This document contains lecture notes on number systems for an open educational resource on basic electronics. It covers various number systems including binary, octal, decimal, and hexadecimal. Conversion between these number systems is discussed along with 1's and 2's complement representations. Examples are provided to demonstrate converting between number bases and performing addition and subtraction using 1's and 2's complement representations. The document is authored by Dr. Piyush Charan of Integral University for a course on basic electronics.