Air/Fuel Ratio Meter Question

Discussion in 'The Projects Forum' started by gandalf0531, Sep 12, 2008.

  1. gandalf0531

    Thread Starter New Member

    Aug 26, 2008
    4
    0
    I am just now learning a little about electronics, and want to build this A/F ratio meter. I have read the data sheet for the LM3914, and trying to understand how to vary the range. Is the 680Ω at R4 what determines the range of the meter, in this case .01 to1.0 v.

    Thanks!!
     
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    I believe it is pot 1 which sets the range, R4 determines the LED current.
     
  3. gandalf0531

    Thread Starter New Member

    Aug 26, 2008
    4
    0
    Thanks for the reply!! Also, the voltage from the O2 sensor runs from .01v to1.0v. Is 1 meg (R2) not an awful lot of resistance for such a low voltage, or am I misunderstanding?

    Thanks again!!
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Not at all. The high resistance insures that the sensor output voltage will not be pulled down by having to supply too much current to the 3914.
     
  5. gandalf0531

    Thread Starter New Member

    Aug 26, 2008
    4
    0
    Ah, I understand!!! Like using a resistor in an LED circuit.

    Thanks again!!
     
  6. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,440
    368
    I'm not convinced about the need for the 1M series resistor, the input to the 3914 will have a very high (Megohms) impedance since it's the input of an op-amp.

    I have a Megasquirt ECU on my car which was consuming O2 sensors and I could see no reason why. I added an op-amp buffer between the O2 sensor and ECU (with an LED which flashes when the sensor is working correctly) and since then everything has been OK. The buffer has an input impedance of 1M.
     
    Last edited: Sep 13, 2008
  7. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    R2 and C2 are a lowpass filter to prevent the reading from jumping too much. The O2 sensor has an oscillating output that needs a lowpass filter in order to show an average reading.

    If your mixture is too lean (are you fooling with the hydrogen-oxygen electrolysis scam?) then the valves, pistons and O2 sensors will get burnt.
    If you have a turbo then your fuel pump or its wiring are too small. The lean mixture will also burn the turbo.
    You will have too much pollutant nitrous oxide in the exhaust and it won't pass an emissions test.

    Have you noticed the stink of pollution coming from little kids' Honda little cars without a muffler, with blacked out glass, lowered and with an added turbo??
    They get the emissions tested online (just a payment, no test).
     
Loading...