Recently [Manuel] published an article about creating logic gates from anything. He referred to a web site about ladder logic. While you can indeed create logic gates using switching logic (i.e. two switches in series is an AND gate, two switches in parallel is an OR gate), this is not the only way. If you are connecting large circuits, there are some advantages to using conventional modules. Many discrete switching element based computers work like this: you have a PCB with a certain number of basic gates (like two input NAND gates) and then the logic is how you wire them together. In this case, the SPDT relay is used as a two-input multiplexer (or multiplexer).
If you think that relaying should be relegated to the curiosity of history, you should know that there are still some applications that are better tools for the job. If you do not believe in ordinary macro relays, then work is underway to create micro relays in microcircuits. Even without using relays to achieve this, some FPGAs use multiplexer-based internal logic. Therefore, it is worth delving into the past and seeing how a simple switch between two connections can make a computer.
How to convert a 2-input multiplexer to an arbitrary gate? It’s easy if you pay attention to the banner image. (or try it interactively). The multiplexer symbol shows inputs on the left, outputs on the right, and selected inputs on the bottom. If zero is selected, input “0″ will become the output. If 1 is selected, input “1″ is routed to the output.
Obviously, a relay with two poles can work as two valves (the inputs at the bottom of both valves must be the same). You can also define a buffer (by swapping the inputs to a NOT gate), or A OR NOT B and A AND NOT B.
Relays can also be used for operations such as “wired OR”. For example, suppose you have ten AND gates made as shown above. If you want OR outputs, just connect the output wires. The operation of any one (or more) logic gates AND leads to a high level of the output signal. Alternatively, you can leave the land at 1 and float at the maximum. This has the advantage of working better with ICs and other circuits that draw more current than they give out. The relay coil is always connected to a positive power supply and must be grounded to complete the circuit.
You can also use other tricks. Diodes can perform some simple logic functions, although this might be considered cheating if you’re trying to build a real relay computer. The resistor can turn a normal relay into a latching relay, just like an auxiliary contact. If you set two logic levels to the actual voltages, you can make a joke of powering both sides of the coil. This makes for a great XOR scheme – think about it. If you don’t mind using two relays, it’s easier to create an XOR. Many modern demonstration relay computers use semiconductors for memory and control circuitry.
This is not only from the field of theory. Many relay computing devices were built in the last century. There are also a few modern examples, although they are mainly intended for demonstration and not for practical devices. For example, there is a nifty 8-bit computer that only uses 83 relays. Watch it in the accompanying video. But to be honest, semiconductors are used for the memory and the front panel. However, even if you don’t want to build a computer yourself, the written architecture is instructive. You can watch a video of him in action below.
[Paul] has a project on Hackaday.io that forgoes using diodes for logic and has up to 32 bits of memory. To save on relays, he used a 1-bit ALU. There are many others, including [Simon Winder's] impressive build for calculating the square root of a phone dial (see video below).
We’ve covered a few other interesting relay builds in the past, including this 8-bit wonder that uses 152 relays and uses optical recognition to read their programming from paper. There is also a larger computer that even has its own online emulator.
If you’ve ever thought about building a computer with relays, this should inspire you. Remember that relays are deceptively simple: they are imperfect devices made from spring steel coils and rods. For example, sparking between contacts is bad, right? rely on. Some contact materials rely on sparking to remove corrosion. Others just get stuck and fail. Relays have many subtleties, and much of their seeming unreliability is actually just misuse. Of course, they are not as reliable as modern semiconductor devices, but a well-made relay, properly designed for its intended application, can be very reliable.
https://hackaday.io/project/11798-relay-based-projects Also thanks to [Yann Guidon] he has a lot of relays.
Thank you! I really should finish YGREC (whygee relay electric computer) https://hackaday.io/project/18757-ygrec16-ygs-16bits-relay-electric-computer For this project I created a “new”(?) logic type: CC- PBRL relay logic with pre-bias and capacitive coupling (using hysteresis as in one relay memory block) https://hackaday.io/project/14628-ambap-a-modest-bitslice-architecture-proposal/log/48940 – related ladder logic with pre-bias https://cdn.hackaday.io/images/7203251478848620093.png
But thanks to Ted Yapo, I was lured into the realm of transistor novelties… https://hackaday.io/project/19333-cbjt-logic
There are many PLCs out there, although I think the basic relays are noisy in operation. Next comes the mercury delay line.
PLCs are no longer based on relays… of course they are usually programmed for “ladder logic”, but no relays are actually involved.
This is often a transistor output, but relays are also often used. Some things need to work with more current than the transistor output can provide, or with a different voltage, etc.
