I want to be able to detect when a 8V signal goes low and send a signal to a raspberry pi over GPIO. A java app will be listening on the GPIO pins for any changes.
I would check if the voltage drops via the pi directly, but I need to check 10 signals and do not want the pi (and 3v LDO voltage regulators) absorbing 3v per signal constantly while dissipating the other 5v. Dissipating 50v and absorbing another 30v will probably produce enough heat to melt the pi without some major cooling.
I have seen examples using optocouplers or NPN transistors, but need to get the output signal to within the pi's tolerances of between 2.0v and 3.3v.