Re: Air/Fuel Mixture Meter


"John" <welcomehowcome@xxxxxxxxxxx> wrote in message
news:wISdnXDx5aSUQfjfRVn-qw@xxxxxxxxxxxxxx
> "JazzMan" <No_Spam@xxxxxxxxxxx> wrote in message
> news:4265D4A7.1272@xxxxxxxxxxxxxx
> > John wrote:
> > >
> > > Hello All,
> > >
> > > I'm pretty new to electronics and would like to take this chance to
> learn
> > > more about electronics and at the same time create a useful gadget.
> > >
> > > I would like to create an A/F (air/fuel display). Here are some of the
> > > constraints, notes, and parts that will be used for this project.
> > >
> > > -- air/fuel mixture will be fed from the O2 (oxygen sensor) located at
> the
> > > header (it's for a 92 civic dx).
> > > -- O2 sensor provides 0-1V
> > > -- would like to use an LCD display of some type with 3 digits
> > > -- would like to display the actual voltage provided by the O2 sensor
in
> > > millivolts up to three decimal places
> > > e.g. O2 sensor reads 0.15V, I would like my A/F meter to display a
> > > reading of 150
> > > -- would prefer not use a PIC since I don't have a PIC writer (i can
> program
> > > in assembly)
> > >
> > > Later on would like to add three leds, one red, one green, one
yellow...
> > > red=volts<0.2V, green=0.8>volts>0.2, yellow=volts>0.8V... this would
be
> a
> > > quick glance indicator of what's the current status of the A/F mixture
> > > without having to read the actual display.
> > >
> > > I'm looking for any suggestions as to what components to use and how
to
> > > build this project. Also, any reading that might help me learn more
> about
> > > electronics while building this project would be helpful.
> > >
> > > Thanks!
> > >
> > > John
> >
> > Some issue to consider. Most O2 sensors are non-linear, which means that
> > the voltage they put out is not proportionate to the amount of O2 in
> > the exhaust stream. As a result, almost all cars using standard-type
> > O2 sensors use them as rich/lean switches rather than actually trying
> > to measure the amount of exhaust O2. Another issue is that the measuring
> > device need to have very high input impedence, otherwise there is a very
> > real chance of damaging the sensor permanently.
> >
> > You mentioned that it's a Honda? If it has the V-TEC engine then your
> > O2 sensor is of a type referred to as a Wide Band O2 sensor. This kind
> > of sensor is designed to measure O2 directly and within a fairly wide
> > range of fuel/air mixtures. This kind of sensor requires special control
> > circuitry because it's heated, among other reasons. It is a fairly
> > advanced project to try designing for a wideband sensor, it would be
> > much easier to just buy either a kit or an assembled unit.
> >
> > Back to the regular sensor. Outside of the stochiometric ratio of 14.7:1
> > where the O2 sensor voltage is 450mV the voltage is not really
> > meaningful.
> >
> > JazzMan
>
> It's not a wideband sensor. It's the regular type on a non-vtec engine
> (D15B7).
>
> Are there any good articles on Honda sensors or regular sensors in
general?
>
> Why doesn't the regular O2 sensor measure linearly? Also, would an A/F
meter
> like the one I'm planning for a regular O2 sensors be of any use? I think
it
> would definitely be an interesting project to work on even if it's not as
> accurate.
>
> Also, what do you mean by "having a high impedance input" as far as the
> meter goes?
>
> Thanks!
>
> John

Hmm.. high input impedance... I think I know what you mean. If the measuring
device has low impedance, the current might flow towards the sensor instead
of the metering device which might damage the sensor, is this what you mean?

Well, from my basic understanding of electronics, I was thinking I could
grab a very small sample from the sensor signal and amplify it with a
transistor at my meter side. So basically, I could place a high resistor
value in parallel with the signal which would drive the transistor and by
amplified for the meter to read and display. But then again, the transistor
will require at least 0.7V in order to open up (I think saturate is the
proper term here) which is pretty much what the O2 signal can provide. So
how do I go about making this work without inversing the flow of current and
damaging the O2 sensor?

Thanks!

John


.

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