I'm a robotic engineer, but I am oddly limited in this case as I can't design the system or modify it much. For an expo they want to hook up real industrial style joysticks to a gaming joystick board for a 3D simulator of our machines, they have already designed the software around the gaming joysticks and assumed that dropping in these other ones would be easy, now they are out of time and gave it to me. They want me to quickly patch the industrial joysticks into the gaming joystick board. I have the analog axes figured out, but the buttons are an issue:
The buttons connect to the main IC by 6 wires, I can't look at it's code, but it seems to be using matrix multiplexing. One wire branches to 4 diodes going to 4 buttons, another has 2 diodes going to 2 buttons, the other 4 wires are returns from those buttons. They are switching at high frequency. The wires from the joystick buttons are 5V normally and go to 0V when pressed.
I've tried just putting NPN transistors across the button contacts and connect the signal to the base (with a resistor) and it seemed to be working, at least on the 4 buttons that share a lead, when I add the transistor with either of the 2 buttons on the other lead, or both for that matter, they all stop responding. I don't know why that would happen, but I need to get it working by tomorrow evening. I'll be working on it and I'll post an update if I find anything new. I have basic transistors and resistors on hand, and a few support ICs like inverting buffers.