Variation of the Arduino board

When starting with Arduino, one soon has much more Arduino boards then RS232 or even USB ports on the computer. So that, it seems more useful to put the signal level converter to an external adapter than directly on the Arduino board.

Schematic of the reduced Arduino Board (click to enlarge)

Only a few components on a breadboard are a fully usable Arduino

The board

Usually, one has (I have) a big basket full of ATMEGA8 and ATMEGA168 with Arduino bootloaders and one idea. So I need a simple and fast-to-build Arduino board. When the bootloader is already programmed, one does not need the ISP port. Also, I prefer an external RS232 level converter than to build it on the Arduino board, since then the usage is more flexible.

When removing these from the board itself, the partlist becomes very short:

For references, see [1].

Schematic of the RS232 adapter (click to enlarge)

That's how it looks like on a breadboard.

RS232 Adapter

The converter from TTL to RS232 signal level is a simple application of a MAX232 and fits inside a connector's housing.

Cute USB Adapter

A look inside

USB Adapter

The idea came from [2] where a CAT-cable for a Sagem mobile was used as CAT cable for a ham radio transceiver.

When connected to a PC via CAT, many mobile phone use a serial protocol and TTL signal levels. At many of these CAT adapters, the connexion to the operating system is done with a virtual COM-port. So we may have exactly the thing we want for our Arduino: A COM-port with TTL signal levels via USB.

CAT-cable for the following cellphones have been tested:

• Sagem MC922
• Nokia 3510i

One can get those cables for about 5 Euros. The important thing is that the drivers provide a COM-port.

The first step should be the installation of the COM driver. When opening, one needs a way to find the meaning of the connectors. One possibility is to look out for a scheme of the connector and mark the wires. Another, maybe faster, way is to identify the pins by oneself:

• The first thing to find out which is the ground pin. Usually, it's the one having the most connections with the connector. It's not necessarily connected to the ground or shield of the USB plug!
• Then, plug-in the connector and watch out if one of those pins is a 5V VCC pin. Should be easy to find out with a multimeter, when you know the GND pin. If there are only three wires, you can do without this step :-) .
• Write a sketch which makes Arduino write a line to serial port frequently and upload it to Arduino. Something like:
  void setup()
{
Serial.begin(9600);
Serial.println("Setup ready");
}

void loop()
{
Serial.println("Loop");
delay(500);
}

  Connect ground to your Arduino. Use a terminal programm (e.g. Arduino environment) to the new COM-port. Connect one of the wires to the TX port of Arduino (pin 2). If you can read the message or something weird in the terminal, you found the RX pin.
• The remaining pin is the TX-pin.

If you want to power the Arduino via USB, you probably have to connect the USB ground to the TTL ground.

References

• [1]  http://www.arduino.cc/en/Main/ArduinoBoardSerial   Arduino Board - Serial Interface
• [2] http://www.db1bmn.de/projects-Dateien/USB_Datenkabel_CAT_Interface.pdf   Cellphone data cable as USB-RS232 converter with TTL levels
  (link unavailable)