Simulate 9-pin joystick output

Question

Arduino Joystick Recorder

I have built this little setup, which is supposed to record and replay input from a classic 9-pin Joystick. I connected the 9 pin connectors to the Arduino, read the input on the left side from the Joystick, and then passed it to the right side by setting the corresponding pins to HIGH.

It does work, and I can play games on the Commodore 64, however, I noticed that with this setup current is passed to the Commodore 64, which is very likely harming the machine a lot (I stopped immediately after noticing).

So my question is: how do I need to modify this setup so that no current is going from the Arduino to the Commodore 64 anymore?

Picture of the setup:

Setup

Answer 1

I would isolate the two devices using opto-isolators.

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Turning the output ON will illuminate the infra-red LED in the opto-isolator which turns on the photo-transistor connecting the input to GND. Both devices are then completely galvanically isolated (unless of course you take power from the C64 for the Arduino).

Another option, if you don't care about galvanic isolation, is to just use a simple transistor (small signal NPN or logic level N-channel MOSFET) to make an Open Drain output:

schematic

^{simulate this circuit}

Again, set the output HIGH and the transistor will turn on connecting the C64 input to ground.

• Esshahn: Thank you (again, I guess, as you pop up with solutions for every single question I ask ;) ) Majenko. I will look into both options, that was very helpful.
• Michał Stępień: @Majenko can it be transistor 2N2222?
• Majenko: @MichałStępień Yes, that is an ideal and common transistor.