ok... just connect them this way:

Phone               Arduino
 4(rx) <----------- 1(tx)
 5(tx) -----------> 0(rx)
10(gnd) <-------> gnd

you don't need level converters, both are ttl-level ports.

just remember that some people has reported some troble using pins 0/1 serial with USB cable attached... sound weird to me because of the 2 1k resistors between the ftdi and atmega... anyway, if it don't work at first try, check this.

What's your phone model? I think the easiest (and safer) way is googling it... or, is very possible that the GND is connected to pin 5 on the serial cable... if the 3 wires you get are straight to the phone connector, they are probabilly ttl...

The major difference between BC547 and 2N2222 is the current (I) they can handle... BC547 is rated for 500mA, 2N2222 800mA.

If you really need isolation, why don't you use an optocoupler (i.e. 4N25)? Something like this:

                                      4N25
                        47R        __  __                   10K
          3V >-----/\/\/\-----|o U   |--       ------/\/\/\-----< 5V
                                     |        |         |
       GND >----------------|        |-------+----------------< Arduino pin
                                     |        |
                                  --|_____|------------------------< GND
                                    
   Video switcher side         <|>          Arduino side

Yeah, of course.

For the sender, the basic of the basic will be a lot of pieces of code like this:

if ( !digitalRead(2) ) // 2 is the pin where the push-button is tied to, don't forget pull-up resistor. Just change pin nuber as you need.
{
    Serial.print("D21") // "D"igital pin "2" actuated
}


In the receiver:

if ( Serial.available() == 3 )
{
    kindof = Serial.read();
    addressof = Serial.read();
    valueof = Serial.read();
    flagrcvdata = 1;
}

if ( flagrcvdata && kindof == "D" ) // enter here if complete data packet received and is an digital reading
{
    digitalWrite(addressof, valueof);
    flagrcvdata = 0;
}


Of course, this is not the most elegant or pro-like way for code this, but from here you can figure out the way it will do what you need.
You will probably want to add some kind of header and better control in the serial part of it.

Don't forget that you may will need to add some delay between readings, otherwise you will fill the RX serial buffer faster than you will could process it.

Hope have helped.

In fact you just need to connect GND between the two boards (NEVER interconnect +V of different power sources). Unless you want to use only one power supply and power the "remote" Arduino from the local one. That's the way I use mine, a "master" unit, with power supply, and the "slaves", that are all interconnected over an 4-wire RS-485 network (+V, A, B, GND). This way I can have them up to 1200 meters away... so far away than the standard 5~15 meters of RS-232 (even less using only TTL signals).

Best regards,
Celso Fraga.

I think the easiest way to do this is using the serial to connect them (i'm actually doing this over RS-485, one "master" Arduino controlling some "slaves" (Arduinos too). I suggest you to create some kind of communication protocol, i.e. you read digital inputs of one arduino, when one of them is actuated (by a press on an push-button, maybe?) send over serial (in ascii) "D41", that can mean "D"igital input number "4" actuated ("1")... the other Arduino receive this message and based in your programmed logic do an digitalWrite(4, HIGH), powering an led... for the pot, same logic... send the analogRead value, the other Arduino will decide what to do based in what you programmed.

Maybe some confuse text, but i think it's understandable...  :

Yes, it's possible... For pots, tie the ends to +5v and gnd and the tap to one of the digital inputs. For PBs, one pin to any Arduino's digital input and the other to +5v or gnd, maybe needing pull-ups or pull-downs, according the config your choice. To send it to a Linux running machine, I think that the easiest way is using some of the examples of simulating PS/2 keyboards, there are plenty of them here in the forum.

Take a look on what these guys are doing...

http://www.batsocks.co.uk/products/Other/TellyMate%20Shield.htm



Schematics, source code, precompiled hex files and examples available.

Yes, I have sucessfully used that for bootloading both mega8 and 168... Have you configured the fuses and lockbits?

I'm using v0011 under Vista, it's working ok... What king of problems are you having?

Yeah, i2c is a good choice... if you really want them "together"... i2c is intended, as its name says, Inter-Integrated Circuit communication... not intended for long busses (or long wires)... if you want to put them away, you must use some sort of serial arrangement, rs-232 or rs-485, according to the desired distance.

Have you used the "Run as Administrator" context menu to install the driver?