##### Keyboard Builders' Digest / Projects

## reJESK

*T.G. Marbach* released the *reJESK*, a diodeless monoblock split keyboard with 70 keys.

**KBD.news**

Published October 4, 2024

The **reJESK**, created by *T.G. Marbach* aka *triliu*, is a Redox-inspired 70-key unibody split keyboard that uses a diode-free design based on the JESK graph.

Following on from my diode-free keyboard releases of the Heawood42 and JESK56 earlier in the year, I decided to see how far I could push the unconventional key matrix concept. The result is the reJESK, which uses 28 GPIOs for 70 keys and has good key rollover characteristics. With no diodes, and options for hotswap and direct soldering, the keyboard is straightforward and fast to self-assemble – triliu.

Going diode-free can reduce the complexity and length of time to make your own board. The *reJESK* is part of the geometric diode-free keyboard project, which is creating keyboards with great key rollover characteristics without using diodes, based on a modification of the JESK graph. The graph was found by *Exoo*, *Kolokolnikov*, *Janssen*, '*Salamon*; see their arXiv paper or final publication.

## Specs

- 70 keys
- MX, hotswap or soldered
- Pro Micro RP2040
- no diodes

## How does this work?

According to *dusan69*'s comment, a diodeless circuit translates to a undirected graph as follows: a pin becomes a node (vertex), a switch becomes an edge. As a pin is either input or output, the graph is bipartite. A regular matrix becomes a graph with girth 4, i.e. there are 4-cycle. In such a cycle, if 3 switches are pressed simultaneously, current can flow from any pin to all pins, meaning that one can't tell which switches are pressed. On the other hand, if only 2 or fewer switches in a 4-cycle are pressed, one can tell which ones are pressed by a suitable test.

The smallest cycles in the JESK graph (girth 6) are 6-cycles. If 4 or fewer switches are pressed simultaneously, one can tell which one is active.

Another explanations by the author:

When you press one switch, you connect two GPIOs, say A and B. In order to have ghosting, you have to create a chain by pressing multiple switches that connects A to B, say A to x to y to z to B. In typical grids, you might have a switch connecting R1 and C1, and so you have to be worried about connecting R1 to C2, C2 to R2, and R2 to C1, as this connects the chain from R1 to C1.

The way that the particular design of these boards is made is so that most chains that create ghosting have five hops, meaning that five keys need to be pressed to create ghosting.

However, I also set these up so that each of these chains runs along the columns twice, so to have ghosting, you would have to hit two keys in a column with one finger, and two keys in a column with a second finger.

The new board loosens these conditions a bit for the top row, as presumably these keys are the number keys, and are less likely to be simultaneously pressed with several other keys as much as the letter and thumb keys (but still, you have to have a hard-to-press chain of length four to achieve this).

## Resources

*Published on Fri 4th Oct 2024. Featured in KBD #176 (source).*

### Related

### JESK56

*T. G. Marbach*'s

*JESK56*is a diodeless 56-key ortholinear keyboard, using a single RP2040-based microcontroller thanks to some fancy math and graph theory.