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Multiplexing temperature sensor

Discussion in 'Electronic Basics' started by Ed, Nov 18, 2005.

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  1. Ed

    Ed Guest

    My engine has a temperature sensor on the right bank for the dash gauge, and
    a different one on the left bank for
    the electronic fuel injection system. Both sensors are just temperature
    sensitive resistors of some kind.
    I would like a temperature reading for both banks. Rather than drilling and
    tapping a hole for yet another
    sensor for the left bank I was thinking about multiplexing the EFI sensor.
    Is there an simple circuit that I could
    cobble together to do this, perhaps based on an IC? The circuit would
    probably also have to modify the
    output to match an available temperature guage.

    Any ideas?

    TIA

    Ed
     
  2. John Fields

    John Fields Guest

     
  3. Ed

    Ed Guest

    Here is the EFI sensor resistance curve:
    Deg C Ohms
    10 3700
    20 2500
    30 1700
    40 1180
    50 840
    60 600
    70 435
    80 325
    90 250
    100 190


    Here is the the curve for the sender to the dash temperature guage. If
    the output of the
    multiplexer circuit produced something close to this I could switch
    manually back and forth between
    the two, which would be fine:
    'C ohms
    0 1117
    10 871
    20 680
    30 530
    40 412
    50 319
    60 245
    70 187
    80 141
    88 112
    90 105
    92 99
    100 76
    110 54


    I used to have access to a Web page with the ECU internals, but it
    seems to be down at the moment,
    or perhaps has gone away. I'll continue looking for it, but maybe in
    the meanwhile the above
    will be sufficient to offer suggestions, of course with caveats.

    TIA

    Ed
     
  4. Bob Masta

    Bob Masta Guest

    I'd be very careful of messing withthe EFI circuit. You should
    read the voltage across the sensor with a high impedance
    circuit, so as not to disturb the EFI operation. So, the
    actual resistance curve of the sensor is not the issue, it's
    the voltage output at whatever current the EFI wants to
    drive it with. This may not be a constant current, since
    the EFI could be turning it on only during measurement
    cycles at whatever update rate it requires... maybe not too
    often, given the thermal time constant of the big hunk of
    metal it's attached to. So your first order of business
    would be to look at the voltage across the sensor while
    the engine is running, preferably with a scope. If it
    is constant (other than varying with temperature!) you
    can proceed to curve-fitting to match the other sensor.
    If it pulses at ahigh-enough rate, you may be able to
    filter it (after buffering, to not disturb the EFI) to get
    a suitable signal.

    But curve fitting by analog means can be a bear.
    If you want to go whole-hog, you can use table
    look-up with a small processor chip. But my guess
    is that installing a separate sensor may be a lot
    easier in the long run.

    Best regards,


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  5. Ed

    Ed Guest

    Thanks, Bob.

    This is a very early EFI system, namely the Jaguar adaption of the
    Bosch
    D-Jetronic system, circa late 1970s. The ECU is an analog
    implementation with discrete
    components. It is of course driven by a pulse stream from the engine,
    picked up from
    a trigger board in the distributor. The primary inputs are engine
    speed (derived from the
    pulse stream), manifold pressure, coolant temperature, and air
    temperature. It is described
    at http://www.jagweb.com/aj6eng/djetronic.html if you really want to
    get into it.

    Also, I have now found the site at which the Porsch 914 implementation
    of D-Jetronic is
    fully reverse engineered. It is at:
    http://members.rennlist.com/pbanders/ecu.htm. From that site
    you can navigate to http://members.rennlist.com/pbanders/ecu2.jpg
    which shows the portion of the
    ECU schematic where the cylinder head temperature sensor (since the
    914 engine is air cooled). It
    looks like the sensor feeds into the base circuit of a transistor. I
    have read that this is eseentially the same
    as the Jaguar/Lucas implementation, although there are some
    differences.


    The coolant temperature sensor (CTS) is a thermistor with resistance
    curce as given previously. One
    side is grounded and the other connects to the ECU. I just put a scope
    on the the ungrounded
    pin of the CTS in situ with the engine idling, warm. What I see is a
    very clean 2.2 volts, varying
    only by a negative 0.2 volt blip every 37 ms.

    So, are you saying I might be able to just connect a gauge to the CTS
    through high impedence,
    no multiplexing required?

    Thangs again for your interest.

    Ed
     
  6. Bob Masta

    Bob Masta Guest

    On Sat, 19 Nov 2005 19:49:00 GMT, "Ed"

    That should work fine, as far as not disturbing the EFI.
    You can then use nonlinear circuits to linearize the
    voltage response for a digital meter, or you can make
    a custom face for an analog meter. (I'm assuming
    that you won't have an analog meter that is calibrated
    to match the EFI curve.) Or you can digitize and use
    lookup tables.

