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So I am trying to add a button on my Game Boy that will "press" A+B+SELECT+START at the same time. I have found the test points for these buttons on the PCB and connected them to a button with the other end to ground, tested it and it worked just as I expected. But then I tried to press B by itself and all the buttons are pressed again, makes sense seeing how all the test points are now connected on one end of the button.

Do they have buttons that are 5 inputs that are not connected until the button is pressed, or is there a way easier way to do this that I am not thinking of?

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    \$\begingroup\$ You might get somewhere with diodes to each point and a common switch. \$\endgroup\$ – Chris Stratton Jul 28 '16 at 4:04
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As you have seen, if you connect all the buttons together, pressing one is like pressing them all. You can solve this with diodes:

schematic

simulate this circuit – Schematic created using CircuitLab

A, B, ... are the old buttons, and you hook your diodes directly to them or to a convenient test point. CHEAT is your new cheating button!

The diodes prevent current from flowing "back" from one point to another, while still allowing the voltage to sink low enough to trigger a logic input (roughly 0.6 volts).

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