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wirewitchviolet · 1 year ago

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How a Computer Works - Part 5 (Input and Output)

It's been a bit since I've updated this series, so to quickly recap, we've been over a few fundamentals of how we can make electricity do fun tricks for us and the history of that, we've talked about logic gates and how to latch in data to save for later, we've talked about how people make those gates and other fiddly bits nice and tiny and well-organized, and we've even explained how you can run a current through a pile of transistors and end up with the answer to a basic math problem. but it's hard to get really excited about making a pile of electronic components add numbers together unless, at the very very least, we can easily change which numbers are being added, and see a display light up with the answer. And of course like everyone doing anything involving computers, we really want to eventually get neat games running where we're pushing buttons then seeing and hearing (maybe even feeling) cool changes happen with our fancy display and speakers and such. So today we're going to talk about various ways to input data to a computer, and have it output something back to you.

And like always, before we get into that, I'm going to post this link so you can maybe input money to my bank account, and I can output it to my rent checks and grocery bills, so I can continue to input food into my mouth and not be output by my landlord onto the street.

Switches and Secretly-Still-Switches

The most basic and easy to understand way to interact with a computer, or really any other sort of electrical circuit is a switch. We have a wire making a connection between two points, and we just physically sever that connection by severing the wire. Then we have some moving piece we can put back in place and reconnect it. This can be as simple as holding two wires and touching them together (ideally well-insulated ones, but you can skip a step and bridge the gap with your own body just by grabbing metal if we're taking precautions that it's a load you can safely handle -- and hey just to be absolutely clear the electricity that comes out of your wall is NOT a load you can handle safely, same goes for a lot of parts inside the average computer). Usually we get a bit fancier and make a little metal lever covered in a non-conductive material we can move, maybe we get all fancy in the design as as we cross a certain threshold the switch finishes throwing on its own with a satisfying click and keeps anything dangerous from happening while the metal bits inside are just-barely-not-touching. These are pretty intuitive.

How about buttons? Well, buttons are really just switches. Most buttons have a spring inside so that the gap in the circuit is only getting bridged while the button is actively being held down and breaks again as soon as you let go. That's officially called a momentary switch. Sometimes though, again, people get fancy with button design and have them physically latch into place with a spring or a magnet or something until you press them again, so they function like a standard switch. And nothing's stopping us from putting either of these on a wire which sets a logical latch and functionally does similar.

What else can we make a circuit react to? There's all kinds of special sensors right? Like... how does a theromstat work? Well, a thermostat, and honestly a shocking number of other things, contains something called a bimetallic strip. You literally take two (that's the bi part) different metals (you got it), and you stick them together in a strip. You know how heat makes metal expand? Well, different metals expand (and contract) at different rates based on the temperature, so either the top or the bottom is going to want to stretch and take the other with it which causes the strip to bend up or down depending what's going on (it's much easier to see this and make use of it if you coil the whole thing around a bunch). So you just mount a strip like that inside whatever device you want to have react to temperatures and under the right conditions it'll curve one way or another and either directly connect (or break) your circuit or press up against something that will, and tada, we're using the temperature as an input. This is how theromstats work, and circuit breakers. And a shocking variety of other things honestly, including some old clocks and motors even.

You may be thinking you don't necessarily need two metal strips for this. Thermal expansion can make something swell to a point it makes contact with something. For that matter, when ice forms it ends up filling a larger volume than liquid water because the shape of the molecules makes them line up together rather than pack tightly, and that could press a button. Some things even use tri-metallic strips for some more fine control over things.

Technically Not Just Secret Switches

What are some other common sensor types we have today? Well there's various sorts of photosensors, that's how digital cameras work. Apparently, and I got pretty lost in the weeds looking this up, this sort of thing works by way of photons and/or UV radiation bopping electrons around inside a sensor material to flip it from conductive to non-conductive, in a way very similar to how modern semiconductors are made. And I suppose there's different versions of this for different wavelengths of light, letting digital cameras detect various colors. Sorry for being sketchy here, this one's just out of my wheelhouse.

