current on analog pin input

Hi

I am interfacing a sensor and i need to work out the resistance across the probe, it is an analogRead with 3.3V, how do i work out the current that the arduino is putting through the probe?

Sorry this may seem silly to you but i can't really find anything on the internet.

I know V = IR , but i dont know the current.

thanks Jonny

If you have a multimeter (you have one, right?) you can just measure the resistance of "the sensor" or measure the current flow.

Is your question really about level shifting inasmuch that you are using a 5V Arduino with a 3.3V peripheral?

An input pin doesn't have any current. Well less than you can measure (think less than 1nA typically at room temperature). CMOS circuitry is all MOSFETs and MOSFETs inputs (gates) are isolated by a layer of silicon dioxide, a very good insulator.

I know V = IR , but i dont know the current.

and i need to work out the resistance across the probe,

=> I = V/R (algebra ?) V = 3.3V ? R = ?

Analog inputs draw something like 160 nA, so we consider that to be "no current".

Since you haven't told us what the sensor is or given us a link to the vendor , there's not much else we can do.

Not really sure what the problem is . Do you have a multimeter for measuring voltage and resistance ? (and current )

To measure the resistance of the probe you need to supply it with some current, for example a resistor from the analogue input to 5V. The value of the resistor will depend on the expected resistance of the probe. Do you have the data sheet for the probe? If not measure it's range of values with a multimeter and work out an appropriate value from your results.

Russell.