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

--- In opendiag@yahoogroups.com, "Adam Bradley" <adam_j_bradley@y...>
wrote:
> Frank,
>
> Good point! I think the link was too long. Try browsing to the only
> file in this folder ->
> http://groups.yahoo.com/group/opendiag/files/AJB/
>
> Thanks for trying!
> Adam
>
> --- In opendiag@yahoogroups.com, "Frank" <frank@b...> wrote:
> > The link doesn't seems to work, so it's difficult to review
> something that
> > isn't viewable
> >
> >
> > -------Original Message-------
> >
> > From: opendiag@yahoogroups.com
> > Date: dinsdag 14 oktober 2003 5:54:55
> > To: opendiag@yahoogroups.com
> > Subject: [opendiag] 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.
> >

Not sure what you're saying, but it looks to me like you've gotten
that backwards.

From ECU to uControllen is a simple voltage divider so that you get a
voltage swing of approximately 0-4 V on RX when the ECU gives a swing
of 0-12V on the K-Line. During reception from the ECU the TX line must
be high, which is pretty standard behaviour for asynchronous
transmitters in idle.

Data to the ECU is sent through the TX line. When TX is high the
K-Line is held high by the 5.1k resistor. When TX goes low the
transistor opens and the K-Line is grounded by the TX-Line. The data
you send will also be looped back to the RX-Line.

At least that's how I understand the circuit, admittedly I'm not very
good at this kind of stuff so it's possible I've misunderstood the
idea behind it all.

> > 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?
> >

I don't think I would use a circuit like that. There's a pretty neat
design my Jeff Noxon at http://www.planetfall.com/~jeff/obdii/
That design is for conversion between RS-232 and K/L-Line so you can't
use it straight away, but it can perhaps serve as inspiration. Please
note that on RS-232 the signal polarity is inverted compared to what
you get out of the uController.

/Henrik

 
Received on Tue Oct 14 15:30:46 2003