mini expander v1.1.0 Rev C. - DIY Edition

<- Click here to order the Mini Expander PCB from PCBWay!


The Aquarius Mini Expander was commonly bundled with the Aquarius, along with a pair of Aquarius Control Pads. But many later collectors, particularly outside the United States find it difficult or expensive to acquire these units. The goal of this project is to open source the device so that anyone can make their own.


I took for granted the ability to acquire and use the Mini Expander when I got back in to Aquarius in late 2018, but I soon discovered that many of my peers in the UK and EU found it difficult and expensive to acquire the unit. It commonly comes with two Aquarius Control Pads, but even those are difficult to find outside the United States. If owning a Mini Expander were merely about adding joystick ports, that would be one thing, but the Mini Expander also increases the number and complexity of audio channels on the stock Aquarius, a feature that SHOULD have been included, but was not for cost-cutting reasons. Additionally, it allows the use of BOTH a RAM expansion cartridge and a program ROM cartridge at the same time... rarely an issue on the dozen or so ROM titles available for the Aquarius, but a nice feature nonetheless.

So after I reverse-engineered the PCB for the Aquarius motherboard, I decided to do the same for the Mini Expander.

BOM (Bill of materials)

There are a couple of ways you can build the Mini Expander yourself, in a range of complexities and costs:

  1. Complete build - This will require ALL the components listed below in the BOM. This will PROBABLY result in you paying more for a Mini Expander than you could finding a used one on eBay and having it shipped to you, regardless of where in the world you live. It will also take some time and skill to complete.

  2. PCB-only build - This will result in a bare PCB that you can plug into the Aquarius, but will have exposed components, and will not be as structurally sound as the original contained in a case. Benefits are that it is possible to make a Mini Expander for less than a used one will cost. Downsides are that you'll have to get creative with sourcing your parts, and you'll have a slightly more "hacked" component in your collection.

  3. Combination build - This is more likely what many will chose to complete. Add as much or as little of the complete build as you want, potentially improving components as you have time and money to do so.

Aquarius Mini Expander v1-1-0 Rev B - Bill of Materials

GitHub Repository Link - This is the collection of source files for the project, including original Eagle board and schematics, Gerber files (to print your own PCBs), and 3D models for the enclosure to print your own.

By using files from this link, you agree to the GNU General Public License v3.0 for this project.

Component Overview

The Aquarius Mini Expander has a number of components and sub-components to it:

  • Stock Mini Expander

    • Case

      • Bottom - This will be 3D printed (available for self-printing; I will not be offering these pre-printed)

      • Top - This will be 3D printed (in development; don't hold your breath)

      • Top trap door assemblies - This can be 3D printed or simply not used (not started). Alternately, we'll try to make a "plug" that can be 3D printed and used instead of the more complicated set of doors, hinges, and springs from the stock unit.

    • PCB

      • Circuit board - The board can be ordered inexpensively from eBay, or the Gerber files can be used to order your own PCB, or you can modify the Eagle schematic and boards to create your own custom version of the PCB. This PCB also includes two small break-away PCBs for the Control Pad interface, below.

      • Ports - These components are the most critical.

        • The male DB9 ports at the back of the Mini Expander are the most expensive components, but are required to allow the Control Pads to be used on the system. A user COULD alternately use the MALE end of an Atari/Sega joystick extender cable and hard-wire it to the PCB rather than the more expensive board-mount ports. For that matter, a user could wire their Control Pads DIRECTLY to the PCB if they so choose (or add pin headers, etc.)

        • The cartridge connector ports need to be short enough to allow clearance from the Trap Doors of the Mini Expander (less than 16mm tall from PCB to top of connector). They also need to be no wider than about 62mm to allow for stock and after-market cartridge shrouds. The thickness of the connector is rarely an issue, with most being about 9.4mm. If you're not interested in using either the plastic case or the trap doors, only the width should be a concern.

        • DEVELOPERS: Rev C of the board has silk screen markings on the rear Expansion Bus header that can accept a 2x20 @ 0.1" / 2.54mm standard female header. Additionally, a 2x10 @ 0.1" / 2.54mm breakout header for the AY-3-8910 I/O ports has been added, the same type as is used on the Micro Expander.

