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'[PIC]: magnetic coil input to PIC'
2001\02\16@072546 by Roman Black

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Just a quickie, has anyone connected a magnetic
coil pickup, as the typical sensor used in car
speedos, to a PIC??
-Roman

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2001\02\16@095549 by Chris Eddy

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Roman;

The challenge is to discern the low amplitude sine wave at low speeds.
Most folks want to pickup to as low as 5 or 10MPH.  The cleanest solution
is to run the signal into a comparator.  Asuming a 12V supply, generate a
half supply voltage with a divider and buffer it.  Use this 6V to drive
one side of the mag pickup.  The return line goes to the comparator.  Of
course, the other pin of the comparator goes to the 6V line.  Tie the mag
return to the 6V line with both a resistor (to pull it to center) and
capacitor (not big.. just to crush EMI).  At higher speeds, the signal
may get pretty big.  In this case, clamp the return line to the +12 and
ground lines with diodes.  You must add a bit of series resistance to act
as burden.

Just sos you don't wind up camping out in the driveway with a cot, I
suggest that you run some tests on the signal and characterize it
entirely, including source impedance.  Then you can retire to the safety
of the lab.

Play with these values until it works just the way you want.  If you use
a comparator such as LM311 or equal, you can just pullup the output to
VCC and get a 5V signal.  This ought to satisfy the RB0 pin very nicely.

Chris~

Roman Black wrote:

> Just a quickie, has anyone connected a magnetic
> coil pickup, as the typical sensor used in car
> speedos, to a PIC??
> -Roman
>
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2001\02\16@221403 by Roman Black

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Thanks Chris for the good info! Running tests
and gathering info on all the different vehicles
is just about impossible. I have a comparator
design in the works, but not sure if it justifies
the size or cost. I was really hoping someone
would have actually done this and knew the
expected signal level from most standard auto
sensors or something.

I also may not have the luxury of accessing both
wires to the coil pickup, it will probably be
limited to just tapping into the "out" wire
from the pickup as the other wire is usually
12v (or gnd).
-Roman


Chris Eddy wrote:
{Quote hidden}

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2001\02\16@235626 by Charles Morgan

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At 11:20 PM 2/16/01 +1100, Roman wrote:
>Just a quickie, has anyone connected a magnetic
>coil pickup, as the typical sensor used in car
>speedos, to a PIC??

Roman, I needed to build my own Vehicle Speed Sensor for an engine swap.  I
originally used a magnetic coil pickup driven by the speedometer cable.
The coil fed a Harris HIP9020 "Programmable Quad Buffer with Pre and Post
Scaler Dividers".  It's a zero-level detector and limiter made specifically
for such applications.  I then used used a 16F873 to adjust the frequency
to that required by the vehicle computer.  Unfortunately, the output of the
coil is useless until a minimum speed is reached.  I replaced the coil
pickup with a Hall sensor that I mounted in the differential cover and
triggers whenever one of the ring gear teeth passes it.  The Hall Sensor
feeds directly into the PIC.  It provides a usable signal down to about 0
mph and is much cleaner.

Charles Morgan
P.S.  If you're stuck with the magnetic coil pickup and can't find a
HIP9020, you can also use a National LM2907/2917 as a coil-to-PIC buffer.

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2001\02\17@090841 by Roman Black

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Charles Morgan wrote:
{Quote hidden}

Thanks Charles! This is the situation I'm in. Most
of the motorcycles use a hall sensor with a nice
squarewave output. We are trying to expand our range
to suit the rare bikes (and MOST cars) that use
a coil sensor.

I can build a front end amp and use some type of
zero corssing detector, but obviously it would be
nice to know what average ac voltages these coils
produce in most normal cars. I was sort-of hoping
they would produce a few volts so I could just
connect it to the PIC with a diode and simple
filter. Adding an amp will mean a new PCB manufacture
which is an added expense...
-Roman

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2001\02\17@110555 by ken

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Roman,
I've just finished playing around with serveral
methods of hooking up a pic to vehicles VSS.
I'm know EE by any means but here's what I came across.
I started by making my own vehicle speedo with LCD
output.

The sensors I played with only produced a few AC volts.

Trying to use cheapest, least amount of parts
with op amp, diodes, resisters, etc. to create a zero
crossing config I could never get to work (maybe someone smarter
could) Tried configs with serveral diferent op amps etc.

Main problems;
Noise, Noise, Noise.
Mostly ground noise causing a lot of chatter.
(yea, I know what your thinking, good signal filter,
caps, offsetting, etc., thats what I thought too)
So you will end up trying to create multi-level
op amp noise filters, tried several different zero crossing
circuits, good luck. I had none.

Also since I was taping into the wires directly
while still in use for vechiles computer,  impedance,
drawing to much current etc. problems.
Vehicles computers are very sensitive.
To isolate you start increasing inline resistance
and lose to much for your op amps, less restance and you start
screwing up you vehicles computer, etc.

The only solution that ended up working (for me)
was also mentioned by charles, using a zero
crossing device, ie LM2907/2917
these ic's are very signal sensitive, very configurable
as far as shaping your signal output and allow offset.
With alittle tweaking these chips turned out quite reliable
signals.

I would suggest a bench set up to start.
I purchased a VSS sensor from my local ford
dealer for $14.00 added a cheap dc motor $3.00
a washer with a notch cut in it and a pot for variable speed.

