Re: Q: 5V<->12V level conversion

--- In opendiag@yahoogroups.com, "Adam Bradley" <adam_j_bradley@y...>
wrote:
> Henrik,
>
> After a bit of bashing about I've made a couple of changes that make
> the circuit a little better. Rather than a voltage divider, I've
> used a pull down resistor to tag the voltage at either 0V or 5V
> irrespective of the input voltage (almost anyhow!).
>
> Can you have a look at
> http://members.optushome.com.au/adambradley/electronics/LevelConverter
> /LevelConverter-v7d-cnv-candidate.jpg
>
> Also, if I want to optoisolate this circuit (in both directions) is
> all that is required to add two opto's on both the Rx and Tx lines?
>
> and let me know what you think?
> Thanks!
> Adam
>

Took a while to grasp, but the only way I can make sense of that is if
you have turned the Q1 symbol upside down. The battery symbols are
definitely turned the wrong way. If my suspicion about Q1 is correct,
then it will act as an inverter, high on K-line gives low on Rx which
is not the way the original design worked and not the way Tx works.
But maybe I'm wrong. As I said, I'm not very good at this stuff.

Actually, if you're going to run the entire design powered from the
electrical system in the car, opto isolation might not be such a big
issue. If you look at the design I pointed out in my previous post
you'll see that it takes two opto isolators and you will probably need
to have two transistors on the K-line side. You must make sure the
uController Tx line is capable of driving the LED in the opto
isolator, otherwise it must be buffered. You also have to consider the
polarity of the signals. Most schematics that you'll find for this
kind of interface are designed to be connected to a serial port on a
PC where the signals are inverted compared to what you get straight
out of the UART. Those designs invert polarity of the data going in
both directions, your design probably shouldn't.

/Henrik

 
Received on Tue Oct 21 08:06:45 2003