Relay Logic Modules

If you love retrocomputing and considered building digital circuits with electromechanical relays then these kit modules are for you! These 4-bit relay building modules are similar to CD4000 series integrated circuits and could be used to build more complex relay based digital circuits all the way up to a full blown computer. The relay modules offered by Catahoula Technologies pull back the curtain on the inner workings of silicon chips by replacing the logic elements with electromechanical relays that you can see and hear being turned on or off. They are great for a wide range of educational purposes to teach or learn digital logic and binary arithmetic.

All relay modules are standardized on Omron G2R-2-12VDC DPDT relays with a clear case for visible switch action. The modules are designed to operate on 4-bit nibbles and have the ability to be cascaded for larger bit widths. There are two different ways to design relay logic; the first way is to have CMOS style '0' = ground and '1' = 12VDC as used in Basic Relay Computer, and the second way is to float the zero in which '0' = no connection and '1' = 12VDC as used in Relay Computer Two. This single fundamental design decision impacts how all the relays are used and connected together to implement logic functions. All the modules offered here are designed using CMOS style and more closely resemble how actual silicon is designed, so all modules generally require all of their inputs to be connected to ground for '0' or 12VDC for '1' in steady state to function correctly. All I/O on the modules have LED indicators.

You can do a lot of relay based logic circuits with these modules ranging from a simple introductory level to functional clocks or even a computer! Learn more here at Relay Logic Projects.

There are a series of modules available to implement digital logic or storage functionality available for purchase below. While all modules are standardized on 4-bits, the modules have the flexibility of being populated with fewer relays to implement widths less than 4 bits to optimize on relay count if desired.

NOTICE TO CUSTOMERS (MAY 2021): Complete kits with all components are not available yet as supply chain logistics need to be worked out especially with industry wide electronic component shortages, but presently G2R-2-12VDC relays can be found on Ebay for about $2 a piece. I do not recommend purchasing relays from major electronics component distributors because of their price markups.


RL4008 - 4-bit binary full adder

RL4008 is a 4-bit binary full adder with a ripple carry implemented with 2 DPDT relays per bit for a total of 8 DPDT relays. The module can be cascaded for larger bit widths by connecting the carry between modules.

Datasheet: RL4008_ds.pdf
Schematic: RL4008_sch.pdf

PCB only: $16.00
Status: IN STOCK

Complete Kit: TBD
Status: COMING SOON


RL4067 - 4-bit to 16-channel mux/demux

RL4067 is a 4-bit binary to 16-channel multiplexer or demultiplexer implemented with a total of 8 DPDT relays.

Datasheet: RL4067_ds.pdf
Schematic: RL4067_sch.pdf

PCB only: $16.00
Status: IN STOCK

Complete Kit: TBD
Status: COMING SOON


RL4079 - Quad 2-input logic gate

RL4079 is a quad 2-input logic gate implemented with 1 relay per gate for a total of 4 DPDT relays. The module could be populated with fewer relays to implement less than 4 gates if desired. Each logic gate on the board can be individually configured to perform XOR (CD4070), OR (CD4071), AND (CD4081), or NOT (CD4049).

Datasheet: TBD
Schematic: TBD

PCB only: TBD
Status: COMING SOON

Complete Kit: TBD
Status: COMING SOON


RL4100 - 6 switch control panel

RL4100 is a 6 switch control panel designed for convenience of connecting switches to many relay modules. The board contains 4-bit data switches and 2 additional switches to control other functions if needed.

Datasheet: RL4100_ds.pdf
Schematic: RL4100_sch.pdf

PCB only: $5.00
Status: COMING SOON - August 2021

Complete Kit: TBD
Status: COMING SOON


RL4108 - 4-bit binary incrementer

RL4108 is a 4-bit binary half adder with ripple carry implemented with 1 DPDT relay per bit for a total of 4 DPDT relays. This module is typically used to increment a 4-bit number and could be cascaded to produce wider incrementers.

Datasheet: RL4108_ds.pdf
Schematic: RL4108_sch.pdf

PCB only: $9.00
Status: IN STOCK

Complete Kit: TBD
Status: COMING SOON


RL4175 - 4-bit D-type flip-flop register

RL4175 is a 4-bit D-type flip-flop register implemented with a total of 6 DPDT relays. The design relies on "sampling" capacitors to capture the voltage values at the input and then transfers the capacitor charge to the storage relay coils to update their states on the rising edge of a clock. The storage relays take advantage of the fact that relays require less holding current after a switch occurs. The capacitors are sized sufficiently to hold enough charge to force the coil to switch and then a holding resistor is used to maintain the coil current in it's current state. With this approach, the register is able to achieve D-type flip-flop behavior with a minimum amount of relays. This design is more reliable and a significant improvement over the early (and not-recommended) design documented in Basic Relay Computer.

Datasheet: RL4175_ds.pdf
Schematic: RL4175_sch.pdf

PCB only: $16.00
Status: IN STOCK

Complete Kit: TBD
Status: COMING SOON


RL4519 - 4-bit 2-input multiplexer

RL4519 is a 4-bit multiplexer implemented with a total of 2 DPDT relays. It is used to select between two 4-bit inputs to produce a 4-bit output.

Datasheet: RL4519_ds.pdf
Schematic: RL4519_sch.pdf

PCB only: $6.00
Status: IN STOCK

Complete Kit: TBD
Status: COMING SOON


Other accessories

The following accessories are not offered for sale here, but are suggested to help complete your project assembly with these relay modules.

Suggested PCB mounting hardware:

Power supply and wires (Ebay):