Q: 5V<->12V level conversion

I'm after some validation of this cicuit -

http://f3.grp.yahoofs.com/v1/sGaLP_qiX3XBN-0F61-
_E5a_BTiLgo0FvJy7wqIIa50c0x15sxgZ0tgWwMMNXAjYGd2V_2zKvf69vodJ6xH68z-
E9V-xV7vHQ2giBQ/AJB/cnv.jpg

I'm after a way of sending/receiving from my car ECU a 12V digital
signal using a uController (AVR).

A kind sole presented the above circuit. Based on the labelling, to
transmit from the car (12V) to uController(5V), place 12V (or 0V)
on the collector of Q1 (labelled K line) and read on Tx.

To transmit from the uController, place 5V (or 0V) on Rx and read
from the K line.

I've also discovered placing 5V (or 0V) on the emitter of Q1
(labelled Tx) works! This freaks me out! Why does this work ?

Anyone got any general suggesions as to whether this is a good
approach. Also, if I want to include optoisolate this circuit (in
both directions) how would I go about doing this given a single line
carries Tx and Rx information?

Thanks in advance!

 
Received on Mon Oct 13 20:54:48 2003