Cheap? FTDI Adapter

Well... the Duemilanove is awesome, but sometimes you just need an FTDI cable for another Arduino that doesn't have built-in USB. I added a switch to my Duemilanove that turns it into an FTDI adapter. With $22 Duemilanoves, it's hard to justify $15 for an FTDI cable.

I added a switch that ties the reset pin on the host arduino to ground via its 10K resistor and switches the DTR from the FTDI from the host arduino to the slave arduino.

I mounted a triple pole dual throw switch next the the USB port and broke the reset-enable solder bridge. After those two modifications it's just a matter of soldering wires. Oh, you will need some wire and a connector for the client Arduino.

Host Pin0 RX Out
Host Pin1 TX Out
Host 5V 5V Out
Host Gnd Gnd Out
FTDI DTR(from Reset-Enable) throw1 center
Host Reset(from Reset-Enable) throw1 bottom
DTR Out(Reset on Client) throw 1 top
Host Gnd throw2 center
Host Reset(from Reset Button) throw2 top

When the switch is in the top position and nothing is hooked to the cable the Arduino works normally. When the switch is in the top position the Arduino works as a FTDI adapter(with full DTR/auto reset capability).

If someone wants to add a really stand out feature to their next Arduino clone, this would be a really handy feature. You could just substitute a switch for the reset button. Flipping the switch to the bottom and back to the top will reset the host arduino.

Great! Seems you can turn an Arduino into every kind of adaptor... burn bootloaders, ISP-programmer and now that.
It would be nice of you to provide some better pictures, as those are quite blurry.
I just don't really understand which cable goes to where (except for 5V, GND, TX and RX, these are quite obvious). But what about the other ones? What exactly is the FTDI DTR and the DTR out?

Thanks!

This should show what I did better.

This disables the host arduino by holding its reset line low while passing all the signals to the slave arduino.

Where you tie into ground, 5V, RX, and TX doesn't really matter, but the reset/DTR points matter. The reset switch uses a 10K resistor while the "RESET-EN" point doesn't.

*** UPDATE ***
I updated the image... it's correct now.