Tested on:RPi3; Raspbian Jessie; MoPower firmware v0.93
Prerequisites:Raspberry Pi with Raspbian Jessie and internet connection (see steps 1-7 in the Introduction)
Hardware:Case: wood and tools, hinge, screws and drill, aluminium sheet (, Sugru)
Soldering equipment, multimeter, cable, connectors
(5V active speakers with USB power supply)
(12V wallwart)
USB mouse and keyboard
Official RPi touchscreen
Solar charge controller
Solar panel
12V bike battery
MoPower UPS
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BRX1000: autonomous RPi solar computer with MoPower for energy management

BRX1000: autonomous RPi solar computer

BRX1000: autonomous RPi solar computer

The small, lightweight, inexpensive and energy efficient Raspberry Pi is a great base for autonomous solar devices. In this tutorial, you’ll learn to build the simple solar computer BRX1000 with a Raspberry Pi, the official touch screen, a MoPower for energy management and an affordable solar setup. This allows for instance setting up an autonomous gaming station with RetroPie.

Nacked solar computer
The solar computer consists of

  • Raspberry Pi, keyboard/mouse, touchscreen
  • MoPower UPS Raspberry Pi HAT
  • Solar panel (here: 17.5V, 4 W, 230mA, 31.5*31.5cm with aluminium frame)
  • Solar charge controller (here 12V, max. 20A, e.g. from Ebay)
  • Bike battery (here NP7-12: 12V 7AH)

Altogether, that goes for slightly more than 150€. Here’s a picture of the solar computer without the case:

BRX1000 nacked

BRX1000 nacked

The solar charge controller is connected to the solar panel and the batterie. Additionally, the battery is directly connected to the MoPower, which sits as HAT on the RPi behind the touchscreen.

Case assembly
When building the case, ensure the bike battery fits in and length and width match the solar panel. For me, that’s

  • 2 planks 31,5 * 15,5 * 1,8cm
  • 2 planks 28 * 15,5 * 1,8cm
  • 1 bottom plank 28,2* 28,2 * 1,8cm
  • 1 plank 18,2 * 17 * 1,8cm with a centered cutting of 18 * 10,5cm
  • 2 small pieces aluminium sheet for attaching the touchscreen
  • 2 hinges with wings sized 5 * 1,8cm
  • 1 Y-cable from the battery to charge controller and RPi
  • Doublesided tape for stabilizing the battery

For setting screws into the wood, pre-drill with a small drill. Since the hinges didn’t hold in the aluminium frame, i used a thin layer of sugru between frame and hinge:

Hinge with Sugru

Hinge with Sugru

The RPi and the charge controller are both powered directly from the battery. This requires a ustom cable, battery terminals to open for the charge controller and to a 2-pin header for the MoPower:
Y-cable

Y-cable

The touchscreen with the RPi on its back can be attached easily to a wooden frame with two small pieces of aluminium sheet:
RPi frame

RPi frame

The frame is attached to the case with simple screws: the outhere planks were drilled with a drill slightly thicker than the screw thread. By this, the frame can be rotated easily for 180° and allows quick GPIO access.

Software
For startup, simply install the MoPower software like shown in the tutorial MoPower on Raspbian Jessie. Afterwards, the RPi can be started and shutdown with the power button on the MoPower HAT.

Energy budget
Here are a few ideas for approximating the energy budget; live test results will follow.
The battery gives 7AH at 12V. The Raspberry Pi including touchscreen, MoPower, periphery and charge controller takes around 0,65A at 12V. Therefore, the fully loaded battery allows an uptime of around 7AH * 0,65A ~= 10,7H * 0.8 ~= 8,6H (* 0,8 because the battery can only be discharged for 80%). But how long does it take to charge the battery?
The empty battery contains about 1,4AH at 12V; 5,6AH are missing. The solar panel maximum current generation for 12V is (17,5V /12V) * 0,23A ~= 0,34A. For charging the battery to 100%, the solar panel needs to charge under ideal conditions for 5,6AH / 0,34A ~= 16,5H. Given 7,6 hours of direct sunlight per day in June, the battery would need to be charged for more than two days. Without a sun tracker, more than double that is more realistic.

ToDo

  • Casket lock
  • Improve RPi frame
  • Handles and feet
  • (Integrate Speakers)
  • External power jack and main switch
  • Solar panel and RPi: arrester, idealy with adjustable angle
  • LiPo-batteries instead of lead
Links
MoPower on Raspbian Jessie: http://www.knight-of-pi.org/mopower-on-the-raspberry-pi-3-with-raspbian-jessie/
Retrogaming with the solar computer BRX1000: http://www.knight-of-pi.org/emulator-gaming-with-retropie-update-using-two-keyboards-with-a-solar-powered-raspberry-pi/

Knight of Pi

Johannes Bergs aka Knight of Pi. Diploma in Bioinformatics, some Webdesign and Python coding then. Living in the beautiful city of Vienna.

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