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'[EE:] Display circuit sanity check'
2004\07\19@212456 by Anthony Toft

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Here I am asking for a favour again...

I'd really appreciate it if someone could take a few moments to sanity
check my circuit. It can be found at

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

It is basically an augmented serial to parallel converter, the serial
cable comes from a PC in the trunk, it is modified to supply 12v from
the PC. 7805 provides regulation (it will have a heatsink on it) The
steering wheel controls are on an analog input, the rest of portA are
digital, the Axiom GPS module will be accessed via bit banged serial
(it's CMOS levels). The "lights" port is digital input 12v-ish at the
top end, the 2N2222 is to convert that to 0v and 5v.

The parts that concern me the most is the impedance of the steering
wheel controls (I don't know how to calculate it beyond adding the
reciprocals together at minimum and maximum resistance), the transistor
part (it's the first time I have used a transistor this way) and the
MAX232 part (again the first time)

Any comments are appreciated as is all the assistance that has been
given by folks to get me this far.

Anthony
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2004\07\19@215449 by Jinx

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> the transistor part (it's the first time I have used a transistor this
way)

Is there a transistor there ??? (sorry if I can't see it)

> and the MAX232 part (again the first time)

I usually have an extra 3 caps across 15/16, 2/16, 6/15. You can
get a 232 version that needs fewer

> Any comments are appreciated

I be prepared for noise filtering in an automotive environment. This
could include a grounded metal case, a choke or pass-through on
the 12V line, ferrites on signal lines, Schottky diodes to ground on
inputs to protect them from under-voltage spikes, a spike filter on
the i/p to the 7805 etc etc etc

Could I also suggest IrfanView to make gif images instead of jpeg.
gifs are so much better for circuit diagrams

http://www.irfanview.com/

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2004\07\19@223040 by Anthony Toft

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> Is there a transistor there ??? (sorry if I can't see it)

It is bottom left, converting the 12v panel light input to 5 volt (and
inverting it in the process)

> I usually have an extra 3 caps across 15/16, 2/16, 6/15. You can
> get a 232 version that needs fewer

Caps are not a problem, although the hobby budget isn't unlimited (don't
we all wish it was) it can stretch a few cents to make it right

> I be prepared for noise filtering in an automotive environment. This
> could include a grounded metal case, a choke or pass-through on
> the 12V line, ferrites on signal lines, Schottky diodes to ground on
> inputs to protect them from under-voltage spikes, a spike filter on
> the i/p to the 7805 etc etc etc

Ewww, I am hoping I can get away without all of this!


> Could I also suggest IrfanView to make gif images instead of jpeg.
> gifs are so much better for circuit diagrams

Sorry, I have corrected this, and thanks for your time...
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2004\07\19@230230 by Jinx

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> Caps are not a problem, although the hobby budget isn't
> unlimited (don't we all wish it was) it can stretch a few cents
> to make it right

Have a look out for discarded appliances and strip them for parts.
VCRs are full of usable components like caps, so are monitors.
Service centres, inorganic rubbish collections, dumpsters..... I've
got many drawers full of parts that are so handy. Saves a lot of
petty cash and trips to the store for one or two components or
more exotic parts that normally would be difficult to buy OTS

> > be prepared for noise filtering in an automotive environment
>
> Ewww, I am hoping I can get away without all of this!

Automotive environments are notorious for being noisy. You may
be lucky, but you'd need to get a scope on the wiring to check it
out. I'm just saying that circuits take no notice of wishful thinking !

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2004\07\19@234001 by David P Harris

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Jinx wrote:

>>and the MAX232 part (again the first time)
>>
>>
>
>I usually have an extra 3 caps across 15/16, 2/16, 6/15. You can
>get a 232 version that needs fewer
>
>
Look at the polarities of these caps carefully -- easy to get them
backwards.

