Evelands Dot Net

A keyboard is a pretty basic circuity based on the idea of a matrix of wires. Pressing a single key on the keyboard smooshes two wires togerther, connecting a given row and column of the matrix. For instance, the scan on the left shows the two layers of the matrix for the keyboard I used, one color for each layer. The circles correspond to the keys.

In practice this matrix is usually made of two sheets of transparent plastic with wires running across. The layers are separated by a third sheet with holes where the keys will push the sheets together. When a key is pressed, the two sheets are pressed together in the spot under the key, completing the circuit. To emulate the space bar being pressed, I needed to identify what it was connected to on the keyboard I was using and connect those two wires.

To create the circuit I wanted, I first opened the keyboard and removed all the guts. It consists of a top and bottom plastic case, a small PCB and three sheets plastic with wires that form the key matrix. To identify what two pins on the PCB I needed to connect to signal the space bar I needed to trace the spot under the space bar key to the PCB. So I cheated. I scanned in the two layers of the matrix after circling the space bar points. I then opened the images in the Gimp and did a flood fill on the points I had circled. This traced the wires back to PCB.

The next step was to connect wires to the PCB on the pads I had identified in the step above. Then I needed to make sure that if I touched the wires together, I got a space bar on the computer. After soldering a wire to each pad, I used hot glue to secure them to the PCB. I then plugged it in and verified that it was still recognised as a USB keyboard and that connecting the wires generated a space bar keypress.