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.
Two ideas:
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.
Edit:
The two variable resistors (R1 and R2) are the heating wires, and are about 15 ohms each, but do vary.