David

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2004\07\19@234418 by Martin Klingensmith

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Anthony Toft wrote:
{Quote hidden}

You don't have a resister on the base of the transistor. 10k would do.
You need two more capacitors one each on your MAX232 VS- and VS+ lines.

Good luck!

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2004\07\20@062449 by hael Rigby-Jones

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{Quote hidden}

The first thing I see is the transistor you are using to sense the panel
lights (Q2).  You have no current limiting resistor in the base circuit, so
the the 2N2222A will fry the first time the lights are switched on.  It's
good parctice to have a resistor pulling the base to ground as well, and if
you are feeling really paranoid a diode to prevent reverse breakdown of the
BE junction from any negative spikes.  Can't be too carefull in automotive
environments!


           10k          |/
Panel  ----/\/\/--+---+--|  Q2
Lights            |  _|_ |\>
                 \  /_\   |
             10k /   |    |
                 \   |    |
                 |   |    |  0v
                 +---+----+---

I would also suggest some filtering and transient suppresion on the input to
the voltage regulator.  A 7805 will be toast above around 35volts on it's
input, and >35volt spikes are commonplace in car electrics.  A simple series
resistor and capacitor to ground will go a long way to damping out most
nasties, but idealy a fast transient suppresor (e.g. "Transorb") would be a
good idea.  The resistor value would need to be deterimed by you maximum
current draw to ensure the voltage on the input to the 7805 does not fall
below the drop out voltage (around 7-7.5 volts IIRC).

I notice you have the PGM pin (RB3) connected to your programming header.
If you are intending to use low voltage programming then 1) you won't be
able to use the pin for anything else, and 2) you need to put a pull down
resistor on the pin.  If you don't the floating pin will put the PIC into
programming mode randomly.

Is AN0 the only analog input you are using?

Regards

Mike


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2004\07\20@064149 by Spehro Pefhany

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At 03:02 PM 7/20/2004 +1200, you wrote:

>Automotive environments are notorious for being noisy. You may
>be lucky, but you'd need to get a scope on the wiring to check it
>out. I'm just saying that circuits take no notice of wishful thinking !

Have a look at the roundup of 7 different transient types in an automotive
environment in this powerpoint presentation:

http://www.engin.umd.umich.edu/ceep/tech_day/2003/slideshows/ppt/2003PresentationShen.ppt

The voltage, duration and ENERGY involved are given.

This sort of thing will likely KILL or disrupt your circuit if you don't
design for it.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
TakeThisOuTspeffEraseMEspamspam_OUTinterlog.com             Info for manufacturers: http://www.trexon.com
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2004\07\20@075738 by Anthony Toft

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On Tue, 2004-07-20 at 06:26, Michael Rigby-Jones wrote:
> The first thing I see is the transistor you are using to sense the panel
> lights (Q2).  You have no current limiting resistor in the base circuit, so

Done, thanks!

> I would also suggest some filtering and transient suppresion on the input to
> the voltage regulator.  A 7805 will be toast above around 35volts on it's
> input, and >35volt spikes are commonplace in car electrics.  A simple series
> resistor and capacitor to ground will go a long way to damping out most
> nasties, but idealy a fast transient suppresor (e.g. "Transorb") would be a
> good idea.  The resistor value would need to be deterimed by you maximum
> current draw to ensure the voltage on the input to the 7805 does not fall
> below the drop out voltage (around 7-7.5 volts IIRC).

The 12v power is coming from the PC, not the car, the PC is powered
through an inverter so I think it's pretty isolated from the nastyness.
if the inverted looses it's marbles, a few components will be the least
of my worries (the PC in the trunk will probably fry too). Other than
this, the only thing I am using the car's power for is to sense what the
driver is trying to do (lights on == reduce display brightness)

> I notice you have the PGM pin (RB3) connected to your programming header.
> If you are intending to use low voltage programming then 1) you won't be
> able to use the pin for anything else, and 2) you need to put a pull down
> resistor on the pin.  If you don't the floating pin will put the PIC into
> programming mode randomly.

