The Design And Fabrication Of A Digital Clock

boarddesign

This clock is the first thing that [Kevin] ever made, way back before the Arduinofication of making, and long before the open hardware community exploded, and before the advent of cheap, custom PCBs. It’s an elegant design, with six seven-segment displays, a time base derived from line frequency, controlled entirely by 74-series logic chips. There was only one problem with it: it kinda sucked. Every so often, noise would become a factor and the time would be displayed as 97:30. The project was thrown in the back of the closet, a few revisions were completed, and 13 years later, [Kevin] wanted to fix his first clock.

The redesign used the same 1Hz timebase to control the circuitry, but now the timebase is controlled by a DS3231 RTC with an ATtiny85. The bridge rectifier was thrown out in favor of a much simpler 7805 regulator, and a new board was designed and sent off to OSHPark. Oh, how times have changed.

With the new circuitry, [Kevin] decided to construct a new case. The beautiful Hammond-esque enclosure was replaced with the latest and greatest of DIY case material – laser cut acrylic. Before, [Kevin] would put a jumper on the 1Hz timebase derived from the line frequency to set the clock – a task that makes plugging a clock in exactly at midnight a much simpler solution. Now, the clock has buttons to set the hours and minutes. Much improved, but still an amazing look at how far DIY electronics have come in a little over a decade.

 

O Christmas tree of digital logic

tree

[Chris] over at PyroElectro is getting into the swing of the holidays with a LED Christmas tree build. Unlike the other electrical Christmas trees we’ve seen this holiday season, [Chris] designed his tree entirely with digital logic – no microcontrollers included.

The tree [Chris] constructed on a piece of perf board is a beautiful spiral arrangement of 64 green LEDs.While we’re sure getting all the LEDs soldered to the right height, [Chris] makes it look so easy to create 3D structures with circuits.

The LEDs are driven with a set of eight shift registers, themselves clocked by either a predictable 555 timer chip or a pseudo-random pattern generated with a circuit built from a few hex inverters. By setting the tree to the sequential mode, a pair of lights travel slowly down the spiral of the Christmas tree. If set to random mode, an random number of LEDs light up and walk down the array of LEDs.

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Homebrew FPGAs

Homebrew CPUs made out of logic chips are nothing new, but a homebrew FPGA is another matter entirely. [Joshua] sent in a project he whipped up where he made a single logic cell FPGA.

Despite how complicated and intimidating they are in practice, FPGAs are really very simple. They’re made of thousands of logic blocks capable of transmuting into AND, OR, NAND, and XOR logic gates. These logic blocks are all tied together, and with a somewhat complex hardware design language are capable of becoming a CPU, a micocontroller, or even a video card. Basically, programming a microcontroller tells a chip what to do, while programming an FPGA tells the chip what to be.

To build his single logic block FPGA, [Joshua] used a four-bit multiplexer to hard wire a truth table out of a 74HC174 D-type flip-flop. A bit of Arduino code changes the state of the pins connected to the multiplexer allows for any combination of TRUE and FALSE to be calculated for AND, NAND or XOR logic functions.

Yes, it’s only a single logic block for an FPGA, and if this build were expanded to even a few hundred cells it would be gargantuan. Still, there’s no better way to learn the ins and outs of abstract hardware, so we’ll gladly tip our hat to [Joshua] and his homebrew FPGA.

Building a 4-bit TTL computer

When [GG] was 12 years old, he was introduced to BugBooks, the wonderful ‘introduction to digital design’ books from the early 1970s. It has always been a dream of [GG] to build the TTL computer featured in the BugBooks, and now that he has the necessary time and money available to him, the Apollo181 has become a reality.

[GG]‘s computer is built around a 74181 ALU, an exceptionally old-school chip that provides the core of a computer in a neat 24-pin chip. With a 256-byte RAM and a few additional logic chips, [GG]‘s computer is an exceptional piece of engineering able to perform 625,000 instructions per second when clocked at 2.5 MHz.

This isn’t [GG]‘s first homebrew computer build; last year we saw his incredible Z80 minicomputer. Now we can’t wait to see what’s on tap for next year. After the break, you can check out [GG] loading in operands and operators into his computer and letting the Apollo181 churn away on its program.

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Halloween Hacks: Flickering Jack-o’-lantern

The dollar store is always a great place to find some weird stuff, so when [jethomson] found a flickering Jack-o’-lantern, he thought it would make a great project for the 74xx logic competition.

Instead of using the flickering incandescent lightbulb that came with the blinking pumpkin, [jethomson] decided to rebuild a blinking circuit around a 74HC14 Hex inverting Schmitt trigger IC. The chip was used as a relaxation oscillator by adding a resistor and cap from the input to the ground. After a bit of component selection and some calculation, he had a red and blue LED blinking at 2,6,9, and 15 Hz.

The result is a seemingly random pattern of light that looks like a ghostly blue after image of the handheld Jack-o’-lantern. While it may not be one of the most complex builds for the 74xx competition, it gets points in our book for originality.

Although [jethomson] says his camera doesn’t pick up his project very well, he did post a video of the Jack-o’-Lantern in action. Check it out after the break.

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Over engineering a game of Lights Out

[Bertho] might have outdone himself this time. He built a Lights Out clone for the 7400 logic competition.

Lights Out is an electronic toy sold by Tiger in the mid 90s. The goal of the game is to turn a 5 by 5 grid of light up buttons off. There’s a catch, though – pressing a button toggles the state of the four surrounding buttons. Check out this Flash game that’s faithful to the original.

[Bertho] read a few Lights Out fan pages and set out to design the circuit. Most of the build is made up of shift registers: the ‘game state’ is saved in five 74hc164 shift registers and a 4557 programmable register. The board is set with a random number generator that toggles bits in the game register until a solvable puzzle is found. A truly spectacular build.

For the light-up buttons themselves, [Bertho] found an old Hack A Day post that describes putting tact switches underneath a LED. The project was put into a nice Plexiglas enclosure after 15 hours of soldering. [Bertho] was kind enough to put a video up of the game in action. Check it out after the break.

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A masochistic video card

Have a penchant for pain? Why not destroy your fingertips with a wire-wrap tool building a video card made entirely out of discrete components.

When [Chris] decided to build an entry for the Dangerous Prototypes 7400 logic competition he already had his hands full. The 74xx chips he had on hand had a maximum clock frequency of 25MHz, but the VGA pixel clock runs at 40MHz. dividing the H sync timing by 4 means the maximum speed required of the video card is only 10MHz, albeit with a reduced resolution.

The video card was constructed on perfboard with wirewrap sockets. An 8-bit DAC was included, allowing the card to display 256 separate colors but only the three primary lines were wired up to the VGA cable. As is, the card cycles through 8 different colors in a constant loop, not bad for a pile of chips.

VGA out has been done on everything from an ARM to an ATtiny, but rarely, if ever, has VGA been done with discrete components. While this video card may not be our first choice for bitcoin mining, it’s still a very impressive build. Check out the walk through video after the break.

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