Using computers RS232 serial port as input

I'm building a project which needs to interface with a computers serial port pin 4 (DTR) pin to trigger the Arduino or attiny (eventually) to start a sketch.

I've found that the 3rd party software has the serial port DTR pin set high by default +6.4V and when the software triggers the event it pulls it to -6.4V. I plan on using a diode to stop the negative voltage and would like to use this 6.4V signal on an input pin on the Arduino or attiny84 so that when the pin goes low it starts a series of events in my sketch.

How would you guys drop the 6.4V to something more safe for the Arduino or attiny84 like 5V?

From my research a voltage regulator is out of the question as it needs at least 7v to drop to 5v. What about wiring two or three diodes in series to drop to 5v or just below?

** forget to mention i'm using a USB to serial adapter that's why i'm getting 6.4v instead of 12v signals if that matters **

A voltage divider using two resistors should be adequate. Like this for example

+6.4v ----Res 1k----- -----Res 3k3------- GND | | +4.9v

But please check my calculations.

Another option would be to use a resistor and a Zener diode.

...R

I’d try the diodes to keep things simple. I’ve got an arduino that ran at ~8v for a short period of time and did not die.

histo: ** forget to mention i'm using a USB to serial adapter that's why i'm getting 6.4v instead of 12v signals if that matters **

I didn't notice this before - I thought you had a genuine RS232 port. Why not just get a USB - TTL adapter?

...R

Didn't think about that at the time of procuring parts. Now i'm against a time crunch. So i'll either go with two diodes in series, unless someone has a reason why I should use a voltage divider over the two diodes?

If you haven't converted the rs232 signal to TTL, the below might be of interest.

http://www.scienceprog.com/alternatives-of-max232-in-low-budget-projects/