DIY Single-Board Computers

Countless technological innovations have certainly made the earliest personal  computers long obsolete. As Circuit Cellar contributors Oscar Vermeulen and Andrew Lynch note:  “Today there is no sensible use for an 8-bit, 64-KB computer with less processing power than a mobile phone. “

Nonetheless, there exists a “retrocomputing”  subculture that resurrects older computer hardware and software in DIY projects. It may be sentimental, but it can also be instructive.

In their two-part series beginning in July in Circuit Cellar, Vermeulen and Lynch focus on that strain of retrocomputing that involves designing and building your own computer system from a “bag of chips” and a circuit board.

Part 1 describes a simple single-board CP/M design that uses just one high-capacity RAM chip and is compatible with a serial or PC terminal.

Here is a homebrew N8VEM system with a single-board computer (SBC) and disk/IDE card plugged into the ECB backplane.

“It is easy to create a functional computer on a little circuit board—considering all the information now available on the Internet,” Vermeulen and Lynch say in Part 1.  “These retro machines may not have much practical use, but the learning experience can be tremendously valuable.”

Some “homebrewed” computer creations  can be “stunningly exotic,” according to Vermeulen and Lynch, but most people build simple machines.

“They use a CPU and add RAM, ROM, a serial port, and maybe an IDE interface for mass storage. And most hobbyists run either BASIC (e.g., the 1980s home computers) or use a “vintage” OS such as CP/M.

“Running CP/M, in fact, is a nice target to work toward. A lot of good software ensures your homebrew computer can do something interesting once it is built. As the predecessor of MS-DOS, CP/M also provides a familiar command-line interface. And it is simple. A few days of study are enough to port it to your circuit board.”

But some Circuit Cellar readers may want more from a retrocomputing experience than a one-off project.  In that case, there are online resources that can help, according to the authors.

“Working on your own, it can become progressively more difficult to take the next steps (i.e., building graphics subsystems or using exotic processors) or to add state-of-the-art microcontrollers to create ‘Frankenstein’ systems (i.e., blends of old and new technology that can do something useful, such as automate your home).”

Part 1 of their article introduces a solid online resource for taking retrocomputing to the next level–the N8VEM Google group, which provides a single-board CP/M design meant to engage others.

This is the N8VEM in its $20 stand-alone incarnation.

“N8VEM is not about soldering kits. It is about joining in, trying new things, and picking up skills along the way. These skills range from reading schematics to debugging a computer card that does not operate as intended. The learning curve may be steep at times, but, because the N8VEM mail group is very active, expert help is available if or when you get stuck….

“As the novelty of designing a simple single-board computer (SBC) wears off, you may prefer to focus your energy on exploring graphics systems or ways to hook up 8-bit machines on the Internet. Or, you may want to jump into systems software development and share your experiences with a few hundred others.

“Retrocomputing is not always backward-facing. Making  ‘Frankenstein’ systems by adding modern Parallax Propeller chips or FPGAs to old hardware is a nice way to gain experience in modern digital electronics, too.”

For more, check out the July issue of Circuit Cellar for Part 1 of their series. In Part 2, scheduled for publication in August,  the authors provide a technical look at the N8VEM’s logic design. It also provides a starting point for anyone interested in exploring the N8VEM’s system software and expansion hardware, according to Vermeulen and Lynch.

 

 

Retro Electronics (“Retronics”): Analog, Test, & Micrcontroller Tech

Pop quiz: What was the first microcontroller to leave the Earth? Find out the answer in Jan Buiting’s new “Retronics” webinar. Check out the video below.

The Tektronix 546B

If you read Circuit Cellar and Elektor magazines, you likely have as much passion for old-school electronics as you do for he new, cutting-edge technology you find at events such as the Embedded Systems Conference. Elektor editor Jan Buiting is well-known for his love of both new and old technology, and in his Retronics webinar series he presents some of his favorite old-school technologies.

In the video below, Jan explains how and where he found some of his retronics equipment. He also details how he fixed some of the systems and what he does with them. Examples include:

  • A Heathkit TC-2P Tube Checker that Jan found at lawn sale
  • Old audio equipment
  • A satellite TV receiver
  • An “Elektorscope” from 1977
  • 1980s-era test equipment
  • And more!

