I have this circuit I desiged that tells how much resistance there is between the connector (both contacts are connected to one sense wire) and ground.
Then, if the test point (a brass elbow in a co2 gas stream) gets wet (blood, most likely), we know the machine has had a biological contamination.
So normally the ADC pin (A1_Fluid_Sense) reads about 32000 When J5 sensor is hooked up and open. it reads 38000 or so when J5 is open.
it will read 0 if you dead short it to ground.
My problem is that D34 is getting blown. it is just a sod-323 shottky. I had thought that some transorb or something would be more appropriate there, but I never did figure out something that worked. The intention of D34 was to be like a 5v transorb, but I couldn't get that to work. We need to keep the ADC pin safe from any crap that might blow the processor.
D34 has not blown much (I think about 3, over a year of 200 machines running), but that is too much.
Oh, Vusb is 5.2volts.
Not sure I am getting the schematic png attached...