    Best regards,


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  7. Ed

    Ed Guest

    Sounds easy! I'm wondering, though, about how to get the
    proper relationship between voltage at the sensor and temperature.
    Direct calibration is just about impossible for two reasons. First,
    it would require fitting another temperature measurement of the
    coolant at the same location in the system, which is what I'm trying
    to
    avoid. Second, covering the range at the high end would require
    overheating
    the engine. There must be some way to relate the known resistance vs.
    temperature
    to the voltage at the ungrounded terminal while it functions in the
    EFI system.
    Would this require details such as resistances etc. in the ECU where
    the CTS
    connects?

    Ed
     
  8. Ed

    Ed Guest

    Answering my own question, I would think I could just measure the
    resistance
    between where the CTS pin and the battery voltage pin at the ECU, then
    view that
    resistance and the CTS as a "voltage divider."

    Ed
     
  9. Jasen Betts

    Jasen Betts Guest

    if you can characterise and simulate the probe pulses sent to the sensor
    you can remove the sensor and heat it in a pot of oil with a thermometer while
    feeding it simulated pulses...

    if the EFI sends different pulses to the sensor depending on the engine
    speed or temperature (etc) you have a tougher task.
     
  10. Bob Masta

    Bob Masta Guest

    Measuring the ECU "resistance" probably won't work.
    (It's probably a current source, not a simple resistor.)

    As I recall, you already have the resistance curve for the CTS.
    So shut off the engine and quickly measure the CTS resistance
    and that will tell you its temperature at some arbitrary reference
    point. The voltage across it just before you shut it off will
    allow you to find the ECU driving current from I = V/R.
    Now assuming that the ECU uses the same current all
    the time, you can apply that to the resistance curve to
    convert it to a voltage curve.

    Best regards,


    voltage
    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  11. Ed

    Ed Guest

    Thanks, Bob. Here is the relevant part of the ECU schematic. Could you
    take a look and tell me if it is a constant current source?

    http://www.efsowell.us/ed/jag/electrical/ECU_CTS_connection.JPG


    Thanks, Ed
     
  12. Bob Masta

    Bob Masta Guest

    Not constant current, just a plain old series resistor (plus a
    diode). But I'd avoid trying to measure the resistance of the
    ECU, just because it is a questionable practice in general,
    and also because that diode is in there to mess up your
    measurement. But since it isn't a current source, using
    a voltage divider approach is probably best. Just measure
    the CTS resistance (disconnected from the ECU, of course),
    as well as the voltage when it is connected, and you will
    have the current at that temperature. The effective ECU resistance
    will be 12 minus the CTS voltage divide by the current.
    You can probably assume that effective resistance over
    the whole range, and just use your CTS curve to derive
    the voltage curve.

    Best regards,



    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
  13. Ed

    Ed Guest

    Thanks, Bob.

    Ed
     
  14. Ed

    Ed Guest

    I remember forom analog computer days a device called a diode function
    generator (DFG). As
    I recall these were basically a set of diodes and resistors. Is there
    an IC that does this sort of thing?
    Or, what is the processor chip you are referring to? Microcontroller?

    When I do a Google on "diode function generator" it's like a march
    back in time... pulls up
    all sorts of old papers by analog computer programmers. None of mine
    though... sigh.

    Ed
     
  15. Bob Masta

    Bob Masta Guest

    I have never really used the traditional diode function
    generator... they are a morass of interacting bias resistances,
    not suitable for hand-tuning. Nowadays, I imagine that
    modeling software or even a special diode function generator
    program might make this more straightforward, though.
    An "improved" diode function generator uses op-amps to
    isolate each breakpoint, so they are somewhat easier
    to adjust, but they tend to have sharp corners at each
    breakpoint... not what you want here.

    If you want to pursue this, I'd first take a long hard look
    at the correction you need to apply. You might get
    lucky and be able to use some simple circuit to do this.

    The processor approach is not specific to any chip.
    You read the voltage with an A/D, and use that
    value as an address into a lookup table to get the
    corresponding temperature. If your chip has a lot
    of memory, you can have an entry for every degree.
    If not, you can segment the required curve into
    segments, and take the high N bits of the A/D
    value to select the segment, and apply linear
    interpolation to get the degree reading. Since
    temperature changes so slowly, even a slow
    chip should be plenty fast enough to do a lot
    of calculations. Note that I'm no expert at
    this, and have no specific chip recommendations here
    If you are not already comfortable with embedded development
    then you can either use this as an excuse to go off
    on a long, involved learning quest, or just fall back to
    creating a face for an analog meter.

    Best regards,


    Bob Masta
    dqatechATdaqartaDOTcom

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Home of DaqGen, the FREEWARE signal generator
     
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