The Apollo guidance computer & the NOR gate

If you are familar with modular synthesizers, you will probably be aware of logic gates.
But did you know of their connection to the Apollo mission to the moon?
It's nearly 50 years since those heady days.
Apollo changed our lives in so many ways & the technology used then, permeates our society to this day.

Above is a pic of the AGC - The Apollo guidance computer.
 (The original uploader was Grabert at German Wikipedia. [Public domain])
 
This was the computer that took those 3 men to the moon and back. In addition, it was one of the first computers to use integrated circuits (ICs). The AGC was in fact the world's first "embedded system".

Today ICs are everywhere. But back in the late 1960's they were a revolution 
NASA's goal was to use components that were low weight, had low volume & operated on low power. Coupled with this, they had to be extremely reliable.
With these goals in mind they decided to use a single simple integrated circuit for all logic functions.

Since just one chip was used, NASA could really test it under whatever extreme conditions it liked.... to make sure that it could survive everything which space travel could throw at it.

AGC dual 3-input NOR gate

  This single chip contained a three input NOR Gate. It's incredible to think that it's possible to carry out all logic functions to take men to the moon with just a NOR gate. 

  This gate gives a positive output only when both inputs are negative.

The beauty of the NOR gate, is that it is what is known as a universal gate.
In other words, it can be combined with itself to make any other  kind of logic gate

For example a NOT gate can be made by joining the inputs of the NOR gate.

 

The OR gate can be made by inverting the output of a NOR gate.

 The AND gate is made by inverting the inputs of a NOR gate.

 We can go on and on, but I think you get the idea.

The AGC dual 3-input NOR gate schematic

The above schematic shows the makeup of the NOR gates using discrete transistors.
They are just really simple NPN/resistor designs.
You could possibly build this using any NPN type transistor (for example 2N3904, BC547, BC548, BC549 etc.)
 The use of transistors for the construction of logic gates uses their ability to act as fast switches.

 NASA used about 5,600 NOR ICs in total for the later AGC versions. 

Getting back to synths, the 4001 IC is  a useful chip to have in your bag.
It's a CMOS IC containing four NOR gates

So in theory, you can build any logic module with just CD4001s

Below is a Serge module... one of my favorite Synth logic modules.
At first glance, there doesn't seem to be a NOR gate.

 AND: Output goes "high" when all inputs are "high".
 OR: Output goes "high" when at least one input is "high".
 XOR: Output goes "high" when a odd number of inputs is "high" and the rest is "low".

The logic inverters can change the functions into NAND, NOR and NEXOR.
 Note: a logic inverter turns a "0" into a "1" or a 1 into a zero
 It inverts the logic state, not the voltage value (i.e. +10V will not turn into -10V).

Links.
+ Schematics of the AGC
+ Ljunggren Audio
+ The Boolean Serge
+ CGS 39 - Ken Stone's Quad logic gate
+ Talking electronics - about the CD 4001
+ About CMOS
+ Pittsburghmodular logic banks
+ Transistors - basic info
+ Making logic gates from transistors