resistance problem

Hello everyone!
Thanks in advance!
I'm trying to connect the Arduino to a system that includes A calculator that reads signals and a sensor. I want to connect the arduino between the sensor and the calculator so I can get information from it(from the sensor as well). Regardless of the communication it has with the current system.
once the sensor detects something, its resistance changes.( I hope I've been able to explain properly).
I want to connect my Arduino to this working system without the Arduino Will damage the resistance of the circuit.

Thanks very much for the helpers! And if someone did something similar I would be happy for link or anything else.

barakweizman:
A calculator that reads signals

Please elaborate. I've never seen such a thing.

I hope I've been able to explain properly

Sorry it is all a hand waving explanation with no specifics at all, so I have no idea what you are talking about.

Please read this:-
How to use this forum

Then fill in the blanks with links to your sensor, calculator and other stuff. I have never come across "A calculator that reads signals" so I guess you are using the wrong words.

Do you have any specifications/information about the sensor?

You can't directly read resistance... An Ohmmeter works by applying a known current and measuring the voltage (or by applying a known voltage and measuring the current). Then of course, it makes an internal calculation (using Ohm's Law) to display the resistance.

That means you can't measure resistance in a "live" circuit, you have to turn-off the power. And you are measuring the net resistance between two points in the circuit which may be less than the resistance of the component you are connected across.

I assume you have a [u]voltage divider[/u] where one of the resistors is fixed and the sensor is the other... If that's the case, the Arduino can probably read the voltage (if it's in the range of 0 to +5V, and if it's high enough to read).

Do you have a multimeter so you can check the voltage?

I want to connect my Arduino to this working system without the Arduino Will damage the resistance of the circuit.

The Arduino's inputs have very-high impedance so they generally won't affect whatever they are connected to. That is, as long as the voltage is between zero and +5V. If you exceed +5V or go negative, you can damage the Arduino and/or the circuit it's connected to.