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.