Single Board Relay Computer, Bare Board DIY Electronics Kit

Description

The Single Board Relay Computer (or Relay Trainer) is a computer where the CPU is made out of electromechanical relays (like Konrad Zuse's Z3 from 1941), but with memory and front panel interface built from modern semiconductors (like the 1970s and 80s single board "trainer" computers, such as the KIM-1).
This absolutely unique computer clicks when it runs!  You will experience computer programming the same way as the early computer pioneers.  This is a great way to learn machine code and assembly language.
The CPU is composed of 83 DPDT DIP relays.  Careful circuit design and architecture keep the relay count low enough to fit the computer on a motherboard sized printed circuit board.  A microcontroller is used to implement the front panel and memory, but the CPU really is all relays.
A video of the computer is available
here
.
Also look for the project website, search for "Relay Trainer".
Features:
Single cycle 8-bit parallel instruction design operates at 10 Hz / 10 instructions per second
The CPU is unlocked and might run up to 20 Hz- a knob is provided to control the speed :-)
Computer has four output bits (relay contacts) and four input bits (contact closures) which allow you to easily interface with buzzers, lights, switches and buttons
The CPU supports operations similar to an 8-bit microprocessor including arithmetic, logic, shift, a complete set of conditional branches, subroutines and indexing.  The instruction set is just as easy to use as that of any 8-bit microprocessor.
LEDs are provided to show the state of the CPU, including PC, current instruction, memory operands and flags
A 7-segment LED hexadecimal display and keypad allow easy editing and monitoring of the memory as well as single stepping
A serial port provides access to a built-in command line assembler / debugger.
The design is open source
This is a bare board version of this computer.  You must purchase the parts and assemble the computer yourself.  The BOM (Bill of Materials), schematics and assembly instructions are available on the project page.  Please search for "relaysbc", click on "Project page" then click on "Files".  [Sorry, eBay complains when I use a direct link].
Your purchase includes:
Bare board
Programmed microcontrollers
The board is all through-hole for easy assembly.  Even though the relays need only 8 pins, the board allows you to use standard 16-pin DIP IC sockets for them.  Although they will add to your cost, I highly recommend that you use sockets to allow the relays to be easily replaced.
13 of the relays must be matched with holding resistors and it is recommended that you use relays as specified in the BOM (otherwise you may have to find different holding resistor values).  The remaining 70 may use any standard 12V DPDT DIP relay.  You may be able to save money by buying surplus ones.
The components will cost you ~0 from Digikey and Mouser (excluding shipping).  The top most expensive items include the 83 relays (6), 24 tactile switches (.43), 102 16-pin DIP sockets (.22) and the 4 SN754410Es (.08).
You will also need a 12V, 1.5 A minimum power supply using 5.5 mm x 2.1 mm barrel connector (positive on center terminal).  I recommend 2A for extra power for peripherals.
Also you will need a USB to TTL serial adapter cable with socket for the standard 6-pin header.  I recommend this one from FTDI: TTL-232R-5V