A challenge during this project will be to specify components which are low cost when purchased in low volume. Mechanical components can be inexpensive with an obvious trade off in quality. Electronic components used in this design will be in expensive individually but quickly combine to create an expensive BoM. Low cost fabrication is available but delivery times are horrendous.

Designing in a microcontroller is the first of these hurdles but with a slightly different twist. The user needs to transfer a binary image from their computer to the microcontroller. It cannot be assumed the user has a selection of programmers standing by so the BoM also needs to include this item. Low cost programmers from vendors come in at around £20 so this out of the question.

The Arduino UNO (Atmel ATmega) is an inexpensive dev board but lack some key functionality desired for this design. The microcontroller is held in a socket so can be programmed and transferred to the design which is nice. The same goes for the TI Launchpad except at a lot lower cost. Open source DIY programmers are available and may come in at a cheaper price but would be a project in its own right.

Hacking a dev board is the way to go. Throughout the range available the ST Nucleo boards come in at a lower price than the competition (the NUCLEO-F302R8 is only £6.48 from Farnell ) and packs a plethora of peripherals. The tricky step to decrease the BoM cost even further is to de-solder a LQFP package from the board and solder it down on power supply PCB. The remaining part of the dev board can be hacked to open the programming interface, add some headers and program the microcontroller in-situ. The user also has a programmer handy to upgrade the design as required.