Cordwood puzzle, second edition

A modern take on an old
assembly method

Saving space back in the day. Source

The completed project

The completed project

Kit contents

In the 1950s and 1960s inventive engineers saved space by using the 'cordwood' assembly, where components were sandwiched in between two circuits boards. This construction became less useful with reduced component sizes, PCB manufacturing advancements, more compact integration, and surface mount technology, but is still a lovely piece of history. Combining both old and new, the Cordwood Puzzle is a tribute to this construction and to the engineers who came up with it.

The puzzle is to correctly assemble the circuit with the components at hand. Once completed, all LEDs light up when power is applied. When connected to a controller board each LED can be individually controlled.

Unfortunately(!), all the comprehensive assembly guides were mysteriously missing from the kits.

The Cordwood Puzzle can be connected to a controller board (like an Arduino) to control each LED

The Cordwood Puzzle can be connected to a controller board (like an Arduino) to control each LED

The kit contains

  • x2 identical PCBs (96.6x16.6 mm each)

  • x2 red 10mm LEDs 1168543

  • x2 green 10mm LEDs 1142474

  • x2 yellow 10mm LEDs 1168543

  • x6 nMOS FET, Fairchild, 2N7000

  • x1 2-pin SMD 2.54mm pitch header

  • x1 6-pin SMD 2.54mm pitch header

  • x6 10KΩ 2W resistors, 1602193

  • x6 150Ω 2W resistors, 9338101

  • x1 1uF capacitor, 1549977

  • x2 24.5 mm M3 standoffs

  • x4 8mm long M3 screws

  • A bit of 0.84mm diameter 20AWG hookup wire


Further information

The Cordwood Puzzle is an open source design, as is most of our work. You can find the design files for the hardware and packaging at our GitHub repository. You can edit the design files using our own PCB design open source software, PCBmodE.

Cordwood Puzzle was Project #3 of the Boldport Club. If you'd like to purchase a kit, become a member and add one to your order through the shop.

The PCBs were lovingly made by Eurocircuits.

Thanks to Erbsland Art for taking lovely picture of the project. Full album is here.

Finally, community contributions for this project are on our community site.



Challenge yourself by assembling the kit without further instruction. Before you do that, however, note the following: When connecting to a board, make sure that the I/Os can tolerate the voltage levels that are applied to the Cordwood! Also, make sure that you do not short power and ground!


The two circuit boards are identical, but function differently depending on how they are wired. You can mark one of them on the white silkscreen patches with a pen to distinguish between them if you like.


Start by placing the six FETs into their holes. Match the shape of the FET to the legend on the bottom side of the board. Don't worry, they are not meant to go in all the way. Solder the pins on the top side and clip them.

On to one of the boards solder both the 2-pin and 6-pin header, making sure that the tips are flush with the circuit board's edge.


Insert the LEDs to their sockets. It doesn't matter which LED colour goes where, but the long leg of the LED must go into the square hole! Make sure that the LED's base sits right against the board and solder all of them into place. Clip the leads.


Place the resistors so that they are against the bottom of one of the boards. Some holes have a gear-shaped mark next to them; that's where the brown resistors go.


Make sure that the boards are aligned such that the brown resistors go to the holes with the mark on both boards. By angling the boards carefully insert each resistor to the second board.


While doing that also insert the capacitor. It should go from ground to power, or a round hole on one board to a square hole on the other.

Line up the standoffs and screw them into place. This may require some coordination, but once done, soldering is much easier.

Aiming to have all the components centred — feel free to use some electrical tape to hold them in place — solder all the components. Clip the excess leads.


Use the wire to connect three signals. These go straight through. Only these three should be connected.


Finally, connect power and ground of the two boards. Here, square goes to square and circle to circle. Make sure that the two wires are not touching!