I'm sure I can forgive an error of 0.1V if the input is 5, and output is 4.9.

It needs to be single supply too - I thought TLV2371 was a good pick?
Why not?

I'm looking at the TLC2272 (or alternatives). What I need is a single supply Op Amp, capable of operating at 5V, and able to Follow a voltage from gnd to +5V.

Looking at the TLC2272 datasheet - it's noted the recommended operating condition is to VDD - 1.5V (so I can only expect an input of 3.5 at the output in voltage follower mode)?
I'm looking for it to have a high input impedance (no interference on circuit being monitored).

I think I can see the TLV2371 op amp as doing it - the datasheet shows that the common mode input voltage range is 0 to VDD.
I take it to mean that if I power it with a single 5V supply, and place 5V on the input, and got the DMM, I'd expect to read 5V on the output.
I think 1000Gohms means 1000 giga ohms of input resistance - so this would mean it would not load the circuit being monitored, therefore ensuring it is as if a DMM was monitoring it..

Am I right with that?

That's interesting, and perhaps introduces a design issue.

So in terms of program control I don't want to say - analogWrite(MOSFET, 127) for an hour?

I'm thinking of two MOSFETs to spread the amp load between the two of them to avoid the current issue - but from what you've noted the best MOSFET is the one sized to be fully on at the fan maximum ?
Is it a good idea to run a MOSFET at 100% for 2 hours (since it acts like a switch in that state)?
i.e. Is it better for heat reasons to use a large MOSFET and switch it at 50% duty - or is that still bad for it?

I'll take a look for a high current gate driver, but this is still all new to me - any links ?

Thanks for the reply but I'm not sure on your values.

150 ohm to limit below 40mA, so 5 / 0.150 = 33mA - OK!

What happens with R1 - in the linked circuit it's a pull up resistor right? So a high value is intended to keep current low so that the transistor grounding wins - or have I got how that works exactly wrong? 

I've got two automotive fans I'm wanting to drive from an arduino.

The fuse that supplies both of them is 50Amp.

So if I take that as the maximum both will draw, I plan to split the fans into two using MOSFETs.

I'm looking at the datasheet for the IRF1405 - and I think this will do, but I'm just tossing up whether I need to add a transistor and drive it using battery voltage or direct from the pins.
The datasheet is here:
http://www.jaycar.com.au/products_uploaded/zt-2468.pdf

I think I need a current limiting resistor between the arduino pin and the transistor - but how do I size that resistor?

See image for the circuit I'm planning to use - any problems / thoughts?
http://www.neufeld.newton.ks.us/files/electronics/mosfet-motor/MOSFET-motor-driver.png

The MOSFET is different in the schematic linked, so I wonder if the resistor values change compared to the jaycar one I linked to? (it can handle higher current so should be ideal with a transistor).

Also, the datasheet shows the gate threshold min max as 2 and 4 volts. Is 2 the off value and 4 the 'barely on' value - and 12 the 'it's full on' value?
Using the transistor (2n3904) - I'll want to get 4 - 12V modulating...

They still seem very low and this is where I'm struggling - I want to pick a MOSFET to drive them.
The fuse is 50A. If the stall current is supposed to be '3 - 8x' the operating current, then 8Amps is very low.

My DMM is fused only for 10A - a 50A fuse suggests I shouldn't measure the current..

What sort of fans are they? If they use brushed DC motors, you can work out the stall current by measuring their resistance and dividing it into the supply voltage. Measure the resistance several times, rotating the shaft a little between readings, and take the lowest resistance.

I think they are brushed DC - typical radiator fan motors.

The resistance differs between them - the measurement I took was 1.8ohms and 2.6ohms

So, taking your calculation, 12 (lowest supply voltage) / 1.8 ohms = 0.15A ?
12 / 2.6 = 0.21A

That seems stupidly low.
They have a 50A supply fuse on them.

I'm looking to use an arduino to control fans.

I'd like to do better than a simple relay, but there's also the issue of stall current. I haven't tackled anything to do with stall current before, but reading this thread:
http://arduino.cc/forum/index.php/topic,5325.0.html

Made me think that those car relays are typically rated for around 30A. It's already got a controller in it that appears to operate them at a medium.

What do typical designs do for start up / stall current? The relays that are in there appear to not have failed in the many years it's been running. Perhaps it's the case the fans combined do not draw > 30 A at start up ?

That fixed the bulk of the errors, there's still some errors hanging around.
'error: cannot convert 'const char*[15]' to 'const char **' for argument to 'byte menu_selection(const char**, byte)'

The function is used to display a menu produced from an array:
Line 3322: byte menu_selection(char ** menu, byte arraySize)

I'm using it as below:

Code:

byte menu_selection(const char ** menu, byte arraySize)
{
  byte selection = 1; // Menu title takes up the first string in the list so skip it
  byte screenChars = 0;  // Characters currently sent to screen
  byte menuItem = 0;     // Menu items past current selection
  boolean exitMenu = false;
  boolean forceExit = false;

  // Note: values are changed with left/right and set with middle
  // Default selection is always the first selection, which should be 'Exit'

  lcd.clear();
  lcd.print((char *)pgm_read_word(&(menu[0])));
  forceExit = delay_reset_button();  // make sure to clear button
 

The array being passed in is as below:
const char displayMenu[6][13] PROGMEM = {"Display menu", "Exit", "Contrast", "Metric", "Fuel/Hour", "Font"};
const char adjustMenu[11][12] PROGMEM = {"Adjust menu", "Exit", "Tank Size", "Fuel Cost", "Fuel %", "Speed %", "Out Wait", "Trip Wait", "Tank Used", "Tank Dist", "Eng Disp"};

They were defined as:
prog_char *adjustMenu[] PROGMEM = {"Adjust menu", "Exit", "Tank Size", "Fuel Cost", "Fuel %", "Speed %", "Out Wait", "Trip Wait", "Tank Used", "Tank Dist", "Eng Disp", };

You should add this to the code to get an idea of what is being seen:
    Serial.print(i);

Get rid of that, it's just giving you needless numbers (telling you it's looping).

      incoming_char = cell.read();
Add Serial.println(incoming_char)

That'll give you a new line with the received character in Serial monitor.

The \0 at the end of at_buffer is a string null termination character.

Is it no array in sketch?

I do not really understanf the readATString function, how the caracter are "compiled" in at_buffer.

I am very block at this point.

Many thankf for your help

You aren't using an array with this line of code:

Code:

if (at_buffer == "+SIND:0") Serial.print("SIM card remove");

You were given a valid answer earlier:

You need to look at "strcmp"

What did you find when you looked into strcmp?

The chip isn't receiving \n from you, so doesn't exit the while loop..

Hmm. How much SRAM does your Arduino have?

But now I have big difficulties ti understand how to store and work specialy with those value

Your post definitely needs some clarity added to it (more of the code), and perhaps some of what you have tried?

The first thing you need to test is the result of 'incoming_char'- so Serial.print it. I bet it's not the string you are testing for, I'd suggest it's just one of the characters of the string you are expecting.