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Q: 5V<->12V level conversion

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I'm after some validation of this circuit ->

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!



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