      • Electronic components - If you already have a stock of common components such as through-hole resistors, transistors, or capacitors, you should feel free to use those, so long as they meet the specifications shown in the BOM. But there are a few components that need special mention:

        • The AY-3-8910 sound chip has several after-market (clone) versions that are widely available. For the most part, they work fine, so long as they are in the DIP-40 (DIL40-6) package. There are some notable differences in how sounds are generated on the newer clones versus the stock components, but most users probably WON'T care. Also, it is recommended that users install a SOCKET for this chip rather than soldering it directly to the PCB.

        • The L1 line filter is GENERALLY not necessary, and most users can simply bridge this with wire. But in noisier environments, or where users WON'T be using the plastic case, adding an original 33uH choke/filter could help. We recommend a ferrite bead to help limit line noise, but usually a wire closing that gap works fine.

        • The Q1 transistor was originally a 1402 NPN transistor, but those are more difficult to find. A more common 3904 NPN transistor works fine, and the v1-1-0 Rev B PCB has been designed to use those newer devices. If you want to use an original 1402, you MUST rotate them 180 degrees so that the proper pins line up. The silkscreen printing on the Rev B PCB shows the position of the 3904 transistor.

        • U1 - U7 logic chips are recommended to be socketed, but CAN be soldered directly to the PCB to save time and a little cost. Make sure to pay attention to the positioning notch of the silkscreen so that your chip is not damaged.

  • Control Pads

    • Case

      • Upper - This will be 3D printed (in development)

      • Lower - This will be 3D printed (in testing; coming soon)

      • Control disc - This will be 3D printed (in development)

    • Cable

      • DB9 cord and cable - An Atari/Sega DB9 joystick extender cable of at least 2m (6') is recommended for this. The cable will be cut at 50mm (2") from the strain relief of the MALE end, leaving most of the 2m remaining for the Control Pad cable. The MALE end can potentially be used as an alternative to the board-mounted DP9 Male connectors for the PCB part of the project. The FEMALE end will be used to plug into the Mini Expander, and the 9-wire pigtail end will be wired to the Control Pad interface board below. The cable can be left straight or wrapped around a wooden dowel (or pipe), and then heated with a heat gun and allowed to cool to create the coil (if desired).

      • Control Pad interface board - This is where the cable pigtail attaches to the control pad body, and allows the mylar "PCB" (below) to plug in. Two of these Control Pad PCBs are included as break-aways on the Mini Expander v1-1-0 Rev B PCB. It also requires a difficult-to-source flex-cable connector (finding a source).

      • Flexible mylar "PCB" with conductive ink - This is one of the most difficult components to recreate from stock. Creation requires use of a laser printer, a laser cutter, and a vinyl cutter (such as a Cricut or Silhouette). As a result, there will LIKELY be two options to recreate this:

        • A set of drawings and instructions on how to use the laser printer, laser cutter, and vinyl cutter to create an exact replica of the original (and possibly a pre-built one that will be made available on eBay). This is complex, but recreates exactly the feel and function of the original. (In testing)

        • A PCB-replacement with micro-tact switches. This will likely require a larger, modified version of the 3d printed case, but will make a more modern, rugged version of the control pads. (In development)

1. Overview

We discuss the Mini Expander, what it does, and talk about its various components and how we will try to duplicate them.

2. soldering and testing the pcb

We go through the build process of populating the Mini Expander PCB and then test it.


  • Even the TE Connectivity edge connectors mentioned in the BOM will need the bottom corner tabs removed for the connector to sit low enough to clear the "trap doors". This will cause the pins to stick FURTHER through the PCB, and might require that they be trimmed a bit on the bottom of the board.

  • YES, a plain WIRE can be used in place of the choke/ferrite bead.

  • To reiterate, the 3904 transistor on the BOM should be soldered AS SHOWN on the PCB silkscreen/printing. You only need to spin the transistor around if you're using the old 1402 transistor, as it's pins are flipped 180 degrees. Recommendation: just use the 3904.

  • You do NOT have to solder the RAM or ROM pads for the system to work, as-designed.

Coming soon