Activation may be from either a high or low input, with the high or low offset (pullup/down) in one of three ways;
- Input held high by external resisted 5V pullup (for grounding switches)
- Input held high by internal uC pullup (for grounding switches)
- Input held low by resisted grounding (for high switches)
Check the Launch Control page
in the Wiki, and the options in TunerStudio for concept of the action. Also review the schematics for your specific board for options that may guide your choices.
In your case with a 12V signal, and input to the uC limited to 5V, a voltage reduction to the LC input would be required. The board would ground the floating
input through a resistor to hold ground until switch activation. The switch would provide a 5V signal to the input to raise input voltage above 3V to a maximum of 5V to activate the function.
An example that includes these options is the UA4C Launch input, which options all three modes above. In this example, the jumper to Ground would be used (solder-jumping pads 2&3), and the positive voltage input from your switch would pull it up for activation going high
. In this example the pulldown is through a 2k7 resistor, so a 6800 input resistor would pull the input voltage at the uC pin to about 3.8V (@13.5V) to 4.3V (@15V) as a voltage divider circuit. There are other options. Hope that helps.
edited to add pulldown voltage divider example
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