Looks like the Maxim MAX488E would be an excellent choice.  They are full-duplex (separate receive and transmit drivers) so you only need one 8-pin DIP at each end.  Digikey.com has them for $3.28 each.  Search for MAX488CPA+-ND

http://datasheets.maxim-ic.com/en/ds/MAX1487E-MAX491E.pdf

I'd put two RS485 transceivers at each end and use two pairs from a CAT5 cable to get full-duplex TTL Serial communication.    I'm pretty sure RS485 is rated for 1000 feet.  Put a USB to TTL Serial at the PC end and connect to pins 0 and 1 at the Arduino end.

RS485 is bi-directional but half-duplex (can only send one direction at a time).  By using one pair for outgoing data and one pair for incoming data you don't have to worry about turning the line around.  The driver chips are under $2 each.  100 feet of CAT5 cable can be had for under $10.

Do you have the connections between the thermocouple and the chip very close to the chip so the connections are the same temperature as the chip?  That's what you have to do with a thermocouple.  If you try to extend it with a cable you are creating more thermocouples!

Read the data sheet for the chip.

Hey johnwasser, please suggest me some more solutions if you know, so that I can compare the pros n cons...

You want more?  Try

http://www.google.com/search?q=Arduino+Touchscreen

I strongly suspect the 'small crystals with integrated caps' is really a ceramic resonator with internal caps.

You are probably right.  I've just never seen a ceramic resonator in a metal can.  All the ones I have seen look like fat ceramic disk capacitors with three pins.  I guess I never saw a surface mount resonator before.

I do tha but the led is not turn off when i pressed the button(pin)4

You changed the program I gave you and made it stop working.  Perhaps you should not have done that.

That crystal is the one for the tiny ATmega processor being used as a USB to Serial converter.

Both areas of the circuit board have pads for both large crystals with external capacitors and small crystals with integrated capacitors.  Your board just happens to have the small version in both places.  The 'official portrait' has a large crystal on the USB interface and a small crystal on the main processor.   Either one will work in either or both places.

Still not clear to me whether the 5x coin count is an INPUT (as implied in the question) or an OUTPUT.  Perhaps that is why I am confused. I would think that a change machine would take in paper currency (counted by the pulses from the bill acceptor) and then OUTPUT pulses to the coin dispenser.

My understanding is that the coin dispenser has an on/off input (represented by the LED) and a pulse output which signals that a coin has been dispensed (represented by the second button).  The program turns on the coin dispenser until the right number of coins have been dispensed and then turns off the coin dispenser.

http://www.hoperf.com/rf_fsk/rfm12b.htm

Data sheet: http://www.hoperf.com/upfile/RFM12B.pdf

Programming Guide: http://www.hoperf.com/upfile/RF12B_code.pdf

Does it look like this:

http://arduino.cc/en/uploads/Main/ArduinoMega2650Front.jpg

The crystal, which is shown in the schematic, is that little metal can to the lower left of the processor.  It looks like in this picture the capacitors are integrated into the crystal package.  It looks like there are also solder pads for a larger crystal and external loading capacitors if the small integrated units are not available.

It appears to be an Arduino Uno clone.   It will probably work exactly like an official Uno.

You can get the official Uno for $29.99 here:
http://www.hobbyengineering.com/H4305.html

He wants to dispense 1 euro coins when someone puts in 5, 10, or 20 euro bills.

For each 5 euro of bill value the bill detector sends a pulse: 1 for a 5 euro note, 2 for a 10 euro note and 4 for a 20 euro note.

For each pulse of the input he wants the coin dispenser to dispense five coins.  Each time it dispenses a coin it produces one pulse.  When done dispensing he turns the dispenser (represented by the LED) off.

So far he has not commented on the code I provided to do exactly that.

I think the FLASH is in a different address space than the RAM.  You can execute the program out of FLASH but the data pointers are all assumed to point to RAM.  If you pass a FLASH address/pointer to a function like Serial.print() which takes a RAM address/pointer you will get the wrong data.  For that reason, all of your initialized global variables and string constants are copied from FLASH to RAM before your program begins.  Then the RAM addresses are used.

You can explicitly put constants into the FLASH address space but you have to copy them to RAM as needed if you are going to pass them to a function that takes a RAM address pointer.

I don't know if the compile/upload will fail if you have more global and static variables than your RAM can hold.  Certainly your program will fail if your data overwrites the stack space.  Unfortunately there seems to be no detection of a collision between heap and stack.

Anyway, i don't understand the Rubenjohnson answer ???

Neither do I.

There must be some kind of motor that will give you the speed and control you want.  A servomotor might do it: http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee4a/bdeee4a_1.aspx

The serial pins on an ATmega328 are 2 (RXD) and 3 (TXD).

The serial pins on the Arduino are 0 (RXD) and 1 (TXD).

Pull out the processor on your Arduino.  Connect either Data Pin 0 or pin 2 of the processor socket to pin 2 of the 328.  Connect either Data Pin 1 or pin 3 of the processor socket to pin 3 of the 328.  That will allow you to use the USB interface of the Arduino to talk to the external 328.  Connect the grounds together.

If the external 328 has an Arduino bootloader you can upload sketches to it the same way.  You will have to use the manual reset.