A vacuum tube computer next to a part of its schematic

This Colossal Vacuum Tube Computer Plays A Mean Game Of Pong

It doesn’t happen often that we report on new vacuum tube based computer designs. Today however, we’re pleased to introduce to you the Fast Reliable Electronic Digital Dot Computer, or Fred.Computer for short. It’s the brainchild of [Mike] who also brought us ENA, which we featured earlier.

Fred is a new design that reuses the parts that made up ENA. It has an 8-bit CPU, 16 bytes of RAM, 256 bytes of NVRAM, and runs at a clock speed of 11.3 kHz. With its 560 tubes drawing a total supply current of about 200 A it also provides a fair bit of heating to [Mike]’s study. The main logic is implemented through NOR gates, built from 6N3P dual-triode tubes sourced from Eastern Europe. These NOR gates are combined into more complex structures like latches, registers and even a complete ALU. A total of sixteen machine code instructions can be used to write programs; clever design allows Fred to perform 16, 32 or even 64-bit calculations with its 8-bit ALU.

A PCB with many reed relays
Need some RAM? There’s sixteen bytes right here.

An interesting addition is a new RAM design based on reed relays. [Mike] realised that relays are actually very similar to digital transmission gates and can therefore be used to make a simple static RAM cell. If you thought relays were too slow for RAM cells, think again: these reed relays can toggle at a mind-boggling 700 Hz, making them more than fast enough for Fred.

The main I/O device is a console that contains several pushbuttons as well as a 12 x 8 LED display. All of this makes Fred a fully-functional general-purpose computer that’s even capable of playing Pong (video, embedded below). [Mike]’s website is full of interesting detail on all aspects of vacuum tube computer design, and makes delightful reading for anyone tempted by the idea of building their own.

Can’t get enough of vacuum tube computers? Have a look at this 1-bit MC14500 implementation, marvel at this modern interpretation of an adding machine, or find out how IBM designed its logic in the 1950s.

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Mechanisms: The Reed Switch

Just about everywhere you go, there’s a reed switch nearby that’s quietly going about its work. Reed switches are so ubiquitous that you’re probably never more than a few feet away from one at any given time, especially at home or in the car. You might have them on your doors and windows as part of a burglar alarm system. They keep your washing machine from running when the lid is open, and they put your laptop to sleep when you close the lid. They know if the car has enough brake fluid and whether or not your seat belt is fastened.

Reed switches are interesting devices with a ton of domestic and industrial applications. We call them switches, but they’re also sensors. In fact, they only do the work of a switch while they can sense a magnetic field. They are capable of switching AC or DC at low and high voltages, but they don’t need electricity to work. Since they’re sealed in glass, they are impervious to dirt, dust, corrosion, temperature swings, and explosive environments. They’re cheap, they’re durable, and in low-current applications they can last for about a billion actuations.

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Sensing A Magnet With Local Sourcing

I had a small project going on–never mind exactly what–and I needed to detect a magnet. Normally, that wouldn’t be a big problem. I have a huge hoard of components and gear to the point that it is a running joke among my friends that we can be talking about building something and I will have all the parts we need. However, lately a lot of my stuff is in… let’s say storage (again, never mind exactly why) and I didn’t have anything handy that would do the job.


If I had time, there are plenty of options for detecting a magnet. Even if you ignore exotic things like SQUID (superconducting quantum interference device) there’s plenty of ways to detect a magnet. One of the oldest and the simplest is to use a reed switch. This is just a switch made with a thin piece of ferrous material. When a magnet is nearby, the thin piece of metal moves and makes or breaks the contact.

These used to be common in alarm systems to detect an open or closed door. However, a trip to Radio Shack revealed that they no longer carry things like that as–apparently–it cuts into floorspace for the cell phones.

I started to think about robbing a sensor from an old computer fan or some other consumer item with a magnetic sensor onboard. I also thought about making some graphene and rolling my own Hall effect sensor, but decided that was too much work.


I was about to give up on Radio Shack, but decided to skim through the two cabinets of parts they still carry just to get an idea of what I could and could not expect to find in the future. Then something caught my eye. They still carry a wide selection of relays. (Well, perhaps wide is too kind of a word, but they had a fair number.) It hit me that a relay is a magnetic device, it just generates its own electromagnetic field to open and close the contacts.

I picked up a small 5 V reed relay. They don’t show it online, but they do have several similar ones, so you can probably pick up something comparable at your local location. I didn’t want to get a very large relay because I figured it would take more external magnetic field to operate the contacts. You have to wonder why they have so many relays, unless they just bought a lot and are still selling out of some warehouse. Not that relays don’t have their use, but there’s plenty of better alternatives for almost any application you can think of.

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