What is a billion (or two) transistors?

The iPhone has 2 billion transistors in the CPU! 
Wait, first, what is a transistor? If you are feeling intimidated by the question, or even worse, comfortable with not having any idea, then you are on the un-empowered side of the very real digital divide in society. If you are on the empowered side of the digital divide you might describe a single transistor as a kind of switch that allows electricity to flow or not flow depending on an input signal. The idea is that the transistor is always in an on or off state, a 1 or 0 state: a binary system, true or not true, high or low.

This summer in the St. Raymond Makerspace we have been developing curriculum that introduces a single transistor on a paper circuit to control a small motor. Students can build this circuit and gain some confidence and better understand the digital world around us.. What I wanted to cover in this blog post is one step after that. When someone knows what a transistor does, then how do we scale that up to understanding many transistors on a computer chip a.k.a. integrated circuit (IC)?

A great first integrated circuit (IC) to study is the venerable 555 timer chip. The different functions can be understood at a high level right away and a first project might take only a few minutes to complete on a breadboard. The problem is the person learning can not **see** the transistors and with only a nascent knowledge of how a single transistor works we go from practical knowledge to the world of theory because we are literally working with a tiny black box. Circuit schematics give us detail, but only build a mental picture. We can not physically probe the inner working of the tiny black box with a voltmeter.

So why not just build a 555 timer out of full size transistors and resistors? There are not that many to put together and we do have a schematic:) Once you get to understanding how a single transistor works then build this full size 555 chip. You can start to get a sense of what is going on inside of a simple IC. This is literally a "dis-integrated circuit" which is a lot of fun to say as a tongue twister when trying to emphasize the right syllable so as not to sound like "disintegrated".

When someone builds this full size model of a 555 IC they can then start to absorb some of the block schematics of the IC as meaningful information. Next year this will be a required project for every 8th grade student. Those students can then appreciate statements like "there are two billion transistors on an iPhone processor" because they actually built and played with exactly the same thing, admittedly with a few less transistors, but not less complex operating theory. To build this oversized is to help students and teachers jump the digital divide by going from ""tiny theoretical black box" to "large guts exposed and poke around in it box".

 The original 555 IC in the upper left .vs the larger than life version below made from full sized transistors and resistors. 

The original 555 IC in the upper left .vs the larger than life version below made from full sized transistors and resistors.