How to measure 2.5 volts out of 5 volts and serial resistors?

Question

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:

1. 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.

2. 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:

This is the schematic:

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

Answer 1

The low current is a small issue, but I believe your best solution is to use a low-side current sensing amplifier (like this one from TI) or just put together your own with an appropriate op-amp.

Place a small resistor at the source of the MOSFET, and amplify the voltage drop up to a usable range. Keep in mind that at these low voltages op-amp input offsets can be a problem, so pay attention to that.

As you have a ~200mA current, a 0.2ohm resistor would give you ~40mV. With a gain of 50 this would become an easy to read 2V.

If you can tolerate a much bigger drop, you could use a couple of transistors or a transistor and a resistor in a crude current mirror configuration that pulls down a pin when a high enough current is circulating.

If you want precision in your measurements, there are resistors designed specifically for this purpose. And, as the impedance of the traces/wires can be an issue a 4-point connection would be a must.

Answer 2

Instead of measuring voltages, you really want to measure the current. You can do that with any number of specialized integrated circuits, or you can build your own with a sense resistor (or use one of the series elements) and a comparator.

You can also run the resulting voltages into some ADC inputs and do some math.