Then we have touchscreens. Old touchscreens absolutely worked as switches. You're either pressing something down to make a connection, or there's a grid of emitters and sensors your finger breaks as you tap the glass. What's most common lately though are capacitive touch screens which... work just like capacitors. I'm still a bit fuzzy on how capacitors work to begin with, but we've got glass as an isulator, half a capacity on the other side, and your finger acting as the other half. The electrons vibe and do their charge-y thing in between, and you don't actually have to make physical contact for that to work, just get close, which i nice since you don't get your greasy greasy fingers all over your screen.

Accelerometers and Potentiometers

Accelerometers are another one where sadly looking up some technical documentation went way the hell over my head. I assume though the basic principle is, I move a sensor, some floatinginternal component lags behind the casing, a potentiometer determines how far off from the center point it is.

Now potentiometers I do get, and we covered them a bit in part one. we have a chunk of material that provides some electrical resistance, we have a wire coming in that brushes up against it, and we can move where exactly it's making contact, usually by rotating a knob to move it along using gears of some sort. At one end we have the current flowing all the way through this resistive material and getting weakened, at the other end we're just barely passing through it. So it's basically a variable resistor. These are used all over the place.

The Ol' Keyboard and Mouse (and Game Controller)

Now the most common things we use to input stuff into computers seem like they just combine some of the methods above in some pretty simple ways. A keyboard is just a big ol' array of buttons, right? We just have a wire under each key with a break, we press the key down, and it completes that circuit. And... well yeah, that's what's going on. But your average keyboard has what? A hundred or so keys? If you look at the end of the plug for it, you're going to notice significantly less than a hundred wires in there. So, what's the deal?

Well, really crappy keyboards have a cheat where we just have wires running through on a grid. You'll have a horizontal wire running down each row of keys, and pressing a given key down connects either the positive or negative end of a circuit to that wire. Simultaneously, each rough column of keys is doing this with another wire. We end up with active signals on like row 2 column 3 and we know that intersection is the W key, and with enough logic gates we can work with that. The reason these keyboards suck though is, well, let's say I simultaneously hit oh... W and G. We're connecting row 2, column 3, row 3, and column 6 or so. That matches up with both W and G, but it ALSO matches up with S and T. Or all four at once. And we don't really have a way to work out what's what, so we're probably going to get some wrong characters.

A good keyboard absolutely does track a separate connection for each key, so you can hold any given combo. Important for games and such. But again, that's way too many signals to pass down a cable. So what we do, basically, is have a binary representation for each key. Let's say 7 bits, I think 128 keys is probably enough. and now we can load all of those values as we hit them into a little memory chip within the keyboard. Then we just pull in a clock signal, and set up a shift register. I've covered though right? Handy for when you want to double a number since you just slide every bit one position to the left? Yeah so we just dump everything into memory. Whatever key is first in line is in the first 7 bits. We pulse the clock, we send those 7 bits down the line. Then we shift everything 7 positions to the left. Pulse the clock again, hey, here's the next keystroke we need to process, send it down next, and so on. We call that a serial transfer. It sounds like it'd be slow enough to get annoying, but remember, we run the clock at absolutely stupid speeds, so nah, it works out fine. Cuts down how many wires we need to run down that cable too. Hell more often than not we actually just send one bit at a time, even.

Now how about a mouse? Well, the buttons are buttons, simple enough. The actual X and Y position, those are a little trickier. Oldschool ball mice had a pretty simple and elegant solution. We've got a wheel for each axis, they spin as the ball rolls around And basically, inside there's a cylinder where for each bit we need to track, and we can get by with just 2 if the clock's fast enough, either we've got a hunk of metal making contact with something and completing a circuit, or we've got a gap, no contact. We can kinda get there just cutting segments of the cylinder in half with an offset. Physically it's a little tricky to build this if we're doing a proper binary representation though, so instead we go with a Gray code. Named after the creator, Frank Gray. His whole idea was that for weird cases like this, rather than standard binary numbers i.e. (00 01, 10, 11) it's less error prone to use a system where you only ever alter one bit at a time. So it goes 00, 01, 11, 10, and back around to 00 if we're just handling two bits (it gets a little harder to keep straight when you need more. Again, hey, just stick in two half-cylinders, with one of them rotated 90 degrees. We can roll up, we can roll down, and it's simple to tell which direction we're going just by which bit just changed. You need to build some logic to interpret it at the other end, but there's no weird case like jumping from 01 to 10 where if the reach happens at just the right time and/or they corner where those meet isn't perfect you accidentally catch 11 or something.

