For years I’ve been wanting to make mechanical logic gates that i could put together into mechanical logic circuits. I want to do this mostly for novelty purposes, but I also think it could be leveraged into either an entertaining learning device, or some sort of game or puzzle.

Since I’m a software engineer, one of the main problems I’ve had is my inability to manifest my ideas in a physical way. Thanks to some modern technology, I think I might be able to partner up with my good buddy Eric and finally make this idea into a reality!

I’m sure some of the things I’m trying to figure out are solved problems, but I want to try and figure it out as much as I can before researching the right way to do it hehe.

Here is some basic background knowledge relating to what I’m trying to achieve, as well as my plans for moving forward.

Logic Gate Basics

Logic gates are the basic building blocks of logic circuits and computers. You can put them together in various ways to make lots of different things happen such as adding or multiplying numbers together, or comparing two numbers to see which is bigger.

The basic logic gates themselves take in either 1 or 2 inputs, and give one output. The inputs and outputs are binary where each one is either a one or a zero.

The 3 basic textbook logic gates are AND, OR and NOT.

AND: this takes in two inputs and gives one output. If both inputs are 1, it gives a 1 as output, else it gives a 0 as output.

0 AND 0 = 0
0 AND 1 = 0
1 AND 0 = 0
1 AND 1 = 1

OR: this takes in two inputs and gives one output. If either input is 1, it gives a 1 as output. If the inputs are both 0, it gives a 0 as output.

0 OR 0 = 0
0 OR 1 = 1
1 OR 0 = 1
1 OR 1 = 1

NOT: this takes in one input and gives one output. If you give it a 1, it gives out a 0. If you give it a 0, it gives out a 1.

NOT 0 = 1
NOT 1 = 0

If you are able to do these 3 things, and combine them together arbitrarily, that is considered “Turing Complete”. That basically means it’s capable of doing anything a computer can do, but check out this wikipedia page for more information http://en.wikipedia.org/wiki/Turing_completeness.

Mechanical Logic Gates

To make mechanical versions of these things I was thinking of having each logic gate a self contained object that had sticks for input and output. If a stick was pushed “up” it would be a 1, if a stick wasn’t pushed up, it would be a 0.

I then set out to make a wooden prototype of each gate to make sure I had the basic designs worked out.

NOT

To make a NOT gate, you would have a box with an input stick going into it and an output stick coming out of it. If you pushed the input stick up, the output stick would go down.

If you pulled the input stick back down, the output stick would go up again.

I made one of these out of wood by making a box with two sticks going into it, that inside were attached with a cross peice that could rotate but not move.

This way, when you push up the input stick, the crosspeice rotates and the output stick goes down. When you pull down the input stick, the reverse happens.

It worked fairly decently.

OR

To make an OR gate, I made a box with two input sticks going in and one output stick coming out. I had the output stick attached to a cross peice that rested above the input sticks. This way, when you pushed either input stick up, the output stick would be pushed up.

One thing I didn’t like about this design was that it relied on gravity to reset the output after changing the input pins. I’d prefer the mechanical logic gates made as few assumptions about their environment as possible so that there are less points of failure. That way, the mechanical computer would work in space, or on it’s side, etc. A spring could be used, but springs wear out and that’s just another moving part that can fail.

It wasn’t 100% ideal but it worked “ok”.

AND

To make an AND gate, if you push up one input pin, nothing should happen to the output pin and it should remain at zero. If you push up both input pins, the output pin should then push up.

The best way i could think of how to do this was to have the output pin rest on a slack string. The string should be slack just enough such that if you push up one input pin, it takes the slack out of the string and rests against the bottom of the output pin. If you push up the second pin, it should make the output pin raise up to the full output pin level.

Unfortunately I never got this working correctly – I’m really bad at making things, I told you! haha.

I wasn’t a fan of the string since it was a finicky, error prone part of the design that could easily fail.

Also, you would need to attach the string ends to the side of the box or something to make sure that as you raised both pins, you didn’t just re-introduce the slack in the string between the sticks. It sounds in my head like it would work, but again, unfortunately I’m a SOFTWARE engineer and this is not really my forte so I’m not sure hehe.

Connecting Logic Gates, Metrics and Physical Requirements

So, assuming the mechanical logic gates actually work like they should, there are still some problems to solve, as well as some in general design requirements about how these gates need to work.

