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I have the Arduino Mega 2560 R3. I know that it has a recommended Voltage of 7-12v with limits of 6-20v but is this over all pins or per pin.. ??

The data sheet mentions that DC Current per I/O Pin is 40 mA but the analog pins are inputs.. Aren't they..??

I'm using the Analalog pins as I'm reading temperatures from multiple rooms and then analyzing this via a vb program.

Lastly each room has 2 feeds. Ground & 4-10v Dc. Can each probe use as single Analog pin but they all share 1 of the 2 Ground pins..??

Sorry.. New to this world...

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Arduino Overview

The Arduino Mega 2560 is not a specific component with specifications, but rather a development board with ratings based on the embedded components. The Arduino hardware is a development board (usually) based around Atmel AVR microcontrollers. The microcontroller on this particular board is the ATmega2560 (datasheet). This component has a maximum operating voltage tolerance of 2.7V - 5.5V, depending upon model and clock frequency.

I highly suggest you browse through the datasheet for the ATmega2560 chip. You don't have to read every word, but at least become familiar with the various internal peripherals to know what this chip is capable of.

Input Voltage

The Arduino does have an on-board voltage regulator to drop the input voltage down to a suitable level for the embedded ATmega chip - part IC1, an NCP117 LDO (datasheet) according to this schematic. It is this voltage regulator which has the 7 - 12V operating range you are speaking of.

I/O Pins

There are numerous I/O pins on the ATmega chip which will work as a digital input or output port. Some (but not all) of these pins can also be used as analog inputs. The input value on any of these pins should not be higher than the operating voltage of the ATmega chip, which is 5V DC. You can also set the "reference voltage" of the analog pins to other values less than that by changing the ADC register values.

Each digital pin can sink (output low) or source (output high) 40mA; however, the chip as a whole (power supply pins: VCC and GND) can only handle a total of 200mA at any one time. Using a pin as an input should only use a tiny amount of current because of the high input impedance.

If you are trying to use the analog inputs with a higher voltage than 5V, you will need to reduce the voltage using a resistive voltage divider. This will scale the sensor range down to a reasonable level. Each sensor will need its own analog input pin (ADC channel) unless you use an external analog multiplexer. The ground (common) line for each sensor needs to be connected to the ground line for the Arudino board, even if they are powered from a separate source.

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The voltage rating is for determining the ranges that can be used to power the board. The analog input range is actually mentioned under the I/O section of the datasheet:

The Mega2560 has 16 analog inputs, each of which provide 10 bits of resolution (i.e. 1024 different values). By default they measure from ground to 5 volts, though is it possible to change the upper end of their range using the AREF pin and analogReference() function.

The last sentence is a little misleading, elsewhere in the site it mentions that the top limit of the analog input can be a maximum of 5V. To get your readings in the right range you would need to setup a voltage divider (this one would scale the voltages in half, 2-5V):

schematic

simulate this circuit – Schematic created using CircuitLab

For the current rating, typically when you are making measurements, the input, in your case the analog input port, is a really high impedance, so the currents are small. The reason for this is because you want the measurement to affect the read voltage as little as possible.

Lastly, I would say yes, your grounds can be shared. They will both be at the same potential, so it shouldn't be an issue.

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  • \$\begingroup\$ Hi Team... Thank you for the above info... I'm a little more the wiser now... Thank you for your time explaining this to me. \$\endgroup\$ – Mark Fellowes May 9 '13 at 3:29
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I'm having a similar issue where I'm trying to read a digital sensor that is either at 10VDC or ground.

This article has a number of options for handling higher than expected voltage in digital inputs.

Options include:

  • bilding a resistor bridge as already pointed out
  • an external 4.7V zenner diode

Protecting Inputs in Digital Electronics

I used a resistor bridge consisting of two 4.7k ohm resistors.

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