Modern mice still use this setup for handling the scroll wheel, but the actual X and Y position are tracked optically. How that works is... honestly kind of just insane. We have a camera pointed down at the desk, taking something like 1000 photos a second, and a whole little processor on board comparing those pictures and looking for little imperfections like specks of dust or wood grain lines, calculating how much they moved by between frames, and updating position data based on that. It's just bonkers we're doing that much work and also that these are as cheap as they are, but, yeah, that's how today's mice work.

Sound

OK, this is the last common input method I can think of (analog sticks and triggers are potentiometers, shoo), speaking or blowing into a microphone. Glancing at wikipedia, you can design a microphone using... honestly basically any possible property of electricity you feel like, but the basic idea is always the same. Sound is a vibration, it travels though the air. You set up a very sensitive wobbly bit that gets knocked around by the sound waves. This moves... whatever really. Capacitors, magnets, potentiometers, one of those crystal oscillators we use for clocks, lasers and photosensors, it's all good. This screws with the signal they send down a wire, now you just have to measure the changes there. Pretty simple? And if you want a speaker... you just throw that into reverse. An electrical signal goes through whatever bit, makes a membrane twitch, which sends ripples through the air, bam, it's sound again.

Other Outputs

OK, what else can we do with electicity? Mainly, we can run it through neat little components that freak out in various ways when we do that. Some things get real hot when you run a current through them. That's nice if you just want to warm things up (maybe one of those bi-metalic strips, or a heater). Some things full on glow. That's how old lightbulbs work. Some things toss electrons off, that's how vacuum tubes work, we covered those. It's also how old TVs work. You get the electrons going off, then you have a bunch of magnets wrapped around the outside to direct them into a beam, and mess with the magnets to sweep it around, they hit phosphor dust, it glows briefly. Stick the same phosphor in a diode, you got yourself an LED.

Then there's electromagnets. We can do all kinds of nutty stuff with those. Way back when we talked about how you can make relays out of them, get those switches to throw themselves around. You can flip other magnets upside down, that's handy for flip-dots, like buses use to show where they're going. This is also how those "e-ink" displays work on fancy little e-book readers so you're not staring into a light. Little balls painted half-black half-white with a magnet inside. This is also how an electric motor works. Just keep flipping magnets and make something spin. You can use that to move anything.

And hey, you know what else you can do with magnets? You can take a big magnetically sensitive film, like a piece of tape, or a spinning disc, use magnets to magnetize bits of it as it travels by, then use something magnetically sensitive later to read that back into electrical signals. That's how... basically all long-term storage media used to work. Tapes, discs, hard drives, if you wanna get real real oldschool, core memory (it was magnets on this woven lattice)... I don't actually know how modern stuff like flash memory and solid state drives work, but I assume it also comes down to using magnets to tweak something inside.

The only non-magnetic storage I'm aware of really is when you go even further back, and we just punched holes through stiff cards or long strips of paper tape. Just use a motorized punch to place them, and then blowing compressed air that'd either pass through a hole and hit something sensitive or get blocked to read'em. This was also how player pianos worked.

I didn't QUITE cover monitors here in great detail, they used to use electron guns and phosphor dot masks, now they're just tons of LEDs. You store all your data in a big array of RAM for a whole frame and turn stuff on or off accordingly, basically. And... yeah I think that covers all the bases.

There should be less of a gap before I post the next part of this series, where I'm just going to try and put all this together so we can build a full, feature-rich computer as a thought experiment or you know, from whatever materials you have on hand, and that one MIGHT be the end of the road unless I start the real deep dives into real serious data structuring and programming and such.

What I'd REALLY like to move onto though is my project of designing a new video game console, and freely providing everything you need to order and populate the circuit boards, 3D print a case, and assemble the whole thing on your own, as a nice hands-on project. That's currently stalled out because... no really I am BROKE. I am writing my last rent check in a couple days, past that I don't even know how to keep a roof over my head. I really need to bulk up the patreon numbers to where I'm treading water, and go up from there so I can afford the components to really prototype everything and make that project real, so I would REALLY appreciate some serious donations and people spreading the word about this stuff.

#computers #computer science #mice #keyboards #education #electronics #monitors #speakers #microphones #thermostats

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