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Cosmac Elf

Updated on February 25, 2014


My first computer was a COSMAC Elf. I built mine from a kit, but Popular Electronics Magazine had an article on how to build one from scratch. With modern computers having billions of bytes of memory and spectacular video displays, the idea of a computer with only 256 bytes of RAM and no video display or keyboard might seem quaint, but I learned a lot about computers, electronics, and programming from the simple little ELF. If you're jaded by modern technology and would like to try something fun, build yourself an Elf. You'll learn a lot and have some good clean fun.

The Original COSMAC Elf

The most elegant personal computer ever designed...

The original COSMAC Elf was designed by Joseph Weisbecker and was published as a 2 part construction article in the Aug. and Sept. 1976 issues of Popular Electronics magazine. At the time, a single 8 bit CPU (Central Processing Unit) chip could cost as much as $350; in contrast, the entire COSMAC Elf could be built for around $80 including the cost of the CPU. The low price came at the cost of simplicity. The Elf only had 256 bytes of RAM, input was by toggle switches, and the only "output" was a pair of TIL311 hexadecimal displays and a discreet LED (which could also be hooked to a speaker for making music). A third installment (March 1977) showed how to expand the memory, add a cassette interface for program storage, replace the TIL311's with discreet LEDs for even lower cost, and add a hex keypad. The final installment showed how to add a PIXIE graphics display to the Elf. All in all, the original COSMAC Elf was the most elegant personal computer ever designed; affordable, easy to build, and easy to program. If you want to build your own computer, you won't find a better design than the COSMAC Elf.

Building an Elf

Either original or enhanced, it's fun to build an Elf...

Building a COSMAC Elf is easy if you have a schematic and instructions. The coolest Elf is the original, published in Popular Electronics in 1977. You can find copies of the original PE Elf articles (including schematics) at Besides plans for the original Elf, the have designs for some really nice upgraded 1802 computers, lots of 1802 software and documentation, and tons of links to other 1802 COSMAC resources. If you're an 1802 or COSMAC fan, is the best resource you'll find on the net so be sure to check them out...

Elf II by Netronics

An expandable COSMAC Elf

Shortly after the original COSMAC Elf construction articles were published, Netronics R&D released a kit for a personal computer based on the Elf design. The basic circuitry was the same - even down to the TIL-311 Hex displays - but the Elf II substituted a hex keypad for the toggle switches of the original Elf. The Elf II added built-in provisions for expansion (up to a full 64K RAM!) and a PC board (making construction much easier) that included a 5 slot, 44-pin expansion bus for add-on cards. The Elf II sold for $99.95, an incredible bargain considering the built in expansion capabilities and the time saved by having a PC board.

Quest Super Elf

A more refined version of the COSMAC Elf

The Super Elf was a more refined version of the COSMAC Elf made by Quest Electronics. Quest added display circuitry to show processor processor status, a more sophisticated 24 key keypad, full 16-bit address display, and a ROM monitor (32 bytes!). The Super Elf only had 1 44 pin expansion slot (compared to 5 on the Elf II), but the Super Expansion Board option gave the Super Elf something the Elf II lacked - 2 S-100 expansion slots.


The heart (or brain) of the Elf

The COSMAC Elf was designed around the RCA CDP1802 microprocessor. The 1802 is a very simple chip that had the unique feature of a "load" mode, which allows programs to be manually toggled into memory. This feature eliminated the need for permanent memory (ROM or EPROM) and made the Elf much cheaper to build than other computers of the time.

Memory and Support Chips

The COSMAC Elf may be the simplest computer ever...

Besides the CDP1802 chip, the original COSMAC Elf used 2 memory chips and just 7 support chips (you'll need to add one more if you use discreet LEDs instead of the TIL311s), making the Elf possibly the simplest computer ever designed.

TIL311 and Other Elf Displays - The most "authentic" display for your Elf...

The original Elf used TIL311 hexadecimal displays which are expensive and use quite a bit of power. Other options are to use standard 2 segment LED displays with a hex decoder chip, or if you are comfortable with binary you can use discreet LED lamps. The TIL311's have a nice vintage look to them though, so they're my preferred choice. If you decide to use them, you'll need at least 2 to display the data bus. If you want to display the address bus as well you'll need 4 more (for a total of 6). The TIL311 is out of production but they come up for sale all the time on EBay.

Soul of a New Machine - Fascinating read for computer geeks...


Please Sign my Guestbook - Got a Cosmac Elf question? Comment, suggestion, or story to share?

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      wrobin422 3 years ago

      Thanks! I started learning to program on a CDC1600 mainframe with IBM punch cards in Fortran 4 back in 1970. In 1999 I took a programming language course - a later version of Fortran! - and wrecked the curve for that class. I own an Elf II, a Super Elf, the basic STG Elf 2000 and am building a Lee Hart 1802 Membership Card. Wish Spare Time Gizmos would re-release the SGT Elf 2000 and add-on boards.

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      anonymous 5 years ago

      I aint gotta Elf, but I have the rack system with CPU, Ram and i/o cards. Havent worked on it for a while, my terminal packed up so I will have to drag out an old IBM and set it up as a terminal.

    • RandomChuck profile image

      RandomChuck 5 years ago

      Great lens. I've been interested in building rudimentary game systems, and this is sorta related. Thumbs up.

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      anonymous 6 years ago

      I built an 1802 cosmac computer system 1977/1981 and am in need of a copy of 'Super Basic' operating system as my copy is damaged and won't load. Can anyone help?


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      anonymous 6 years ago

      My first computer was a Netronics Elf II. I still have it and it still works.

    • pixelposy profile image

      pixelposy 6 years ago

      Ha I still work with Hex in my job. :) Had not heard of the Elf before though.

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      anonymous 7 years ago

      The COSMAC Elf! ~ Well, I learn something new everyday on Squidoo....