Complex circuits include those that have microprocessors within them.
This is because even through some processors only have 8 connections, each of the input/output connections can be configured as inputs, outputs or serial busses with the possessor itself as the master or as a slave. (A master initiates all of the requests, and a slave will wait for a request before carrying out any activity.) With most of those circuits, we would be able and wiling to advise what best to use, and how to connect to it.
Most other circuits can be easily broken down into sections that each section would can be classed as a simple/middle-complexity circuit. We can advise on this if needed.
The Aduino is a circuit that allows complex circuits to be simplified.