CircuitCellar.com is an Elektor International Media publication.

Simple Circuits: Turn a Tube Radio Into an MP3 Amp

Want to give your MP3 player vintage tube sound? You can with the proper circuits, an antique radio, and a little know-how. In addition to generating amazing sound, the design will be an eye catcher in your home or office.

Here I present excerpts from Bill Reeve’s article, “Repurposing Antique Radios as Tube Amplifiers,” in which he provides vintage radio resources, simple circuit diagrams, and essential part info. He also covers the topics of external audio mixing and audio switching. The article appeared in the May 2012 edition of audioXpress magazine.

Manufactured from the 1930s through the 1960s, vacuum tube radios often contain high-quality audio amplifiers at the end of their RF signal chain. You can repurpose these radios into vintage, low-power tube amplifiers—without marring them in any way or detracting from their original charm and functionality as working analog radios.

Wood-cased radios have especially good sound quality, and the battery compartments in antique “portable” radios (like the Philco 48-360 or the Zenith Transoceanics) provide perfect locations for additional circuitry. When restored properly, large furniture-style radios that were built for “high fidelity” (like the late 1930s and early 1940s Philco console radios) can fill a room with rich beautiful sound.

Simple Circuits

The simple circuits described in this article perform two functions. They mix an external line-level stereo signal (typically from an MP3 player or computer) and reference it to the radio’s circuit. They also use the radio’s on/off knob to switch this external signal to the radio’s audio amplifier.

There is not one circuit that will work for every antique radio. (Original schematics for antique tube radios are available on the web www.justradios.com). But the circuits described here can be adapted to any radio topology. All the parts can be ordered from an electronics supplier like Digi-Key, and the circuit can be soldered on a prototyping printed circuit board (such as RadioShack P/N 276-168B).

External audio mixing

Figure 1 and Figure 2 show some examples of circuit schematics that mix the line-level stereo audio signals together (almost all tube radios are monophonic), while providing galvanic isolation from high voltages within the radio. Figure 1 shows an inexpensive solution suitable for most table-top radios.

Figure 1: An inexpensive circuit for mixing an MP3 player’s stereo audio signals safely into an antique radio. None of the component values are critical. (Source: B. Reeve, AX 5/12)

These radios have relatively small speakers that are unable to reproduce deep bass, so an inexpensive audio transformer (available from on-line distributors) does the job. I picked up a bucket of Tamura TY-300PR transformers for $0.50 each at an electronics surplus store, and similar transformers are commercially available. Alternatively, the Hammond 560G shown in Figure 2 is an expensive, highquality audio transformer suitable to high-fidelity radios (like the furniture-sized Philco consoles). A less expensive (and fine-sounding) alternative is the Hammond 148A.

Figure 2: A high-fidelity circuit for mixing external stereo audio signals safely into an antique radio. (Source: B. Reeve, AX 5/12)

I use Belden 9154 twisted, shielded audio cable for wiring internal to the radio, but twisted, 24-gauge wire will work well. An 8′ long audio cable with a 3.5-mm stereo jack on each end can be cut in half to make input cables for two radios, or you can use the cord from trashed ear-buds. You can route the audio cable out the back of the chassis. Photo 1 is a photograph of a 1948 Philco portable tube radio restored and used as an MP3 player amplifier.

Photo 1: A 1948 Philco portable tube radio restored and repurposed as an MP3 amplifier. (Source: B. Reeve, AX 5/12)

Audio switching using the radio’s on/off knob

After creating the mixed, radio-referenced signal, the next step is to build a circuit that switches the voltage driving the radio’s audio amplifier between its own internal broadcast and the external audio signal.

Figure 3 illustrates this audio routing control using the radio’s existing front panel power knob. Turn the radio on, and it behaves like the old analog radio it was designed to be (after the tubes warm up). However, if you turn the radio off, then on again within a few of seconds, the external audio signal is routed to the radio’s tube amplifier and speaker.

