I have a circuit that's being run from a 5v and is essentially a series of resistors. I want to be able to "tap" it to get 2.5v - 3v out of it, which I think would be straightforward, but I want to do this at the end of the circuit.
Specifically, I have a raspberry PI 5v output that goes through a resettable fuse (PPTC), then some heating coils (Kanthal wire) of various resistances, and then to a MOSFET drain. In total, the resistance is about 30 ohms. Source is connected to ground, and the Gate is controlled by the GPIO output of a Pi. This works fine.
However, I'm concerned that something might happen, like blowing out the fuse, and I want to monitor that it's working properly. To do this I was planning on connecting the circuit to a raspberry Pi GPIO input checking that the pin goes high when I turn on the circuit (via the MOSFET).
Simply, how do I connect this? I'd like this check to encompass as much of the circuit as possible. By that I mean that I only want to get the signal on the input if the PPTC and the wires are "working".
Since the 5v is not switched by the MOSFET I can't really tap that to check that the circuit is working. Notwithstanding the that, I considered taping after the PPTC and then using a voltage divider, but that would only confirm that the PPTC is working, and wouldn't confirm the heating elements. I considered tapping after the last heating wire and before the MOSFET, but at that point the voltage measures close to 0v, which wouldn't drive the GPIO input.
Connect right before the MOSFET Drain. Also connect to the 3.3v line through a large resistor to pull up the pin. Then when the MOSFET opens up and the circuit is properly working then the /tap/ should get pulled to ground because it's close to 0v at that point.
Set a transistor right before the MOSFET Drain that can be driven with the small voltage that will exist there, then let the output pull up the pin.
The two variable resistors (R1 and R2) are the heating wires, and are about 15 ohms each, but do vary.