1. Gate pins have to be able to connect to eachother somehow. An input pin needs to be able to connect to an output pin in a rigid way such that if one moves, the other one moves as well.  It would be nice if there was a way for the peices to easily snap together and easily be taken apart again. During normal use they should stay snapped though obviously!
2. The difference between zero and one needs to be standardized. For instance, the difference between a pin being pushed out and pushed in could be one inch, and all logic gates and all logic gate configurations need to obey this measurement in both their input and output pins.  Some metric needs to be decided on when prototypes start getting designed.
3. The different logic gates should be the same dimensions (or compatible dimensions) so that you dont end up with a situation where you are trying to connect two logic gates together but can’t because there is a gap between them.  We’ll have to figure out the metrics when prototypes start getting designed.
4. If i have a gate which takes two inputs, changing one input value should not change the other input value. For example, if in the OR gate, the input pins were rigidly attached to the cross bar, pushing up a pin on either side would cause the other input pin to also change from 0 to 1.  Gates will have to be designed so that this is true.
5. It would be nice if the gates were set up such that you could see their internals working. This is just for fun to be able to see the computations at work.  We’ll see if we can make this happen when making prototypes.
6. The gates should be as simple as possible, and manufacturing should be simple. each gate should be made of similar basic building blocks with as simple design and as few moving parts as possible.  We’ll have to keep the design simple when making prototypes. I want to stay away from strings, springs, wheels etc. Nothing that wears out quickly or has any significant fail rate.
7. When you push a pin up that results in a cascading change across several other gates, friction / resistance shouldn’t be so hard that it gets unreasonably hard to push a pin up, or cause damage to the gates themselves when they are used reasonably.  Hopefully this will be true if we use a light but strong material.
8. It would be nice if gates liked to rest at whatever setting they were at (1 or 0), as opposed to easily sliding between values (0.8 for instance), or some gates having a tendancy to want to rest at zero instead of one.  I’m not sure if we are good enough engineers to make this happen haha… we may just have to say that these mechanical gates have to operate on their sides or something unfortunately…We’ll have to see!
9. Changing the input pins should be action enough to change the output pins of each gate. What i mean is that if you had one of your OR inputs as a 1, making it so the output is 1, when you pull down the input pin back to 0, the output pin should also move back to 0, and not require any extra action, such as pushing down the output pin manually.  Just something to be thinking about in the design.

NOR and NAND Logic Gates

There are actually a couple other ways to make logic circuits besides using AND, OR and NOT.

There is something called a “Universal Gate”, which is just a logic gate that can be combined with itself in various ways to make the functionality of AND, OR and NOT (and thus others such as XOR, XNOR, and everything else).

The two universal gates are NOR and NAND. Here are the truth tables:

0 NOR 0 = 1
0 NOR 1 = 0
1 NOR 0 = 0
1 NOR 1 = 0

0 NAND 0 = 1
0 NAND 1 = 1
1 NAND 0 = 1
1 NAND 1 = 0

For more information on how these gates are universal, check out these Wikipedia pages!

http://en.wikipedia.org/wiki/NOR_logic

http://en.wikipedia.org/wiki/NAND_logic

While figuring out how to make a NOR or NAND mechanically would solve my problem of not having a working design for the AND gate, it goes against one of my design goals, which is to be able to give someone AND, OR and NOT building blocks to put together and so learn logic circuits physically the same way they are shown in text books and online.

One possibility would be to take NANDs or NORs and put them together to make AND, OR and NOT gates that came pre-assembled, but that goes against my design goal of keeping things as simple as possible with as few moving parts as possible. It would also add needless visual complexity to the computations, which might LOOK NEAT, but would just add confusion to what was going on.

I haven’t yet decided on a solution to these issues.

Manufacture

For manufacturing, I wanted to work with my friend Eric to get some 3d models made up and get them printed up into 3d objects by http://www.shapeways.com/.

I’m sure it’ll take multiple iterations to get it right, but this seems like a pretty nice solution.

After we have proven our idea on a small scale, maybe in the future we’ll be able to do something like get a kick starter project together to make some kind of mechanical logic gate set you can buy, or some sort of board or puzzle game.

Next Steps

For the next step, I want to think about our logic gate options a bit more, talk to Eric about them, and try and get some prototypes modeled and printed out.

I’ll post an update when we make some progress (: