Arduino Mega 2560 R3

The heart of the SIM! I have 18 of these. They are awesome and come from china. Quality varies greatly! Some refuse to work outright, some individual pins refuse to work. But 3.99 each, there a bargain and you can build a full sim very cheaply.

https://www.ebay.co.uk/itm/ATMEGA16U2-Board-For-Arduino-Mega-2560-R3-Board-Kit-Compatible-With-USB-Cable/263524929566?epid=7004440489&hash=item3d5b4f501e:g:NNgAAOSwNt1agqr~