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'[EE:] Automotive interfacing questions'
2004\07\05@001641 by

I have figured out the resistances from the car circuit diagram and I
have turned out the first shot at that part of the circuit, I think I
have the source impedance right but could someone smarter than I take a
look at it and let me know if I'm ok or not...

Diagram is available at

http://cowshed.8m.com/car/inputs.html

Thanks

Anthony
--
Anthony Toft <toftatcowshed.8m.com>

--

>-----Original Message-----
>From: Anthony Toft [toftatCOWSHED.8M.COM]
>Sent: 05 July 2004 05:17
>To: PICLISTMITVMA.MIT.EDU
>Subject: [EE:] Automotive interfacing questions
>
>
>I have figured out the resistances from the car circuit
>diagram and I have turned out the first shot at that part of
>the circuit, I think I have the source impedance right but
>could someone smarter than I take a look at it and let me know
>if I'm ok or not...
>
>Diagram is available at
>
>http://cowshed.8m.com/car/inputs.html
>

I would be tempted to use an extra resitor before the zener to provide
better protection, this should survive 12V being fed directly into the
circuit with no series resistance.  Note the a 5v1 zener could have
significantly more than 5.1volts across it at higher currents.  The 2k is a
compromise, a higher value would provide more protection, but the PIC ADC
requires a minimum source impedance to achieve their rated accuracy.  If you
are using a 16F part than they are mostly specced at 10k, so the 1k resistor
could be safely increased to say, 8k2.  The 18F parts require a much lower
source impedance of 2.5k

Regards

Mike

----+---- 12v
|
\
/ Steering wheel R
\
|   2k
+---/\/\---+-------+---- To PIC
|          |       |
\         -+-      |
/ 800R    /_\ 4V7 --- 0.1uF
\          |      ---
|          |       |
----+----------+-------+-- 0v

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--

> requires a minimum source impedance to achieve their rated accuracy.  If you
> are using a 16F part than they are mostly specced at 10k, so the 1k resistor
> could be safely increased to say, 8k2.  The 18F parts require a much lower
> source impedance of 2.5k

This is a 16F876-A with a _maximum_ source impedance of 2.5k, mimimum
does not seem to appear in the datasheet. Should I make Vref from the
car source as you described in the other post? and if I do, how will
this affect the use of the comparators (or should I give that idea up)?

{Quote hidden}

Thanks!
--
Anthony Toft <toftatcowshed.8m.com>

--

Normally, you can't make Vref larger than VCC.
So, what you end up doing is scaling your inputs so that they go through only the 0V-VCC range.

You should add your own protection diodes, to VCC and ground at the micro, and have enough series resistance from the input so that current into these diodes will be minimal even with 12V on the input.

Plan for the worst in automotive applications, you won't be dissapointed.

--

To all those who have commented already, thanks, I have made the changes
suggested and uploaded the entire circuit diagram as it is right now. I
still have a little left to do to it, but it's getting there :)

Thanks to all who take the time to help a hobbyist trying to figure this
world out.

Anthony
--
Anthony Toft <toftatcowshed.8m.com>

--