Paul,

        I installed Electronics Int'l capacitive fuel probes in my Lancair.
        They output a PWM signal.  I wanted to use the Blue Mountain
    EFIS/One to
        display fuel quantity in each wing tank.  BMA says this won't
    work.  I
        tried using the two frequency channels on the EFIS/One and they seem
        work great...on the ground.  The problem is that in flight, the fuel
        readings gradually increase.  Once back on the ground, they will
    return
        to normal.  Needless to say, this isn't good.  Others are using a
        digital-to-analog converter for this task , connected to voltage
        channels on the EFIS, and report that they work fine.   Although
    they
        have reported some erratic readings when transmitting.

        I was curious if I could use the PICAXE to read the pwm signals and
        output a 0-5v signal to be read by the EFIS?   Or, is there a much
        easier way to accomplish the task?  I would appreciate your
    thoughts as
        my knowledge of electronics is a bit limited.


        Thanks,
        Mark S.
        14 hrs

        Well you need to find out why the readings are increasing.
        I suspect the probes. Long aluminum coaxial probes contract when
    cold
        and electronic devices can drift when the temperature changes.
    Where is
        the electronic device that converts the capacitance change of
    the probes
        to a PWM physically located. Best is in the cockpit where
        temperature changes
        are a minimum. What brand probes are you using?

        I read this stuff.


    http://www.bluemountainavionics.com/talk/archive/index.php/t-291.html

    <http://www.bluemountainavionics.com/talk/archive/index.php/t-291.html
    <http://www.bluemountainavionics.com/talk/archive/index.php/t-291.html>>

        Sounds like Greg wants a 0-to 5 volt signal.

        No problem using a PicAXE to read, display or condition the PWM data
        to a 0-5 volt signal but the D to A part would be the same as
        a rectifier (diode) and capacitor. The capacitor and resisters
    involved
        need to be temperature stable types. There is no direct digital to
        voltage (D to A) output on the PicAXE.

        It would be much better if Greg's (BMA) stuff took in a frequency or
        a PWM
        as then the system would be totally digital. A PicAXE could
    translate
        almost any frequency to almost any other frequency. It could also
        translated a PWM to a frequency or vice versa. I would like to
        discuss it
        with Greq but I don't want to "join his BBS" :) I don't require
        people to join
        here so he should also not require people to "join his BBS" :)

        Paul Lamar ...No rotor no motor.

    Paul,

    Yes, I'm on the BMA list.  In fact, one of the postings you linked to
    was from me... (Note: Bob is a BMA tech support guy)

    <snip>
    07-27-2004, 09:13 AM
    Did I miss it somewhere that the EI capacitance fuel probes can be
    hooked to the Hi-frequency channels on the BMA, without any
    interface/converter box? I have just spent many hours trying to figure
    out how to do this and I stumbled across this fact almost by accident.
    Seems that this information could/should have been included in the
    installation instructions. What I did get (see previous posting) was the
    statement, "EI probe transducers are PWM which is not supported by
    BMA."
    The EI probe comes with a small circuit that converts the signal to PWM,
    so I don't need another one, and the 0-5v PWM signal can be read by the
    BMA EFIS-1.
    ------------------------------------------------------------------------

    bob
    07-27-2004, 09:59 PM
    No Mark you didn't miss anything. You can't use the high freq channels
    to measure your fuel. You need freq to voltage converters that put out 0
    - 5 vdc and wire to the voltage channels.
    <snip>

    Maybe I should have followed Bob's advice and purchased two of the pwm
    to voltage converters?  My problem was that it was hooked up and
    working, or so it seemed.  It still works, if I could only figure out
    why the readings aren't stable except when on the ground.

