E-paper is a low power, high contrast display technology such as is used in the Kindle ® and other e-book readers. One of the really nice features is that the display panel keeps displaying whatever image was sent last, even when power is removed. I've always thought it would be rather cool to have such a display for the Raspberry Pi.
What is it? The E-Paper HAT
So I made a printed circuit board (PCB) which meets the Raspberry Pi Foundation's HAT specification, including device tree configuration of the required GPIO pins by the HAT EEPROM.
The board is able to drive a 2.7" 264 x 176 pixel E-paper display panel and I'm really rather pleased with the way it looks. It can also drive the 1.44" and 2" display panels from the same manufacturer.
The board also features a DS3231 real time clock (RTC) IC with a CR1220 lithium coin cell for battery backup when the Pi is powered down. The DS3231 is accurate to 5 parts per million, or to within 3 minutes per year. The RTC can generate an interrupt/alarm signal and also a 32KHz clock signal which can be connected through to GPIO pins by solder pad links, if required.
There is provision for you to fit up to four (provided) pushbuttons to the pcb, two either side of the RGB led.
So now I would like to make a lot more of them so that all you folks out there can have one too.
Why did you do it?
When I started looking around to see what was available, my first stop was a development kit from Pervasive Displays (who make the panel I have chosen), then I came across a more compact arrangement from Embedded Artists in Sweden. Unfortunately they both have the drawback that they require trailing cables from the GPIO connector to the board. I wanted something that would just sit right on top of the Pi with no cables to worry about, and I wanted a real time clock on board as well.
What do I get?
- Single - one tested circuit board with display panel attached.
- Twin - two tested circuit boards each with display panel attached.
- 5-pack - not surprisingly, five tested boards each with display panel attached
What can I do with it?
In conjunction with your Raspberry Pi and some power you might make:
- Desk or wall clock
- Weather station
- Twitter feed display
- Anything else you can dream up
It's great for projects where power is at a premium, because it only draws significant current while writing a new image to the display and it retains the last image sent to it even with the power off.
What you can't do is watch movies or fast changing images - it's not that kind of display. The manufacturer gives the operating temperature range for the panels as 0 to 50 degrees Celcius and recommended storage temperature between -20 and +60 Celcius,
Software to drive the display is available from the Repaper GitHub repository comprising a low level C driver which interfaces to a host of Python demo programmes which you can hack to your heart's content. It is possible to draw a completely new image to the panel or to partially redraw sections of the display, using functions from the Python Imaging Library (Pillow) and text can be placed using a variety of system fonts in various sizes.
Detailed instructions on setting up the real time clock and on installing and configuring the GitHub display code will be provided.
The software is provided under an Apache Version 2.0 licence which gives you plenty of freedom to do what you want with it, as long as you leave the original copyright attributions in place.
To be cost-effective, I really need to make a minimum of 250 E-Paper HATs, so get your pledges in now.
Risks and challenges
The circuit is tried and tested and is based on the reference design from the panel supplier. The prototypes are working well as you can see in the video
The main hurdle is likely to be delivery time on the panels, for which I have been quoted up to 16 weeks.
The circuit boards should be available long before the display panels arrive, so they can be tested and configured ready for final assembly.
If this should prove to be really popular, I can see that delivery times might slip for some people, but not excessively. Family and friends will be drafted in to help, if necessary.
I have chosen to use the Royal Mail Signed For service for both UK and international dispatch, which should provide some level of reassurance on actually getting goods to backers.Learn about accountability on Kickstarter
- (30 days)