Home made AVR (Arduino) programmer "cable"
This weekend I made me a "cable" to program my ATMEGA328p chips the Arduino way. As usual by design goal is to use what I have rather than buy a solution, in this case I'm not going for a MAX232 chip for the level shifting, but doing it the discrete way with coponents I had laying around.
The schematic is based on the one found here: http://www.uchobby.com/index.php/2007/06/11/ttl-to-rs232-adaptor-explained/ but I left out their R2 since I could not figure out what good is did there. On the other hand I added a 10K before the AVR pin2 to protect against an eventual sort circuit in the case we a software bug tries to pull up the RX pin2.
Somehow I seems not to be able to make the last image crystal clear, something with scaling I think.
UPDATE 15-Jan-2012: I have lowered the resistor between collector on the lower transistor and "AVR RX". It was 10K now its 1K. This was necessary when using the circuit with my bluetooth interface modules.