If I'm reading the datasheet correctly, the input pins only draw about 1
microampere, which just isn't significant and shouldn't have any impact on
the ignition electronics.  I would at least put a 1k resistor in series
though, but mostly for protection from being handled before and during
installation.

Brett

On Wed, Dec 4, 2013 at 9:07 PM, James <jk_eaton@hotmail.com> wrote:

> The actual Hall Effect sensor doesn't connect the circuit to ground; the
> "Hall Effect" is about the generation of a varying voltage when an magnetic
> field near a coil is disrupted by the passage of a metal surface or blade.
> However, since the Hall Effect sensor drives a transistor (at least) inside
> the Hall Effect sensor package, that transistor can be a 'sinking output'
> or 'open drain' (boy, haven't heard that phrase in a decade) that connects
> to ground.
>
> Sorry, electronics geek coming out.  Most of us don't separate out the
> Hall Effect sensor from the actual package it's part of on the
> distributor.  But it's why some kinds of Hall Effect sensor (packages)
> connect to ground, and others supply drive signals or drive current.
>
> Maybe over Christmas I can get some readings for us.  Having an op-amp in
> the input is just going to be generally a good idea, for the longevity of
> the sensors in our Vanagons.
>
> James
> Ottawa, ON
>
> > Date: Wed, 4 Dec 2013 20:07:02 -0800
>
> > From: brettn777@GMAIL.COM
> > Subject: Re: Arduino and Vanagons
> > To: vanagon@GERRY.VANAGON.COM
> >
> > Yah, whenever I take a measurement with a microcontroller, I take several
> > readings and average them. In this case, I measure the time in
> > milliseconds for 8 pulses and then calculate the engine speed. By
> > unstable, I mean the rpms were mostly jumping around 1300-1800 and
> > occasionally over 2k. My ear and the tach on the dash agree that the
> speed
> > was pretty steady at about 1100.
> >
> > I drug out my laptop & USB scope today and got a quick reading. Here is a
> > link to a screenshot:
> > https://dl.dropboxusercontent.com/u/4930890/testvan1.bmp
> >
> > Kind of a hybrid square/sawtooth wave form. The peak voltage is 2.82 v,
> > while most of the flat part is around 2.4v. For the Arduino, that puts
> the
> > entire peak region below the minimum 3v required to guarantee a positive
> > reading. I'll probably have to add an op-amp to buffer and raise the
> > voltage a bit. I was in a hurry & now wished I had stretched out the time
> > scale to see the noise on the peak region better. If that noise is right
> > near the pin's high/low voltage reading cutoff, it could be causing
> > multiple readings and throwing my measurements way off. If I run a
> voltage
> > doubler using an op-amp(with high rail of 5v), the entire peak would be
> > over 4v, and then trigger on the drop to zero, I should get a pretty
> clean
> > signal to work from.
> >
> > The duty cycle is at 70%, so the shorter off cycle corresponds with the
> > smaller gaps in the slotted distributor shield thingy, which lends
> credence
> > to
> > Ryan's remark that most Hall sensors are open drain. That means that the
> > Hall sensor connects the circuit to ground (drain) when in the presence
> of
> > a magnetic field, but opens the circuit (disconnects it) if there is no
> > magnetic field. The electronic ignition feeds it a small amount of
> current
> > at 5v, and when the slot rotates near the Hall sensor & exposes it to the
> > magnetic field, the sensor connects that tiny current to ground, dropping
> > the voltage level to zero.
> >
> > I'll try adding an op-amp to the circuit in the next couple of days & see
> > if I can get a reliable tachometer out of it.
> >
> > Also, there is no guarantee that a vanagon Hall sensor has the same peak
> > values as the VW Fox Hall sensor that I'm using, so it would be really
> > great if someone could get a scope reading on an actual Vanagon engine to
> > compare.
> >
> >
> > Brett
> >
> >
> > On Wed, Dec 4, 2013 at 3:47 PM, Jim Akiba <syncrolist@bostig.com> wrote:
> >
> > > Also keep in mind the disty output is very unstable just because of
> > > the slop in the gear drive/powerpulses as it's rotating. I saw the
> > > same thing when we built our distributor replacement (replaced the
> > > stock hall sender with a better square wave output hall sender) and we
> > > saw this slop in the readings. Our pickup was given 72 pulses per rev
> > > of the disty so I had lot's of granularity to see it and it needs some
> > > filtering to be used for any real accuracy (like how we were using it
> > > for fueling and spark control). But just rpm readings though you
> > > should be good with some averaging.
> > >
> > > Jim Akiba
> > >
> > > On Wed, Dec 4, 2013 at 4:09 PM, Craig C Forney <craig@opus.com> wrote:
> > > > David is right. Multiply pulses per second times 30 to get engine
> RPM.
> > > 30
> > > > pulses is 900 RPM.
> > > >
> > > > Target Vanagon idle RPM is 880 plus or minus 50, which is 29.33
> pulses
> > > per
> > > > second plus or minus 1.67 pulses.
> > > >
> > > > ICUs working properly typically perform at 850 to 910 RPM, with most
> > > > performing at 860 to 900.
> > > >
> > > > Craig in Cupertino
> > > >
> > > >
> > > > At 11:33 PM 12/3/2013, James wrote:
> > > >>Distributor rotates at half crankshaft speed.... idle is 850 rpm....
> > > >>half is 425 rpm... one Hall Effect signal per crankshaft rotation (
> or
> > > >>is that two per rotation...??).... nah, I'd better try to calculate
> > > >>this tomorrow!
> > > >
> > > > One pulse per (piston @ TDC** on firing stroke) so two per crankshaft
> > > > revolution.
> > > >
> > > > 850 rpm / 60 secs/minute = ~14 rev/second x 2 pulses/rev = ~28
> > > > pulses/second.
> > > >
> > > >
> > > > ** Really at the fixed timing advance point which is a few degrees
> before
> > > > TDC on the 2.1l.
> > >
> >
> >
> >
> > --
> > Brett in Portland, OR
> > "Albert" '82 VanaFox I4 Riviera
>

--
Brett in Portland, OR
"Albert" '82 VanaFox I4 Riviera