74 TTL Series Logic
I was delighted by the revelation at college that you could implement logic functions from a mass of spaghetti wiring into a single chip solution by simply by using a multiplexer & tying up the inputs to +5 or 0V at college (Aided by a truth table).
I walked into a independent electronics store some years later with my notes while looking to create a circuit that would switch a Amstrad satellite receiver (SRD510 if memory serves), using the non existent 7 segment display (on that model) that any channel number over 50 would switch* on a 22KHz tone generator & switch to a second LNB for TV1000 & the like.
The shop assistant took one quick look at my notes & exclaimed "Blimey you really know your Boolean logic".
*I'm aware that there was a link (& fitted on my unit) on the board that would make it think it was a SRD520, that gave a switched output for driving an external frequency extender up to the full 2Ghz (There was also a later mod that gave that function without the use of an external frequency extender, possibly at the cost of some of the fine tuning of the frequency).