Lego Mechanical Laptop – The Weblog of Dr. Moron
Not way back I used to be a part of a dialogue about utilizing mechanical issues to
reveal computing ideas (comparable to constructing a learning Tic-Tac-Toe
game out of matchboxes). This bought my head spinning on alternative ways to
use mechanical gadgets to compute issues, whether or not or not such computation
is beneficial. I figured I had sufficient Legos mendacity round the home to make
one thing fascinating.
I began by considering of a number of the extra well-known early computer systems, comparable to
Pascal’s calculator and Babbage’s difference engine. Nonetheless, I selected
to not implement these. Maybe partially as a result of they’re mechanically
complicated, but in addition as a result of they’re lacking a number of the basic
parts we take as a right in computing. I wished one thing that had a
type of reminiscence that would maintain a state after which carry out an motion primarily based on
that state. I additionally wished one thing that I may “program” by altering the
state transition logic.
I ended up gravitating towards a intelligent toy referred to as Digi-Comp I, which was
manufactured within the 1960’s. The Digi-Comp I is an easy 3 bit machine that
is programmed by including or eradicating pegs in its facet. Because it occurs, a
firm named Minds-On Toys lately began promoting replica Digi-Comp I
kits. A rational particular person would have simply purchased and assembled one. However not
me.
I wished to make my very own system to show to myself that I may. I borrowed
lots from the Digi-Comp design and ended up with one thing that labored
pretty nicely. My laptop solely had two bits as a substitute of three, and I didn’t
have a pleasant clocking mechanism, nevertheless it was compact and operated fairly
properly. Here’s a video of it in operation.
After posting this video, I used to be inundated with requests for extra particulars on
the interior mechanisms. (Nicely, OK, I bought a few feedback.) Anyway,
this publish is me lastly getting round to offering extra element. I’ve
way back dismantled the system, and I’ve no intention of offering
step-by-step directions in any case. Nonetheless, I’m giving the excessive stage
ideas that lead me to the ultimate design. My implementation is much from
good anyway, so there may be numerous room for enchancment.
Primary Design
There are two principal parts to the pc. The primary is a reminiscence
construction that holds some variety of bits (in my case, two) to symbolize the
state of the machine. The second is a management circuit that takes as enter
the reminiscence bits and produces as output the indicators that change the state of
the reminiscence. As a result of there may be this suggestions loop between reminiscence and management
logic, you want a reasonably cautious timing mechanism in order that the output of the
management holds regular lengthy sufficient to set the suitable state of the
reminiscence. That stated, this management mechanism is principally lacking from my
laptop. As an alternative, it depends on the operator (me) performing a number of
completely different motions within the right order to advance to the subsequent state (as you
can see within the video with me flipping a number of controls).
Of those two parts, the management mechanism is pretty easy.
There are a number of methods to construct logic gates out of mechanical parts
like Legos. One of many easiest to implement and most compact designs for
gates makes use of rod logic. Reminiscence, nonetheless, is mostly extra sophisticated than
easy logic. It was the creation of this reminiscence that dominates most of
this laptop’s design.
The Flip-Flop
Reminiscence could be carried out utilizing a particular sort of digital circuit often called
a flip-flop. The flip-flop is designed to vary its state when it
receives a sign after which proceed to carry that state after the sign is
launched.
A flip-flop can be constructed out of standard gates (at least, the
electronic variety) using a feedback loop that makes a circular loop from
the output of gates back to the input of the same gate. If constructed
carefully, this feedback can hold the output state of the gate. This image
here shows an animation of a set/reset (SR) flip-flop. When the set (S)
signal is raised, the output goes positive and stays that way even after S
is lowered again. When the reset (R) is raised, the output goes negative
and stays there.
This circuit is straightforward to build out of electronic logic circuits,
but problematic when the gates are mechanical. The real challenge is in
designing this feedback loop in a way that does not lock up the mechanism.
Added to this challenge is the fact that the overall mechanical computer
introduces a second feedback loop (memory to control back to memory)
that must also be managed.
Fortunately, there is a simple way around all these problems. The approach
bypasses all these problems by building the flip-flop without any gates at
all. A simple device is shown below.
That’s right. The mechanical flip-flop is… a stick. I’ve added some
labels to define the SR implementation and a post to clearly mark the
position, but ultimately the flip-flop is just a stick you push back and
forth. Unlike electrons, sticks (and really most physical things) tend to
stay put, so you can represent state by where they are sitting.
Of course, to use this stick flip-flop in a larger circuit we’ll need to
place it in a holder. And to support the circuit from control back to
flip-flop, we need a gating mechanism that hold the set or reset signal
until the rest of the control logic gets out of the way (literally) and
then activate it with a clock signal. Both of these are demonstrated
below.
The set and reset signals are now bars that block a second pair of “clock”
bars from moving the stick. As long as both set and reset are blocking,
the flip-flop state will remain in place. Set or reset is activated by
moving that bar out of the way, and the next time you push the clock the
state of the flip-flop will change.
The actual flip-flops implemented in my Lego mechanical computer are a bit
different. They are designed to activate the set or reset by pushing in
rather than pulling out, but the basic idea is the same.
Control Circuit
Notice in the image above that this design for a mechanical flip flop
places the set and reset bars perpendicular to the stick representing the
flip flop state. As it happens, this is an ideal position for the
set/reset signals to be in.
The mechanical computer has yet another bar for each set and reset that
slides laterally in and out to activate the set or reset. Because of the
orientation of the set and reset bars, it is simple to add posts to the
flip flop state stick that will block the set or reset from being
activated. These posts are moved in and out of the way based on the
position of the stick (i.e. the state of the flip flop). The operation of
this mechanism is clearly shown in the video.
Multiple flip flop sticks can be stacked to provide posts for each bit of
state. Thus, the control for each set and reset is an AND operation for
the state of each bit. For an arbitrary number of bits, this logic is not
powerful enough to capture every possible state change, but for just two
bits (the number in my computer), it just so happens to be enough to
capture all possible state changes.
The nice thing about this control logic is that it is very compact giving
the overall mechanical computer a surprisingly small form factor. Another
convenient feature is that the computer can be reprogrammed with minimal
reconstruction. It is just a matter of moving the posts in the flip flop
sticks, which are accessible from the front of the device.
The Final Word
The point of this device, if there is one, is to separate out the concept
and theory of computation from the digital electronic devices we most
commonly associate with computing. There is nothing inherently electronic
about computers. It just so happens that it is much easier to design
computing devices using components driven from electricity. At least, that
is the state of the known inventions so far.
Links
Here is a collection of links to other projects concerning famous
mechanical computers or other Lego computer projects (most far more
impressive than mine).