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### Topic: Some newbie questions - Analog readings , maximum current. (Read 475 times)previous topic - next topic

#### shamana92

##### Oct 13, 2012, 12:25 am
Hi , I'm new to arduino and electronics as a whole.
I recently bought my Leonardo , and i have some basic questions.

I want to hook up some termistors , foto-resistors and all those basic analog sensors.
The guides advice the following schematic:

5v --> termistor(for example) ---------> a resistor ----> Ground
|
|
Analog input
My question is :
Hooked like that , it would act as a voltage divider.So with my limited understanding i think that this will limit the range to 0-2.5 volts , instead of 0-5v.
What would be the problem if i hook it up like this:

5v ---> Termistor ----> a resistor ---> Analog input (that way we get to keep the range 0-5 volts)

My second question is - How much current is considered "safe" for the pins? So i can calculate what resistors to put. Does the safe current differ between analog and digital pins.
What are the ways to damage the arduino using the 5v,analog and digital pins?(excluding the famous VIN pin).
Please excuse my limited knowledge , and if I am wrong is some of my statements , as i said - I'm new to electronics.

#### Grumpy_Mike

#1
##### Oct 13, 2012, 12:43 am
Yes it will limit the range, no there is nothing you can do about it. You suggestion will not work.

The resistance of an arduino input pin is so very high that for all intents you can consider it to be an open circuit.

#### oric_dan

#2
##### Oct 13, 2012, 12:44 am
The first diagram is the proper voltage divider ckt to use. The 2nd diagram
will give you "only" 5V readings.

The input voltage range you get depends upon the values of the 2 Rs. As
Rtherm goes less than R, the voltage will tend towards 5V.

As long as the voltage is in the range of 0..5V, you don't have to worry about the
ckt driving too much "current" into the A/D converter pin. However, in general,
you want to use Rs in the range of 1K .. 10K for connection to A/D converter
pins.

#### shamana92

#3
##### Oct 13, 2012, 12:49 am
Thank you for the answers .
So the only thing i should be careful with is not to input into and output pin ?

#### oric_dan

#4
##### Oct 13, 2012, 12:57 am
Quote
So the only thing i should be careful with is not to input into an(d, edited)
output pin ?

It you tie a voltage to an output pin, you can get a damaging current. The simplest
way to protect an output pin is to tie a small value R in "series" with it, eg 150-330
ohms, then if you accidentally connect a voltage to the pin [which happens all the
time], you won't damage it.

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