e-ink) display worn on the skin, like a temporary tattoo.

Hunter 04/03/2017 at 18:542 Comments

Last Monday I finished the block diagrams for the hardware and have begun hardware prototyping. These are high-level, but provide an overview of the various subsystems I will need to build.

Architecture

Plastic Logic is the company that manufactures the flexible e-ink display I’m using for the final prototype and I’ve based the hardware architecture on their evaluation kit. I’ve made some modifications, namely adding an ESP8266 module for wifi connectivity, using a different LDO / battery charger and simplifying all of the connections and access points so that I can fit everything within the footprint boundaries of the display itself and update the firmware wirelessly. I may end up using the ESP-12S since the footprint is only a little larger than the ESP 07 and has a 4MB flash chip. I'd like to utilize this for the MSP430 if it's possible.

MCU

I am using the MSP430 microcontroller to control the display itself. Since PLastic Logic also has an SDK, I am sticking with the same hardware to minimize the amount of firmware that I will have to write from scratch. If I could just run all of this off the ESP module I would, but with only a few months I want to avoid introducing any unnecessary risk and extra work and focus on just getting the thing working.

Power Supply

Note all of the various voltages coming out of the power supply. WTF?!?! I’ve never worked with voltages this high and the idea of placing this right next to my skin is a little unnerving (but exciting!). I did quite a bit of reading into how these e-ink displays work and in subsequent posts I will get into further detail, but the short answer is every pixel has a source voltage (+15 & -15V) and gate voltage (+25 & -32V) connected to the electrodes on the screen through thin-film transistor FETs and these need to be high enough to affect the ink particles in order to fully switch. It seems that the 9V is a reference voltage, I'm not exactly sure why this is, but I will find out.

Data

The display also has a temperature sensor that's needed to adjust the waveforms it uses and I2C is required for that sensor, so I will need both this and SPI in the final design.

Discussions

cruz.monrreal wrote 04/28/2017 at 16:09 point

Woah. Have fun supplying all of those voltages!

I was actually wondering what software you used to make this chart. It looks familiar, but I can't quite put my finger on it.

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Hunter wrote 05/08/2017 at 21:39 point

ha, thanks! I used Omnigraffle to make all my diagrams, let me know if you want the source file and I'm happy to upload!

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