Paul,


    >              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>
    >
    >
   <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.
    >     --
    >     Paul Lamar ...No rotor no motor.
    >
    >
    >     Paul,
    >     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
    >
    >     http://www.kpsec.freeuk.com/components/cmos.htm#4017
    >
    >     Get a couple of these CMOS 4020 and fool around with them
    >     until it works. Dividing by 13,333 by 64 (pin 4) will give
    >     about 208 Hz max. Put the square wave signal from the 555
    >     in on pin 10,  Take the signal out on pin 4, ground pin 11
    >     and leave all other pins unconnected. DigiKey or Mouser.
    >
    >     If it works tell Greg he owes me :)
    >
    >     ----------------------------------------------------------
    >
    >     Frequency  to analog 0 to 5 Volts Converter.
    >
    >     After you divide it down you can use one of the these and use
    >     the stock 0 to 5 volts analog BMA fuel gage inputs.
    >
    >     Paul Lamar ...No rotor no motor.

   Mark Steitle wrote:
    > Paul,
    > I'm not sure how to proceed here.  It appears that you have
   provided me
    > with two options.
    >
    > Option 1:  Use a 555 timer and feed the output into the 4020 ripple
    > counter, and finally to the 2917.  This would make it so that I could
    > feed the signal from the 2917 into the 0-5v channels on the EFIS.
    >
    > Option 2:  Use the circuit shown in the KPsch.jpg attachment.  Is
   this
    > an IC that I can purchase, or would I need to build it from scratch?
    > Would this circuit handle the task all alone or does it need
   additional
    > hardware?
    >
    > Thanks,
    > Mark
    >

   As far as I can tell your current probes are putting out up to 15 KHz
   square wave pulse frequency's. Not pulse widths. Too high a
   frequency for
   the BMA. So all you need do is feed that into 4020, divide it down by 64
   and then feed that into the BMA low frequency inputs. That is option
   one.

   Option two is add a frequency to analog voltage converter (the LM 2917)
   and then feed that into the regular BMA fuel quantity channels that
   require
   an analog voltage between zero and five volts.

   Both are very low cost IC's that can be purchased from Digikey or
   Mouser.
   DigiKey has 25,000 CD4020 in stock for $1.76 each :)


   The LM2917 replaces the KPsch.jpg. The KPsch.jpg is the same thing done
   with discrete transistors and a quad op amp.

   Check this out on the BMA forum with credit to me and see what they say.

   Paul Lamar ...No rotor no motor.

Paul,
Ok, now I get it.  I'll get some of each ordered and try them to see
what works best/easiest and then post the results to the BMA list.  The
new BMA EFIS uses an instrument module that has two capacitance
channels, so this is not a problem for the late-comers.  Worth posting
anyway as there are plenty of us early "Gold Box" owners still out
there.  Appreciate your help.

Thanks,
Mark
Lancair ES-20B, 15 hours

Post it here too Mark. Might be of interest to other people
using the same probes. By the way it is real easy
to build your own probes using two one inch wide long strips of brass
separated by nylon nuts and bolts. Takes about an hour or two.
A 555 will handle the electronics.