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A 4 bit homebuilt computer that is really useful

Most projects like this just flash some leds...This easy to build modular constructed one makes music, drives servos, steppers, DC motors .

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This mini computer is made from TTL chips, transistors and some passives. The display is a VFD display that is commonly available and inexpensive. If you wish a standard led 7 segment display can also be used. Everything is modular. Add modules you want to build such as music, servo drives, stepper drives and motor drives as output. The output port design allows you to interface things and get them running quickly. Each module is simple and inexpensive to make.. The output port allows you to plug in all the modules and select what you want to do. You can even add a simple printer from a 5$ printer mechanism.
The photo shows all the modules I have prototyped so far and looks kind of a mess. That will all get organized very quickly.Useful for education, quick testing of motors, gearboxes and servo projects. No coding and flashing microcontrollers....No software required. Press a few keys on the keypad and press go.

These type of mini computers seem to be everywhere, many folks have built some version using transistors, TTL chips or whatever and while impressive they went to the trouble to do it, never seem to make them do interesting or useful things.

 I have been building these since the mid 70s(that was pretty much all there was then) and love working with the nearly forgotten 7400 series parts. I have built them using led displays, 7 segment displays and VFDs. People probably think I am overdoing it a bit making so many versions of these but for me it is a fun and pleasurable experience. No two are ever close to being alike, although they may share many of the same parts. This version is a hybrid and uses both TTl and transistors.

One thing I decided for this version was to make it not only a fun experience to make, and have some educational experience attached to it was to make it do some really useful things. Nowadays just about anyone can code an Arduino to run a servo or stepper and shields make this task even stupidly simple. What gets lost for most is how things really work and what is happening to make servos move and steppers step.

This little 4 bitter will take you inside the magic and allow every step of the way to be probed and prodded, measured and visualized. Since there is zero code involved, the focus is brought to the hardware itself and for some this can be a new experience.

This project will attempt to untangle some mysteries and put forth a nice little machine to experiment with.

 I use these extensively for test mechanical mechanisms like joints and 3D printed gearboxes as you can just plug them into a port and they can be selected to run. By tapping a few buttons thing go into motion so you can study them make modifications and even"life " test mechanisms by having them cycle on and off for as long as you like....no programming needed.

There are no licenses associated with this project.For now Hackaday is the sole repository for all files and data.

  • PCBs produced so far

    castvee803/22/2018 at 03:35 0 comments

    Here is the VFD display board:

    It mounts the vfd, filament dropping resistors 5 volt regulator the clocking source,and drivers electronics for the VFD. This module can be swapped for another led custom module if desired.

    This module contains a 4 bit7 segment decoder for the display, and some additional filter caps.

    This board contains 4 20 pin octal buffers.

    This is the  4 bit latch module.

    This is the register to latch interface module.

    Remember all the above modules plug into each other-so no additional wiring is needed.Male and female headers on each board allow connections.

    More coming-this page a work in progress....

  • Module 1-VFD display driver and input registers

    castvee803/21/2018 at 16:48 0 comments

    The four digit display is mounted on a pcb which contains the multiplexer circuitry. The display elements are powered by a 12 dc source, while the filament for the tube is run from a 5 volt source through a pair of dropping resistors.

    Several modules that all plug together to for a larger pcb are all connecting through headers along the top and bottom edge.

    You can recognize all the prototypes were made by OSH and they did a great job fabbing. All the chips used are very common types and I always socket everything..

    When the boards reach the bottom a pair of input cables carry bit data from the keyboard and register where it is transferred into the latches((just below the colorful ribbon cable) and transported to the processor.

    The modules and interconnect  points all provide great test points for a logic probe so it can be debugged for any problems as well as an educational experience to follow the bits of data through the system.

    The register module.


