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'Zero Crossing Detector Reposted'
1998\06\27@103657 by

Thank you all for the valuable advise on my project. I think that there are a
few points that I should clarify. I am a hobbyist at electronics, a middle
school teacher by trade. I only took 2 semesters of electronics in college, so
when someone begins to argue about theorems and laws, I'm simply lost.
My goal is to develop an ignition system for my old truck that incorporates a
PIC controlling the timing curve and coil charging.I am aware that this
project has been done several times over, but the learning experience of
building from scratch is what I'm interested in.

Clarifying a few points about the pickup and zero crossing.
The variable reluctance pickup produces a sine wave pulse every time a tooth
on  the distributor shaft rotates in and out of the pickup's magnetic field.
There are four teeth spaced 90 deg apart. I get 2 pulses per rev of the crank
and 4 per rev on the distributor. The frequency and P to P voltage of the
pulse increases proportionally with an increase in crankshaft. The pulse goes
high as the tooth approaches, falls to zero at the center and goes negative
until the tooth leaves the magnetic field. The output V at cranking rpm is
about 2v P to P and about 12V at redline. My goal is to produce an interrupt
at the PIC when a tooth aligns with the center of the pickup. It must also be
highly immune to emf noise.
I'm considering simply running the wave through a germanium diode rectifier
and watch the trailing edge of the first pulse. When the wave falls, the slope
is nearly vertical, getting me close to the zero crossing.

What does everybody think?
Bruce

Bruce Turrentine wrote:

>  I'm considering simply running the wave through a germanium diode
> rectifier and watch the trailing edge of the first pulse.

Think not in terms of Germanium diodes.  They are obsolete, and for
good reason (especially in the heat of an automotive environment).

> When the wave falls, the slope is nearly vertical, getting me close to
> the zero crossing.

It'll be just as good with a silicon diode.  But what's the diode for?
Feed your reluctor through a resistor (mmm, about 33K) into the base of
a transistor, and put a silicon diode in reverse across the base (to
emitter which of course is grounded as is the other reluctor terminal).
Put a collector load resistor to the supply (5V) and you should have a
rising waveform when the reluctor output goes negative.

Cheers,
Paul B.

You are about to enter the real world.

If I was doing it, had the time and equiptment, and enjoyed it, I
would first
use a scope on the pickup at various speeds and see what the
pulse
really looked like. (don't know if it would be wise to hook one
up to
a running engine though).  Then maybe load the pulse down with
different
values of resisters to see what effect a load would have on the
pulse.

Having this information, I would build a digital to analog
circuit and control it
with a PC.  Using this test circuit, test the various circuits
for zero crossing
detection.  And maybe even distort the output from the d to a
some just
to see how the detector would handle it.

Then after the pic is programmed, use the same setup to test the
pic's
program.  Then after all this and if (and only if) the value of
the antique truck
hasn't increassed to the point that you would loose money by
installing
a non stock item in the truck, I would instal it.

But then I allways have had big dreams that I never carried out.
Does
anyone need 27 tons of river rock?

Bill C.   billcornutt.com

As noted in another reply, a diode may well not be necessary. However, if
your circuit needs one, consider using a Schottky diode. Vfwd is small
(similar to germanium). Beware their typically low reverse voltage (see
relevant spec sheet) - excess reverse voltage mya destroy them depending on
the circuit.

Also, you say that

>My goal is to produce an interrupt
> at the PIC when a tooth aligns with the center of the pickup. It must
also be
> highly immune to emf noise.you wish the trigger point to occur

Unless you need this point because it corresponds to some maximum energy
condition (as might be the case with eg a magneto circuit (which is
unlikely to be what you are using)) you should be able to have the trigger
point occur at any position relative to the centre of the tooth, providing
it did so reliably and consistently. This would free you up from having to
force the circuit to work at a point which may not be the most obvious one.
All distributors that I have seen (and I certainly haven't seen them all)
give you near total control over the relative position of the firing point
and the actual crankshaft position. Hopefully you will be able to alter the
distributor position as required.

I haven't followed all this thread - it has probably been pointed out that
if you apply a voltage from a coil across a resistor across the inputs of a
comparator (with suitable attention to device limits, a few resistors will
be needed), then the comparator will switch VERY close to the zero crossing
point.

----------
> From: Bruce Turrentine <TurrbieAOL.COM>
> To: PICLISTMITVMA.MIT.EDU
> Subject: Zero Crossing Detector Reposted
> Date: Sunday, 28 June 1998 02:34
>
> Thank you all for the valuable advise on my project. I think that there
are a
> few points that I should clarify. I am a hobbyist at electronics, a
middle
> school teacher by trade. I only took 2 semesters of electronics in
college, so
> when someone begins to argue about theorems and laws, I'm simply lost.
> My goal is to develop an ignition system for my old truck that
incorporates a
> PIC controlling the timing curve and coil charging.I am aware that this
> project has been done several times over, but the learning experience of
> building from scratch is what I'm interested in.
>
> Clarifying a few points about the pickup and zero crossing.
> The variable reluctance pickup produces a sine wave pulse every time a
tooth
> on  the distributor shaft rotates in and out of the pickup's magnetic
field.
> There are four teeth spaced 90 deg apart. I get 2 pulses per rev of the
crank
> and 4 per rev on the distributor. The frequency and P to P voltage of the
> pulse increases proportionally with an increase in crankshaft. The pulse
goes
> high as the tooth approaches, falls to zero at the center and goes
negative
> until the tooth leaves the magnetic field. The output V at cranking rpm
is
> about 2v P to P and about 12V at redline. My goal is to produce an
interrupt
> at the PIC when a tooth aligns with the center of the pickup. It must
also be
> highly immune to emf noise.
>  I'm considering simply running the wave through a germanium diode
rectifier
> and watch the trailing edge of the first pulse. When the wave falls, the
slope
> is nearly vertical, getting me close to the zero crossing.
>
> What does everybody think?
> Bruce

[snip]

>  It'll be just as good with a silicon diode.  But what's the
diode for?
{Quote hidden}

'Zero Crossing Detector Reposted'
1998\07\01@084901 by
Has someone tried this zero crossing detector:

Power Xformer
secondary       1M               1M
+--uuuu---O-------/\/\/\-----+-----/\/\/\-----O + 5V (PIC Vdd)
|         |                  |
===        v                  v

To rectifier       To PIC pin

This has the advantages of good protection, simplicity and rather accurate
zero detection even with low transforemer voltages. It can also be used
with a center-tapped xformer.

anyone ?

Peter

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