This is a nice educational reference project based on 89C2051, an 8051 derivative. This uses the Uses analog switches to steer test tones and results. Programmed in assembler to produce pure 1khz tone.
“The example meter that we are demonstrating is typical of many economical digital multimeters that you can purchase, hopefully these instructions will be of some use to you no matter what brand or type of meter you use. “
Students and Hobbyists can study all the example projects and tutors in this site to become more skillful. They have pages that show how a PCB is made and things like Circuit Design and Prototyping. This Site also happens to be one of the oldest resources in the web for Electronic Embedded Designs.