Boundaries of the Arduino Uno and its inputs [Noob crying for help]

Hello everyone,

As i'm really knew and learned a lot lately, i might be confuse...

I use a Arduino Uno supplied with a 6V/800mA tranformator.

I read in digitals (and soon analogics) inputs the pulses send by a water meter through a 12m long wire. I would like to calculate how much intensity will use this facilities on one Pin in order to determinate how much identic schems can i install without reaching the limit of 200mA for the whole Arduino and 40mA per input.

I was wondering, if i calculate the resistivity of the wire and the water meter (if there is any for this last one), knowing that Arduino Uno is supplying with 5V, can i just use the famous U=RI ?
Then i'll just add all the intensities i calculated (lenghts of wires might be differents).

I was really bad student in electricity... but i'm trying to understand... might it be this simple ?

Yes indeed it is that simple: U = I * R, so if you for instance do not want to exceed the 40 mA limit of a single pin you would need

R = U / I = 5 V / 0.040 A = 125 Ohms

at least.

Cheers,

Jack

I think you are confusing that the current capabilities you mention refer to output pins, not input pins. Taken from an Arduino Reference page: "Pins configured as INPUT make extremely small demands on the circuit that they are sampling, equivalent to a series resistor of 100 Megohms in front of the pin. This makes them useful for reading a sensor, but not powering an LED." - Scotty

Hi again…
I took an oscilloscop to analyse the system electric features. Results i found are far from what i expected.
Indeed i learned that my arduino uno should not -or cannot- deliver more than 5V / 40mA.

When i use the oscilloscop, i read a pulse of 46V at the boundaries of my 33kOhm resistance (leading to GND).
How is it possible ? i thought i could not deliver more than 5V … Could it kill my arduino uno ?

When i study the tension between the whole système (from 5V to Pin7 - which is the digital input), i have the same results with a 46V tension…
I did this in order to analyse the impact (resistance) of using a cable made of 2wires of 40m…
This would mean that the resistance made by the lenght of the wires and the pulse emetor facility are equal to 0 (or really not significant).

What should i think about the 46V result ???

If you have any answers please explain me a bit of the “why” if you have the time (and knowledge)…

schéma relève pulses.png

BioMomo:
Hi again…
I took an oscilloscop to analyse the system electric features. Results i found are far from what i expected.
Indeed i learned that my arduino uno should not -or cannot- deliver more than 5V / 40mA.

When i use the oscilloscop, i read a pulse of 46V at the boundaries of my 33kOhm resistance (leading to GND).
How is it possible ? i thought i could not deliver more than 5V … Could it kill my arduino uno ?

When i study the tension between the whole système (from 5V to Pin7 - which is the digital input), i have the same results with a 46V tension…
I did this in order to analyse the impact (resistance) of using a cable made of 2wires of 40m…
This would mean that the resistance made by the lenght of the wires and the pulse emetor facility are equal to 0 (or really not significant).

What should i think about the 46V result ???

If you have any answers please explain me a bit of the “why” if you have the time (and knowledge)…

that you don’t read it well :wink:
did you connect the GND wire of the scope with the arduino GND ?

Thanks for answering but i found the problem...
Just a multiplication factor which was set in the oscillosop... I didn't know about this one...
So i should have read 4,6V and 33K Ohms... meaning that i have a 0.14mA current going throught (i'm happy about that, i'm far from the arduino limit).

You were right : I didn't read well !