    This takes key presses from the keypad and stores them momentarily until the user decides all is correct and is ready for processing.The registers are made from simple npn transistors wired in rc configuration to hold the data while being selected and or edited. This arrangement also debounces the pushbuttons on the keypad.Once the user confirms the data line is what is desired  an enter key is pressed and the data is transferred to the latches.

    I have not sent this off to OSH due to lack of funds.

    The processor module.

    This module has four 4X4 register files (74ls670)which hold the data from the latches in order that they were entered. Each is scanned out sequentially and sent to the output as control codes. The data which consists of numbers 1-9 form the instruction set and each tells the sequencer which port gets what data at clock rate. The clock is a 555 timer which has an external control for rate of playback of the bits. The output data is buffered and can be used to flash Leds, turn things on and off as desired according to program as well as run motors, servos etc. Some numbers(such as 9) are special code data which is used for servo control range or stepper direction. These ports are not buffered and are direct access. The zero digit is used as a space in the output port sequence to pause or halt an operation for a clock cycle then allow it to resume. Even if any of this seems a bit complicated....it is brutally simple for a user.

    This module has several revisions needed for new and additional features and will be sent to OSH for a new version when funds are available.While the modules as is can be useable(I had multiples made in china) the revisions make it so much better.

  • Some part info

    castvee803/15/2018 at 21:31 0 comments

    Here is the VFD I used in this machine:

    https://www.ebay.com/itm/Lot-of-20-IVL2-7-5-IVL-VFD-DIGIT-CLOCK-DISPLAY-TUBE-GREEN-NIXIE-Tube-New-/262949520982

    There seems to be a grand surplus of these in Russia these days! I will make available other display options for the construction but these are what are installed right now. I bought 15 of these for 17$ and that included shipping. They were well packaged and arrived very quickly. They have that beautiful aqua blue characters...It is quite obvious even to the most casual observer these were meant to be used in clocks.....

    I found these cheap thermal printer mechanisms:

    http://www.goldmine-elec-products.com/prodinfo.asp?number=G21008

    Pretty easy to hook up and use as an event type printer, or with a little more effort a full blown printer....For 5$ each. Many times during the year they go on sale sometimes for 1$ each...The connector is available from Mouser at about 20 cents each. The web contains a wealth of info about these if you want them for other projects.Here is the connector it needs to be happy:

    https://www.mouser.com/ProductDetail/FCI-Amphenol/HLW30S-2C7LF/?qs=%2fha2pyFaduilzQ54bNrJAI7Y%252biJS1%2fFQKFs73dlUjT4FpOMiq4X57A%3d%3d

    Most of the TTL chips(if not all) can be bought here very cheap:

    http://www.futurlec.com/IC74LS00Series.shtml

    It takes them a while to get here to the US but well worth the wait as the prices are fantastic.

    To be continued......

  • A few of my other 4 bit machines

    castvee803/15/2018 at 21:04 0 comments

    Here is a version I made many years ago:


    SO many chips! I used a 2 line VFD for display. It contains a simple serial output port.

    Here is a slightly more compact design:


    It just used 7 segment led displays. It has very similar wiring but a bit more compacted together.

    This was the first PCB version I made.:


    I was amazed how compact this was! It took a bunch of time to get everything debugged in the wiring but well worth the effort.


    I also made this version using edgecard connector boards.

    Here is a side look:


    Even has a printer!

    This version was the one I developed a function multiport for It has a printer, a tone generator to play tunes and also could drive motors.

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Dr. Cockroach wrote 03/17/2018 at 23:12 point

My DTL discrete build is taking up my time but would like to tackle TTL like yours someday :-)

Great modular layout :-)

  Are you sure? yes | no

castvee8 wrote 03/18/2018 at 02:15 point

If you do find time let me know-I can help you out with parts-I have lots of spares and extras stuff to make these.

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Dr. Cockroach wrote 03/21/2018 at 13:30 point

Sounds great, I retire next fall so perhaps I can get into TTL then :-) I would like to work with it :-)

  Are you sure? yes | no

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