When I was looking to install some lower RAM memory boards in my newly acquired ZX Spectrums I went looking for an open design, one I could make up myself. I was surprised I couldn't find one. They're easy enough to buy, but not to make yourself. There are no open source designs anywhere on the internet for this device. I decided to fix that.

Starting very much from scratch, I created a schematic in KiCAD6 which turned out like this:

It's a beginner's project really, not that complicated. Which is handy because I was very much a beginner when I started it. The gist is that a modern 32K SRAM is used together with a flip flop latch and an inverter. There's a small timer circuit to slow down a signal which comes in from the Spectrum. That's pretty much it.

I created a PCB board, which looks like this:

Once the board is returned from the fabrication company and the components soldered on, it looks like this:

The pins need to be cut to exactly 4.2mm in order for them to plug into the sockets on the Spectrum's PCB while still being low enough for the keyboard to fit over the top. I made a 3D printed jig to aid in the grinding of the pins:

Once it's plugged in to the Spectrum's 4116 memory sockets it looks like this:

The Spectrum keyboard fits too!

All design and source files, the 3D print files and assembly instructions are on my Github page. There's a more in depth commentary and discussion on Spectrum Computing.