How would I write that equation as a programme line [ X = ... ]

..... X = log 18 / log 5

Take logs of both sides, take the exponent to the front (I forget which law number that is), solve for exponent

Edit....

log (5^x) = log 18

x log5 = log 18

x = log 18 / log 5

http://www.bluesock.org/~willg/dev/ascii.html

Holy cow, that takes me back a decade or three.... used to be fluent in Ascii and Ebcdic

GFS's code works as expected for me.... does a half circle, pauses for 10s with LED on, goes back, starts again.

no, vOut and GND are at the same level in your drawing.

MrDumle, I think it's best if you lose all the lines inside your dotted Arduino, leaving that as a "black box" and not try to visualise what happens inside. If you do that, and mark the dot you have marked as Vout as PinX just to indicate some Arduino i/o pin or other, then you'll be good. If you want the drg to be complete, move the Vout label to the dot between the resistors.

What you'll have then, is this tutorial

Sure think, I don't want to apply that voltage into any pin, it should be already at the Vin - it has the same voltage as the input voltage connector, right? So can I read the Vin in software in similar way I read analog pins A0 through A5?

Well no, because it'll be 12v, so you WILL be putting 12v into a pin although yes, it starts out at the Vin pin but you'll be shoving it into an analog pin. Hence CR's response re a voltage divider....

I guess Blink Without Delay might give you some ideas.

But I'm wondering about the wisdom of positioning the servo based on time which is an open-loop approach.... under different load conditions, the servo will move at different speeds and time may not be a good analog for position. Maybe you should consider some kind of encoder to give closed-loop control over the position.

A response with a Fritzing diagram would help.

You first would be MUCH better.

Yep, you really need to show your circuit, and the code you have so far

Code:

pinMode(pin, INPUT);           // set pin to input
digitalWrite(pin, LOW);       // turn on pulldown resistors

There are no built-in pull-down resistors, only pull-ups... pull-downs need to be external.

Depends what you mean by use.... to download a sketch you need to be hooked up to the PC with the usb, but the Arduino will run the previously downloaded sketch when it's powered from a battery into the jack. At that stage the PC doesn't need to see the Arduino...

Presumably the board supplies the 5v to the pot, and reads back the value of the middle pin.... So, it "might" be feasible to put the Arduino ground pin to the Control 0v and then to PWM the Control "INP" pin from the Arduino, which will give it a "pseudo-voltage" between 0 and 5 for speed. Not sure though, more knowledgeable folks than I should comment on that idea before you hook it up... E&OE

l 298
 

L298 eats volts and needs a heatsink the size of a truck.... there are more efficient h-bridges, I've got one of these.

Well the answer has to be YES, but you'll need to be a lot more specific about what you want it to do, what your skills are and so on. Start by looking at the servo library functions and then follow the links to the knob and sweep examples.

but there seems some things to be wrong.
Is there somebody out there who's sees the mistakes and can help me out!?

What things seem to be wrong?- You can't reasonably expect anyone to trawl through that almost totally comment-free code when we a) don't know what it's supposed to do and b) don't know what it is doing.

Does it compile?- if not with what errors?

If it compiles, what's it doing that you don't expect it to do? (Or, not doing that it should?)

Have you tried to isolate parts of the code to try to identify the problem/s?

What circuit isn't closing? What articles?

Maybe what those un-named articles are getting at, is a pull-down resistor?