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Air/Fuel Mixture Meter

Discussion in 'Electronic Basics' started by JazzMan, Apr 20, 2005.

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  1. JazzMan

    JazzMan Guest

    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
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  2. John

    John Guest

    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
     
  3. John

    John Guest

    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
     
  4. John

    John Guest

    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
     
  5. Chris Dugan

    Chris Dugan Guest

    John,

    high input impedance means that as far as the sensor is concerned it sees a
    high resistance from whatever it is connected to. If you connect that O2
    sensor to a low impedance circuit you reduce the output voltage, this can
    affect the operation of the cars ECU as that now sees a lower than expected
    voltage and so compensates by altering the fuel/air mixture.

    There are several high input impedance circuits out there on the net you can
    use, you can buy high impedance single rail supply op-amps which would be
    ideal for this circuit. You would use one to buffer the circuit (and so
    prevent interfering with the cars ECU operation) and then another (normal
    op-amp) as a non-inverting amplifier to boost the signal before feeding it
    into the digital meter. Although you could also do this by butchering a high
    imput impedance DVM!

    Chris
     
  6. John

    John Guest

    Yeah, it's making sense now. I think you said pretty much what I said the
    post after. So that's definitely a good thing :)
    Could you give a little more detail as to how a high impedance buffer works
    and what components/ideas are behind it?

    Based on your description, I think that's exactly what I need. So basically
    I tap into the O2 sensor signal somewhere between the O2 and the ECU, and
    use that as the input to the buffer? As far as the display goes, how would I
    convert that tiny voltage/current(say 0.50mV) to display as 500 on a digital
    display? And what display would work for this meter/gauge gadget I'm working
    on?

    Thanks!

    John
     
  7. Chris Dugan

    Chris Dugan Guest

    John

    Try this link it gives a pretty good overview about op-amps and their uses:

    http://www.answers.com/topic/operational-amplifier

    The specific sections you want are voltage follower, difference amplifier
    and non-inverting amplifier. The voltage follower acts as a buffer and
    provides no signal amplification (called gain) or you can use a difference
    amplifier circuit. Which would give you a variable output depending on the
    difference in voltage between the two inputs (this can be an advantage in an
    electrically noisy environment such as a car as it means that any common
    noise signal in both the ground and signal wire gets ignored and your
    voltage reaching your display will be more stable).

    The final part is to use a non inverting amplifier to boost (or multiply by
    the gain factor) the input signal: e.g for a gain of 6 your 0-2v sensor
    voltage would vary between 0 and 12V provided the op-amp can push the output
    to the power supply limits (not all of them can). You're probably better off
    staying with a 2V range and using a digital panel meter calibrated for a max
    reading of 2V then allmost all your resistors in the amplifiers will be the
    same value. It's better to keep the signal going to the display at a
    reasonable level rather than reducing to your 500mv as noise will tend to
    drown out the signal at that level (think of an old audio amplifier - the
    noise level is pretty much the same and at low volume levels it almost
    drowns out the music).

    For the 3 Red, Green, Yellow LED's you can use three op-amp comparators all
    reading the same signal and just referenced to different voltages. So that
    op-amp 1 turns on at 0V op-amp2 at 0.2V etc or you can get a bar graph chip
    (LM3916 from National Semiconductor) and connect LED's only to 3 outputs,
    calibrating the chip so the LED's are at the switching points.

    For any further circuits just do a google for op-amp schematics or have a
    look at a few manufacturers website for application notes, you'll then learn
    about decoupling capacitors and voltage regulators which you'll need as
    well.

    As for specific devices to use everybody has their favourites; National,
    Maxim etc. and if I told you everything to do there'd be nothing for you to
    work out for yourself would there ;-) Half the fun with electronics is
    learning from your mistakes and learning to keep the magic smoke inside the
    components!

    Chris
     
  8. steamer

    steamer Guest

    --FWIW there was a neat widget for sale in gag shops a few
    years back called the "Fart detector". It consisted of a battery
    powered heated grid; I think the technical term for this is a hot pipe
    anemometer. Anyway, when confronted by something other than air the
    cooling rate of the device would change; this condition caused a
    recording to play. Accompanied by siren whoops it would shout:
    "Warning! Fart detected!" I extended the probe wires and stuck one
    inside the combustion chamber of a potato gun, trying to sense when the
    mixture was right for firing. Results not great, but it made for an
    interesting project, heh.
    --The trick is not so much to detect a mixture, but to detect
    the *right* mixture. Still workin' on that one...
     
  9. Jeroen

    Jeroen Guest

    Maybe this page is of interest to you.

    http://www.ronsoft.nl/index.php/153

    Jeroen
     
  10. JazzMan

    JazzMan Guest


    Here's a kit that I'm contemplating buying:

    http://wbo2.com/

    Comes highly recommended.

    JazzMan
    --
    **********************************************************
    Please reply to jsavage"at"airmail.net.
    Curse those darned bulk e-mailers!
    **********************************************************
    "Rats and roaches live by competition under the laws of
    supply and demand. It is the privilege of human beings to
    live under the laws of justice and mercy." - Wendell Berry
    **********************************************************
     
  11. John

    John Guest

    Excellent! Thank you!

    That's what I was looking for--nice explanation with a schematic.

    John
     
  12. dan

    dan Guest

    Here's a better question... Whats the point? A standard narrow band
    oxygen sensor is next to worthless for giving you mixture readings. They
    have part/part variation. They vary with age and temperature (which is
    constantly fluctuating). They are highly non-linear, and they exhibit
    hysteresis. In otherwords, they read "lean", "rich", and maybe if you
    are lucky stoichiametric. But under normal running conditions the ECU
    will keep the AFR bouncing between rich and lean. The only exceptions
    to this are when the car's cold (at which point the sensor isn't working)
    or you are at wide open throttle, in which case presumably you are running
    rich but you can't know how rich. Anyone who tells you differently is
    trying to blow smoke up your backside. As for your "device", why not just
    use a cheap digital multimeter? That should do exactly what you want, and
    it's probably cheaper, more versatile and better packaged.

    dan
     
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