Daily Archives: March 23, 2015

Weekly update

Dealing with PC issues at the moment so this week has been a bit rough.

My inspiration for the Portable Raspberry Pi came from the Ben Heck Show on Ben’s creation of a Portable Pi. (Part 1 & Part 2) I liked Ben’s design but there were a few things that I wanted to change.

  1. I did not want to deal with having a batteries in series and not be able to easily charge them in the case without disconnecting them from the device. This requirement was due to Ben’s choice of LCD monitor so I am changing the LCD to an Adafruit PiTFT screen which uses SPI. This will be a bit trickier to use but should not be a problem.
  2. I wanted to use the powerful Teensy for more than just the gamepad so I modified the Teensy code to provide a custom gamepad which has only the buttons that I will have plus provides keyboard and raw HID devices. The reason for these additional devices are for power management and control.
  3. Power – I wanted to be able to charge the battery in the device and still be able to use the device while it was charging. To implement this requirement, I turned to Adafruit’s USB LiIon/LiPoly charger.
  4. Power On/Off – The Portable Pi will be used by my children so I wanted to have an easy way for them to shut it down as well as to automatically shut it down when the batteries are low. This is where the additional Keyboard and Raw HID device come in on the Teensy. The plan is to implement a soft latching power switch circuit which can turn on the power to the device when pressed from an off state. When it is on, the Teensy will be able to detect when it is pressed and start an orderly shutdown of the Raspberry Pi. If the button is held down longer, about 3 seconds, then the power will be switched off. This will allow it to function much like the power switch on a PC or laptop.
  5. Power monitoring – I would also like to be able to monitor the battery level and perform an automatic shutdown of the Raspberry Pi if the voltage is too low. This is where the Raw HID device comes in. I plan to use the Teensy to monitor the battery voltage and send periodic updates to the Pi to let it know the level. If the level reaches a min threshold then it will send keypresses through the keyboard device to tell the Pi to shutdown.

These are just a few of the high level requirements I have for the Portable Pi. So far, I have the Gamepad code completed and the hardware on a breadboard. I am currently working on the soft latching power switch. I have built a few circuits which work but only at 5 VDC. I need it to work at 3 VDC and higher so I need to order some parts as I do not have MOSFETs which will switch full on at that low of a voltage. I plan to implement a design found on Mosaic Documentation Web. I have built a similar circuit and found it to work well and meet the requirements that I have. I also looked at David Jones’s design on his EEVBlog but it did not meet all of the requirements that I have. It is a nice simple design so I am certain I will use it in the future on another project.