The circuit shown in Figure 3 uses a transformer to create the low voltage used by the switching circuit. There are many alternative power transformers available, and many methods of creating a transformerless power supply. Use your favorite….

The next photos (see Photo 2a and Photo 2b) show our additional circuit mounted in the lower (battery) compartment of a Zenith Transoceanic AM/shortwave receiver. Note the new high-voltage (B+) capacitors (part of the radio’s restoration) attached to a transformer housing with blue tie wraps.

Photo 2a: The inside view of a Zenith Transoceanic AM/shortwave radio restored and augmented as an MP3 audio amplifier. b: This is an outside view of the repurposed Zenith Transoceanic AM/shortwave radio. (Source: B. Reeve, AX 5/12)

The added circuit board that performs the audio re-routing is mounting to a 0.125″ maple plywood base, using screws countersunk from underneath. The plywood is securely screwed to the inside base of the radio housing. Rubber grommets are added wherever cables pass through the radio’s steel frame.—Bill Reeve

Click here to view the entire article. The article is password protected. To access it, “ax” and the author’s last name (no spaces).

CircuitCellar.com and audioXpress are Elektor International Media publications.   

Elektor June 2012: Nixie Thermometer, PIC Programmer, AVR Software-Defined Radio, & More

Elektor’s June issue is going to be a classic. You’ll read about a wide range  of topics from a Nixie thermometer/hygrometer to a PIC programmer solution to an Intersil IMS6100 vintage dev kit. And much more!

Watch the video below, and be sure to check out the Nixie tubes at the 6:50 min mark.

Here’s a summary of what you’ll find in the issue:

  • Nixie Thermometer/Thermometer: Nixie tubes are used in a retro-looking temperature & humidity meter
  • Preamplifier 2012 (3): A discussion of the LLLL board, the switch boards and the power supply board.
  • Flexible Stepper Motor Driver: If you have concerns about connecting a stepper motor driver to your PC, consider building this one with full electrical isolation.
  • Embedded Linux made Easy (2)
  • Computer-driven Heliostat: Here’s software and some electronics to enable you to use inexpensive servos to track the sun.
  • Dual Hot-wire Anemometer
  • AVR Software Defined Radio
  • Platino, controlled by LabVIEW (2)
  • Electronics for Starters (6)
  • PIC Programmer for Emergencies
  • 2-Wire Interface for Illuminated Pushbuttons
  • Retronics: Intersil IMS6100 Vintage Dev Kit (Series Editor: Jan Buiting)

CircuitCellar.com is an Elektor International Media publication.

Weekly Elektor Wrap Up: Thermometer with Giant Display, AVR Software-Defined Radio Webinar, & More!

It’s time to review what our Elektor colleagues in The Netherlands, France, and beyond worked on and covered this week! As usual, they’ve been quite busy working in the Elektor lab, organizing webcasts, prepping for Design West, and assembling upcoming issues of Elektor magazine. The following is a wrap-up of some of the many things Elektor staffers covered and worked on this week.

Below is a video of a thermometer with a giant display.

The electromechanical display was recovered from a ’60s-era pinball machine.

The thermometer with a giant display

Using the display and some innovative programming techniques, it’s possible to build a water-temperature indicator a swimming pool. After the temperature appears on the 4″ reels, the circuit’s consumption decreases to zero. But the temperature display remains perfectly visible. You needn’t worry about batteries (dry or rechargeable), adjustments, or maintenance. (Published in Elektor issue 424, April 2012 www.elektor.com/110673)

Board for Elektor's thermometer with a giant display

On the event front, Elektor Academy and element14 have teamed up to bring you a series of exclusive webinars covering blockbuster projects from recent editions of Elektor magazine. Participation in these webinars is completely free! All you need to do is register via www.element14.com/community/events/3258. The “AVR Software-Defined Radio” webinar takes place Thursday, March 9, 2012. Click here for more information.

Elektor also reported some interesting electronics news this week. The items that will most interest Circuit Cellar readers are an Uninterruptible Power Supply in a Chip and a Python-Based Tool for Diagnosing Dead-Core Boards.

CircuitCellar.com is part of the Elektor group.