Just beware, when all looks great and you move to
in vehicle testing it falls apart. Noise, Nose, Noise.

I'm now working on putting together a portable hand unit
to start documenting VSS signals from different vehicles.
I had no luck finding such info. If you or anybody does, I sure would
be interested also.


{Quote hidden}

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2001\02\17@121357 by rottosen

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Roman:
I haven't tried it but you may be able to rework a board for something
like this:

                               +5V
                               |
                               /
                               \
                               /
                               \
                               /
                               +------------ To PIC input
                               |
                              /
                            |/
_________||____/\/\/\_______|    2N3904
(         ||            _|_  |\
( coil                  /_\  | V
(                        |       \
(_________||_____________|_______|
         ||                     |
                                V


-- Rich


Roman Black wrote:
{Quote hidden}

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2001\02\18@042338 by Roman Black

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Wow Ken, another very helpful post. You had that
much difficulty with noise??

Our current product is popular with bikes, but
these normally have 3-wire hall sensors. It does
all the work for you! :o)

I haven't had any experience with the coil type
speed sensors, but really didn't expect to hear
that others had found so many problems. The
situation i'm in is trying to adapt our original
board to suit the coil sensors. I could add a
couple of discrete parts on the bottom of the
board but adding a new chip means a new board.

Do you have any info on what voltage these things
normally put out? Obviously I expect V to increase
with speed as the magnet passes the coil at higher
velocity.

Even just an idea of 1v p/p, or 2v p/p or any
"ballpark" figure will help me a lot. I can't
really start choosing a design method without
that figure. :o)
-Roman

PS. If I get a good simple system I will share
it with the list. I'm hoping I can find a one-
transistor solution, etc...



Kenneth Godee wrote:
{Quote hidden}

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2001\02\18@051019 by Roman Black

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Richard Ottosen wrote:
{Quote hidden}

Thanks Richard, I think most of the vehicles
have one side of the coil already connected to
12v or Gnd. I really need to find this out
first...:o)
-Roman

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2001\02\18@200754 by rottosen

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Roman Black wrote:
{Quote hidden}

The coupling caps were just in case the grounding wasn't known from one
use to the next.

It occured to me that you might want to bias the transistor just short
of conducting to reduce the threshhold of the circuit to less than the
base-emitter drop.

Better yet, maybe bias the transistor slightly on to do a DC restore
circuit similar to a TV sync stripper. The cap is already there anyway.

Do remember that whatever scheme you use, the low frequency bandwidth is
better with a low impedance input. Having the pickup coil looking into
the virtual ground input of an opamp is ideal, bandwith wise.


-- Rich

p.s.  By the time you do all of the above, maybe you are really better
off using one of the 5 pin surface mount opamps or some such.



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2001\02\18@202139 by David VanHorn

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>
>Do remember that whatever scheme you use, the low frequency bandwidth is
>better with a low impedance input. Having the pickup coil looking into the
>virtual ground input of an opamp is ideal, bandwith wise.

It kills self-resonance (by murdering the Q, usually a good thing) and
converts the op-amp from a voltage to a current amplifier.
Also, usually a good thing.


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2001\02\19@025516 by Drew Ames

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I don't know if it is of any use, but Silicon Chip (http://www.siliconchip.com.au)
published a Spped Alert/ Speedo project in Nov/Dec 99. Jaycar have a kit
for it KC5279.

This used a home wound coil to get pulses from magnets on the drive shaft.
It may not be greatly sophisticated but it works on my car.

At 22:50 16/02/01, you wrote:
{Quote hidden}

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2001\02\19@025934 by Mitch Stein

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2001\02\19@051945 by Roman Black

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Drew Ames wrote:
>
> I don't know if it is of any use, but Silicon Chip (http://www.siliconchip.com.au)
> published a Spped Alert/ Speedo project in Nov/Dec 99. Jaycar have a kit
> for it KC5279.
>
> This used a home wound coil to get pulses from magnets on the drive shaft.
> It may not be greatly sophisticated but it works on my car.

Thanks Drew but that doesn't help me much.
I need to adapt an existing speedo product
to work with other vehicles that have the
original coil sensors already fitted. :o)
-Roman

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2001\02\20@113131 by John

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Hello Roman & PIC.ers,

I've used the Allegro 3133  Hall-Effect part here, very successfully
in a product.
It's a 3-leg component with an open-collector o/p, schmitt triggered.

BUT...  the #5^&@?!  suppliers have suddenly decided to no
longer stock or import it.
This despite it being a magic part, does a very nice job,
very easy to interface a pic to, etc..

Can you suggest other makers I could try to
tap into for equivalent HE parts?



{Quote hidden}

best regards,   John

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2001\02\20@125346 by Roman Black

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John wrote:
>
> Hello Roman & PIC.ers,
>
> I've used the Allegro 3133  Hall-Effect part here, very successfully
> in a product.
> It's a 3-leg component with an open-collector o/p, schmitt triggered.
>
> BUT...  the #5^&@?!  suppliers have suddenly decided to no
> longer stock or import it.
> This despite it being a magic part, does a very nice job,
> very easy to interface a pic to, etc..
>
> Can you suggest other makers I could try to
> tap into for equivalent HE parts?


Just about all the motorcycles have a 3-leg hall part with
open collector output. It must be common. Try philips, hitachi,
toshiba, mitsubishi, etc etc. :o)
-Roman

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