    I had not considered the variation being related to temperature change.
    These are Electronics International probes which are pretty common with
    the Lancair group.  I haven't heard of anyone else having this
    particular problem with them.  They have a small IC incorporated into
    the short harness which is attached to the probe with two wires.  There
    are three wires out of the harness (red, black, & white).  I tried
    calling EI tech support to get some information on their probes, but
    they were less than helpful.  So, I attached a scope to the output
    (white) and it revealed that it is a pwm signal.   I partially submerged
    the probe into some diesel fuel (safer than gasoline) and the pwm signal
    changed.  So, I tried connecting it to the EFIS/One's frequency channel
    (normally used for fuel flow) and it read the signal, changing as the
    probe was submerged into the fuel, just like it should.  Once calibrated
    (in gasoline) and it is very accurate.  The only problem is the shift
    during flight.

    As for the location of the electronics, the IC that generates the pwm
    signal is in the wing root.  I can try wrapping it with insulation and
    see if it helps things.  If it does, then I will move it into the
    cabin.  The BMA EFIS/One box is under the back seat, about a foot from
    the left probe.  So, that would only require a short extension of the
    harness.

    Something else I hadn't initially considered is to eliminate the PWM
    circuitry from the probe and convert directly from capacitance to
    volts.  Would that be an easier task for the PICAXE to handle?  There
    are a number of 0-5v inputs on the EFIS box that I could use for that.
    A/C Spruce sells a converter (p/n 10-02159) for $50/ea that they say
    works on all plate type capacitance fuel senders.  Do you feel this
    would work?  I would have to add a BNC connector to my probes, but that
    should be easy enough to do.

    Thanks,
    Mark

    I think we have a definition problem here. Mark.
    There is frequency modulation and pulse width modulation.
    If you feed a pulse width modulated signal into a frequency
    input it will register but it won't work right.

    After looking at this chart we need to know if it is really
    pulse width modulation or if it is frequency modulation.

    Note also that voltages V1 and V2 can vary and not necessarily
    go to zero or to 5 volts. The voltages and all the other measurements
    can be done with an oscilloscope. Note also the time between pulses
    does not change in PWM. Consequently the frequency of a PWM signal
    remains constant.

    Now I need to know exactly what we are dealing with here as
    far as what the level probes are putting out.
    --
    Paul Lamar ...No rotor no motor.

    Mark Steitle wrote:
     > Paul,
     > I've attached four shots of the scope trace showing the output of
    the EI
     > fuel probes.  I hear what you're saying on frequency vs. pulse width
     > modulation.  That's why I was surprised when the EFIS registered
    change
     > as fuel was added.  I recently had to drain the left tank.  I was
     > curious to find out if the left gauge was still in calibration, so I
     > left the EFIS on while I drained the tank.  Sure enough, as soon
    as the
     > pump started sucking air, the EFIS' readout went from 0.1 to .0
     > gallons.  No, I don't understand why it works with a pwm signal
    if it is
     > looking for a frequency.  I guess I really don't need to know
    other than
     > idle curiosity.  If the 555 circuit would do the trick, I'm sure
    I could
     > build a couple and try them.   They look simple enough.
     >
     > Mark


    That is definitely square wave Frequency Modulation and not Pulse
    Width Modulation.
    Highest frequency period shown looks like 75 microseconds or .000075
    seconds
    so about that is about 13.3 Khz or 13.3K pps. If my math brain has
    not faded. Looks
    like the $1.00 Radio Shack 555 trick.

    We also need to know what the lower and upper voltages are. I need
    to see the
    whole scope face and the controls to know what that is. Is that 5
    volts per division
    or 5 volts full screen? Make sure the scope is set to DC and NOT AC.
    It is probably 5 volts per division and we are looking at almost 12
    volts
    peak to peak. If so it looks like they are running the 555  off the
    raw 12 Volt
    supply. If so it is dumb thing to do as the circuit might drift with
    supply changes.
    A 5 volt regulator chip is another couple of bucks.

    Now we need to know the frequency range of Greg's Hi-frequency channels
    input and what peak to peak voltage it will tolerate.

    Paul Lamar

I'm enclosing the BMA EFIS/One installation manual.  On page 45 it
indicates that channels 0 and 15 are used for fuel flow and are rated 0
- 220 Hz.  These are the two channels that I used for fuel level.

Mark

OK I will take a look later today. Got to go to the hangar.
220 Hz is not going to work with those probes. We can do several
things. Divide it down with hardware is the easiest.