The original concept began from a Loco Decoder design that can be found on the Model Electronics Railway Group:
The original design was by Mike Bolton and the software written in PIC assembler by Gil Fuchs, both members of MERG at the time.
Using the schematic for the Merg V13 decoder I designed a double sided surface mount PCB.
The V13 decoder schematic can be found here:
Despite being a faithful copy of the V13 schematic I was unable to get the decoder to reliably function.
I also found that my PCB (36mm x 18mm) was not actually small enough to fit inside some of my locos.
I decided I needed to shrink the PCB, and also understand the inner workings of the code.
Several iterations of the PCB took place in the ”virtual world” of Protel, shrinking the PCB further each time.
Along the way I opted for a more modern processor with more capabilities, the PIC18F25K22.
The final PCB measures 21mm x 14mm, using a 28 pin QFN package for the PIC.
As the QFN package has no pins, a soldering iron cannot be used.
This page shows how I solder the QFN package to the PCB.