Project Opto - en
The "Opto" project addresses the challenge of connecting wireless joysticks to the Atari MultiJoy adapter. It can also successfully serve as an interface between Atari and Sega adapters/controllers (which correctly wires power supply), it supports second button using the Joy2Bplus standard (second button or UP direction), or it is possible to use it with the 4play adapter on the Atari ST, which also lacks power on its parallel port.
Technical Description
The MultiJoy interface allows up to 16 joysticks to be connected to an 8-bit Atari computer. However, its design does not support controllers that require a power supply for operation. These are mostly wireless controllers, such as the 8BitDo M30 2.4G, the Retro Turbo 2000 Super Wireless Gamepad from Commotron, and modern controllers from Atari's + series, as well as XBox gamepads with a BlueRetro Atari adapter.
We considered how to connect such devices to the MultiJoy, which led to the creation of the "Opto" device.
Multijoy with wireless controllers
MultiJoy has no VCC support. It selects the active joystick using a multiplexer that pulls pin 8 of the port low (logic 0), simulating standard port behaviour (GND). However, when a different joystick port is selected on the MultiJoy, the power ground pin is disconnected.
Schematics
This is why galvanic isolation is required to power the joysticks - this means isolating both the power supply and the signal lines.
PCB
The power supply section uses an isolated DC/DC converter, which draws power from a USB power bank or power adapter connected via a USB-C port. The DC/DC converter requires a constant power load to function correctly, which is consumed by a LED. The converterR17;s output is connected to the joystick power pins. Switch S2 allows you to select either Atari mode (powering pin 7) or Sega mode (powering pin 5).
Signal isolation is provided by optocouplers, powered by a 3.3V regulator. The current is set to ensure reliable signal transmission without excessive power consumption.
Populated PCB
Switch S1 allows you to configure the second button on the controller to act either as a second fire button or as the UP direction, which can be useful in certain games. This function is only available when the controller is connected directly to the computer and not via MultiJoy.
Switches meaning
Gaming with Opto
When you want to play using wireless controllers together with MultiJoy, connect the Opto interface to one of the MultiJoy ports. Then plug the wireless dongle into the other side of the Opto interface, and do not forget to connect power to the USB-C port, either from a USB power adapter or a power bank. Once everything is connected and powered on, you can simply start playing and enjoy gaming with a controller that is no longer limited by a cable.
Wireless controllers dongles with Multijoy
To download
Documentation for the current version is available under a Creative Commons license -
CC-BY-SA
You will find schematics, PCB designs including a bill of materials, 3D-printable models, and a few photos here:
ProjectOpto-250728.zip