This is HVP only, I am using the Wisp628 programmer, and it is
recommended to ground the LVP line when programming.

> Is AN0 the only analog input you are using?

Yes sir!

Thanks for the assistance to everyone who has commented, this is going
to be very cool when I am finished.
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2004\07\20@081849 by hael Rigby-Jones

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{Quote hidden}

That's fine, but you should still have a reasonably large decoupling cap on
the input to the regulator, especialy if the power wires are relatively
long.  The 7805 datasheet will give the exact recommendations, but a
10-100uf cap should do the job.  Without this the regulators can oscillate
due to the inductance of the supply cables.

>> Is AN0 the only analog input you are using?
>
>Yes sir!

Ok, was just making sure!  Good luck, let us know how it goes!

Mike

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2004\07\20@095749 by Howard Winter

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Anthony,

My level of experience isn't such that I can contribute any suggestions, but...

On Tue, 20 Jul 2004 07:56:49 -0400, Anthony Toft wrote:

> The 12v power is coming from the PC, not the car, the PC is powered
> through an inverter so I think it's pretty isolated from the nastyness.

Is this a "True sine wave" inverter?  I have heard mixed reports as to whether a PC power supply can be run on
a non-sine-wave invertor, so I'd be interested to hear your results, and whether it's a true sine / modified
sine / square wave invertor, please?

Sorry to hijack your thread!

Cheers,

Howard Winter
St.Albans, England

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2004\07\20@113055 by Anthony Toft

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> sine / square wave invertor, please?
>
> Sorry to hijack your thread!

No problem, it is a modified sinewave deal, it looks like 2 square waves
superimposed on each other, and it works great! Mine was about 35USD a year ago
350W continuous (it was the cheapest I could find) it's a godsend too, I use it
for other things around the car (lead light, soldering iron) as my apartement is
about 50ft from the parking lot.

Overall a very useful piece of kit.

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2004\07\20@121249 by Spehro Pefhany

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At 11:31 AM 7/20/2004 -0400, you wrote:
> > sine / square wave invertor, please?
> >
> > Sorry to hijack your thread!
>
>No problem, it is a modified sinewave deal, it looks like 2 square waves
>superimposed on each other, and it works great! Mine was about 35USD a
>year ago
>350W continuous (it was the cheapest I could find) it's a godsend too, I
>use it
>for other things around the car (lead light, soldering iron) as my
>apartement is
>about 50ft from the parking lot.
>
>Overall a very useful piece of kit.

I've used mine to power my trusty WTCP solder station to modify 100
transformers near an airport in another city- in my car- the stuff came
in and went out within hours. It would have been another day to bring it all
back. I often use it to power chargers and other low power devices in the
car, for example to charge NiMH batteries for my digital camera, recharge
my camcorder or run/charge a laptop computer.

I was at a kids' baseball tournament on the weekend and saw a guy with
a minivan who had a bar fridge in the back hooked up to a 350W inverter!
He said it worked great. A compressor is a tough load.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2004\07\20@135638 by Dave Lag

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A real one?  not one with heater/ammonia cycle?

>I was at a kids' baseball tournament on the weekend and saw a guy with
>a minivan who had a bar fridge in the back hooked up to a 350W inverter!
>He said it worked great. A compressor is a tough load.
>
>Best regards,
>
>Spehro Pefhany --"it's the network..."            "The Journey is the reward"
>RemoveMEspeffspam_OUTspamKILLspaminterlog.com             Info for manufacturers: http://www.trexon.com
>Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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2004\07\20@155455 by Spehro Pefhany

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At 01:56 PM 7/20/2004 -0400, you wrote:
>A real one?  not one with heater/ammonia cycle?

It just looked like a regular inexpensive bar fridge, not an RV fridge,
if that's the kind you're talking about. And it was plugged into the
120VAC inverter.

Best regards,

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