Another driver for the project was the fact that I wanted to make a "mobile" satellite receiver groundstation. I could always make a little box with all the pieces in it, but wouldn't it be cool if it was all attached to a hat? There are a few upcoming PocketQube satellites using either LoRa or ISM band radios, such as FossaSAT, Nepal-PQ1 and my own OzQube-1. 

Seeing that the "Flexible PCB Concept Contest" was going AND this project needs to have a flexible PCB to match the curve of the brim of a cap, it was a match made...on Hackaday.io?

CONCEPT

The first goal of this project was to make a LoRa repeater, based on Dave Ackerman's project. It uses a small LiPo battery to power an Arduino Pro Mini through a 5V boost circuit. The Pro mini is connected via SPI to a small LoRa radio tranceiver module. My concept is going to replace the Lipo Charger with a solar powered version, and utilise a Supercap instead of the Lipo.

The Pro Mini will be built directly onto the Flex-PCB substrate, along with the charging circuitry. The solar panels themselves are from IXYS, and measure 44.5x21.5mm. They will be positioned on secondary  flex-pcbs's either side of the main one. See the sketch to get the idea.

I'll also embed a 433mhz spiral antenna in the flex-pcb too. It might look a bit funny having a small monopole antenna on top of your head.

PARTS

The radio module will be a common LoRa module based on the Semtech SX1276 IC or the equivalent from HopeRF, the RFM98W. The power supply will be similar to a previous project I created for the square inch project.

The Supercap will be charged from the first IC - the Analog Devices ( Linear Tech) LTC3105. This IC has MPPT, which will optimise the power generated from the solar panels. The Supercap will then be regulated to 3.3V output through a second LTC3105. Because of the low startup voltage of this IC, I'll be able to use most of the capacity of the Supercap while maintaining 3.3V output.