Many of us have a few Raspberry Pi single-board computers lying around, maybe leftover from a project, or one that we bought for some specific reason but never got round to using it. I have a total of 4, and to be honest, I am not using any... least of all the Pi Zero W.

It looked like a very good deal at the time, but when it arrived, I found it just too limited for serious use...

That was until I had a need for a low-power, small profile MQTT server on my local home network. The Pi Zero seemed like the perfect candidate for that, but once again, after doing a bit of research, I put it away.

Why this time: Battery backup. I stay in SE Asia, and while the electricity is usually quite reliable, it is infamous to go out for any reason under the sun, usually with no warning at all... The reason for this is that no real planning is done by the electricity supplier, and to save costs, they use the cheapest electrical contractors available... making it necessary to completely switch off the supply to the overhead cables... Maybe they have never heard of bypassing a section of cable and isolating only the area that you need to work on... anyway, this makes for a lot of headaches.

I looked at a few commercially available UPS type hats for the PI Zero but found them way overpriced. So, I decided to do my own.

It did however require a few things to make me happy, most of which is not yet built into the version I will present today...

1) Battery Backup from a single 18650 Cell, with NO relays of MOSFETS switching over when the mains supply goes on a break...

i.e. It has to be full ONLINE backup.

I found that the MH-CD42 Module will give me just that, with the added bonus that it can supply current while it is charging.

2) A single button shutdown for the PI, that does not conflict with some of the other GPIO functions

3) Battery level monitoring - not currently implemented, to send a signal to switch of the PI safely when the battery gets low.

I also require full access to the GPIO pins, and I want them grouped together by function. clearly labelled, and while there, add some level conversion as well, as most of my sensors are 5v, or not 3v compatible

This is what I came up with, my apologies that it is not 100% completed at this stage ( Post Covid Logistics is a HUGE problem, and components, although ordered well in advance, still seem to linger somewhere in a warehouse for at least 21 days AFTER customs clearance before being delivered :) )

The idea is to have the Pi Zero/W plug into a 90-degree Right Angle 2x20 Female Pin Header. Access to all of the GPIO pins is then made via the relevant Pin Headers on the baseboard.

White is 5v capable, Black is 3v. This is achieved via two TXS0108E Level converter chips ( U1 and U2 ).

The converters are controlled via GPIO25 ( U1 ) and GPIO24 ( U2 ), which need to be pulled HIGH to enable level conversion. These pins are not broken out to any other headers.

For the time being, only SPI0, SPI1, I2C1 and the UART are level converted. I may add more converters in future improvements of this PCB.

All other GPIO is available on a 3v logic level header.

GPIO26 is configured as a dedicated shutdown pin. Please note that it will not restart the PI again. That is done by pressing the Power-Up Button, SW2, once...

Due to the fact that the Female Pin Headers have still not arrived, I am testing the board with power supplied from the battery Module,

via Jumper wires, Orange (3v from PI, White, Black is ground, Red and Brown is 5v supply to the PI Zero.