The output is illogical… so I’m not sure what you mean. A long time ago, PLCs were based on ladder logic, but as I said, this was not always the case.
More modern seems to be the use of an analog SPDT switch such as the SN74LVC1G3157, which is a single SPDT solid state switch in a SOT-23-6 package. Cheaper than relays ($0.34 each), uses less power to operate and less than relays, but big enough for the average person to handle them easily.
As an electrical engineer, I have dealt with ladder logic at work. Although most of the relays in my design are used as motorized switches, the electrical isolation and fail-safe operation of electromechanical relays is a necessary component of a safety system. I can’t risk the solid state relay failing due to conduction and starting a device that should be de-energized.
I’m not sure where “most fpgas” came from. It should be a little. I will fix it later.
But now I’m playing with a bigger, less efficient, slower, communist era, click and vibration sensitive MUX2 
Thanks to eBay and Alex (Ukrainian remaining seller), http://www.ebay.com/itm/Miniature -relay- 2-8-V-control-Voltage-Box-of-72-New-/351723476566 I now have enough electromechanical parts to assemble my own 16-bit relay computer using CC-PBRL (with these relays were invented with and for these relays ))
I found this article very interesting, so I did some analysis to see how efficient it is to build all possible gates with two inputs:
a AND b => a?b:aa плюс non b => (a?b:a)?0:1 a XOR b => a?(b?0:a):ba XNOR b => a?b: ( b?a:1) a or b => a?a:ba NOR b => a?0:(b?a:1) a AND !b => b?0:a !a AND b => a 0:ba или !b => b?a:1 !a или b => a?b:1 !a => a?0:1
Obviously there are many solutions, but the above examples show the smallest number of multiplexers needed for each solution. So with one multiplexer you can do everything except NAND, NOR, XOR and XNOR (you need two of each).
You can XOR with only one relay, depending on how you connect the two coil signals as shown in the falstad link 
However with CCPBRL you need 2 relays, one to invert one input and the other to select inverted or not inverted.
In the mid-1980s, my father owned a grocery store and had a shoplifting problem. He installed security cameras, but the cameras soon overtook the monitors. He designed and built a telephone keypad-operated switch that controlled a relay to switch video. It can control which of the ten cameras the VCR is recording. He has six monitors, one of them shows the video stream to the VCR. The remaining five show one of a pair of images. If he chooses to record from camera 1, the monitors for cameras 1 and 2 will flip over to show camera 2 because camera 1 is visible on the recording monitor. As suspicious shoppers move around the store, he can keep an eye on them, documenting their activities while keeping an eye on everyone else in the store.
This switch uses a set of old telephone relays wired together and has been working flawlessly for years. I’m guessing he’s just using his usual technique after working in the phone industry after college.
Given that the relay was known (apparently independently invented by several people) by the mid-1800s, and Babbage conceived the idea of a universal computer around the same time, it’s a pity the ideas didn’t cross paths. If Babbage had considered a relay-based design, he would probably have gone further, if only for the reason that the mechanical tolerances of these components were much lower and therefore much cheaper.
I often think that fluid logic is within the capabilities of many ancient civilizations. For example, the Egyptians could use the water of the Nile as energy to build it. Imagine a pharaoh using a giant stone mainframe to process data. You must be wondering what computers would look like today.
The difference here is that in relay computing, the concept and the hardware coexist in the same place at the same time. I should also add that George Boole wrote The Laws of Thought at the same time, so indeed all the necessary elements are at hand. The Egyptians, being decent mathematicians, did not yet have such an understanding of logic.
I have designed several elevator controls based entirely on relay logic, around 200 relays. There are also companies that sell elevators with PLC or ladder logic options, with ladder logic advertised as lower repair and maintenance costs.
When I was a kid, there was an exhibit in one of the science museums that had you play tic-tac-toe against a relay computer. very cool. I think one of the telephone companies AT&T or Bell Labs is behind this.
One of the codebreaking machines used by the famous public school of ciphers and ciphers was imported from the US but built from relays: https://books.google.co.uk/books?id=M2sVDAAAQBAJ&pg=PA294&lpg=PA294#v= onepage&q&f =false First hand reports say they are telephone relays opening and closing every second for weeks, wearing out.
Glad I accidentally opened this tab, there are interesting comments here. Pharaoh had supercomputers, tic-tac-toe, relays with pre-bias, video switching, in fact, the concepts of relays and calculations were close, but did not combine …
I realize this post is a few months old, but I wanted to mention a few success stories. I used to make my own relays (for computing circuits) using a 3D printer. I think one of the advantages of relays is that the process of making them doesn’t have to be complicated.
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Post time: Jul-06-2023