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Wednesday, November 16, 2016

5V Solar Power supply for Raspberry Pi - feasibility phase

Ok, here the deal.
The goal is to have a 5V solar power supply for Raspberry Pi in order to power up a project (fHelper).

Main characteristics

  • Solar charger
  • Battery
  • at least 5v 1A
  • monitoring power and shutting off nicely Raspberry before the battery is depleted
  • Push button for on/off operation (see RPOf project)

In order to achieve this goal some experimentation is necessary in order to figure out if the location where the project will be placed has enough sunlight and how to dimension the circuit.
Some assumption are made :

  • at least an average of half day of direct sunlight is present
  • the load will have an average of 350 mA (the goal will be to reduce the Raspberry Pi consumption as much as possible, however "as is" the current Raspberry Pi for the project, is stable around 350 mA)
  • The power supply is done using "off the shelf" components as much as possible.
    The only part to build is the RPOf.
This article describe the initial test.

The shopping list


Let's start with the shopping list for the project.
Let see first a block schematic of the test environment

Block schematic

A view of the test system. The battery is probably damaged and need to be substituted.
Temporarly an external data logger (USB-500) is capturing the charge/discharge cycle of the battery and a USB voltmeter is displayng the DC/DC output
On the top the Raspberry Pi with the GrovePi board and some sensors.

Components

Here a list of components used to build the solar power supply.
The goal is to use as much as possible "ready made" circuits, so I looked at Adafruit.

DC/DC converter


This is the core of the solar power circuit and is based on the Powerboost 1000 from Adafruit.
  • convert to 5V 1A a 3,7V battery

Solar charger


The second "off the shelf" component is the solar charger.
It connects the solar panel from one side and the DC/DC to the other side providing the power to charge the battery.
Is based on the USB/DC/Solar lithium module from Adafruit.

Solar panel


From Adafruit, a 6V 2W solar panel

Battery

Initially I will use some spare batteries I have around, to determine what best battery should work for the project.
A 3.7V 6600 mAh probably will be used (from Adafruit)


Data collection


This phase is necessary in order to figure out how to dimension the solar power.
In order to collect some data a Raspberry Pi, powered separately, will collect some data.
The sensors used are based on the GrovePi system.
Here a shopping list of the needed hardware for the data logger functionality (GrovePi is also used for the final project).

Mechanic


In order to have some kind of mechanical support, a wooden base is holding a plexiglass where the Raspberry Pi, the solar panel and the rest of the circuits are mounted.
The mechanical base is the same used for the fHelper project (described in separated articles).



Hardware

Software

In order to use the GrovePi system of sensors a special Raspbian version is used, from Dexter industries (the company who produce the GrovePi system).
Follow the instructions from the GrovePi website in order to install the system on a Raspberry Pi.

The code to handle the data logger functionality is in python.
The idea is to collect data about the light presence (full sun, out of sun), the voltage from the solar panel and the voltage on the battery.
The goal of the data collection is to figure out some answers to these questions :

  1. how long the battery can supply the project in absence of the sun ?
  2. is the solar panel enough to charge the battery ?
  3. is the battery big enough to sustain a constant power supply ?
  4. devise an algorithm to eventually shut down the Raspberry Pi and automatically power it back when enough power is available
Stay tuned for updates

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