Is this OK ???
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Thread: Is this OK ???

  1. #1
    1000+ Posts jo proffi's Avatar
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    Default Is this OK ???

    Can someone with an understanding of electrics/electronics let me know if I've done good or bad on the following, please.

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    Its to do with making up a gauge with spare parts.

    I'm using an old ultra cool and groovy LED marine fuel gauge from the 70's and wanted to turn it into an air temp gauge.

    Using the only air temp sensor I had (r21??), the gauge read on an inverse scale to my needs, unless I set the pots out of range, whereby it had a little 'C' as the first digit and it then read the right way (temp rise = numbers getting bigger). The 'c' error bit on the display is a ripper bit of luck because I want celsius anyway.

    But it was still out of range of the pot control so I grabbed my only bit of spare electronic hardware, a 1k resister left over from an ignition mod the other day, and bridged the signal wire to earth,

    Amazingly with this resistor in line the ranging of the sensor and the gauge seemed to be spot on, and as linear and repeatable as is needed, so I'm pretty smug if this is an acceptable way to fiddle with electric signals which I obviously only just understand at the most basic level.

    Have I committed any major mistakes in need of remedy, or is this an acceptable thing to do to a circuit???
    SHould I be buying a fire extringuisher sooner than later???

    Cheers, Jo

  2. #2
    1000+ Posts schlitzaugen's Avatar
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    Jo, the only risk is that if the resistor does not have the capacity to dissipate the heat it needs to, it will overheat and burn out.

    If the resistor is the wound type, it will probably have enough power dissipation capacity. What is the voltage of the signal? Somewhere up to 12V? If so, the maximum current (@12V) will be I=V/R=0.012A, which means the power consumed by the resistor will be P=V*I=0.14W. Say all this is dissipated as heat, then you need a resistor rated at about .25W. This is because (if memory serves) resistors are rated at .125W, .25W, .5W, 1W and so on and .125W would be too small.

    I don't see any other problem.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

  3. #3
    1000+ Posts jo proffi's Avatar
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    Thanks for the informed reply.

    So far no fire. That much is good.

    The card the resistor came out of says 1W carbon 1k0 5%.

    I can hold the resistor with my fingers and it feels slightly warm so i'd guess 40 C would be the temp it is maintaining.


    Jo
    Last edited by jo proffi; 31st August 2012 at 06:36 PM.

  4. #4
    1000+ Posts schlitzaugen's Avatar
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    What are you measuring the air temperature for? Fine tuning your hair dryer for that perfect no-frizz perm?
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

  5. #5
    1000+ Posts jo proffi's Avatar
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    Quote Originally Posted by schlitzaugen View Post
    What are you measuring the air temperature for? Fine tuning your hair dryer for that perfect no-frizz perm?
    What is this, 1984???
    The dog gets more grooming than me, and she's a short hair baldy.

    To be honest, I'm just goosing around today on a day off
    I had the gauge already installed, but after refitting the OE speedo/tacho and all the other guages that go with it, this gauge was no longer needed and just sitting there spare in the pod.

    I figured why not try and do something usefull with it like an under bonnet air temp sensor with bits I have lying around.

    I'm open to other suggestions as to what I can use it for.
    Even if it just sits there on 000 it is so cool it got to stay.


    Jo

  6. #6
    1000+ Posts schlitzaugen's Avatar
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    Mmmmm 1984..........

    One thing I would do with a temp gauge permanently is try to find a point where I can measure the head temperature on the exhaust side. I think you need a range above coolant temp though. More like up to say 300 deg C?
    If there was room under the rocker cover that's where I would try to fit it, somewhere near the exhaust valve.

    Post a picture of it, will you?
    Last edited by schlitzaugen; 31st August 2012 at 08:08 PM.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

  7. #7
    1000+ Posts jo proffi's Avatar
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    So a day has passed and I cant help but fiddle with this little project.

    It became obvious that the resister was not allowing me to tune the pots on the dash with enough control, so I replaced it with a pot.

    That means I can set the range of the gauge with its inbuilt pots and then calibrate a known temp startpoint for the sensor within that range..
    The challenge is going to be calibrating it through a usefull range, but i've got a plan for that using the cars existing calibrated inlet air temp sensor.
    At the moment absolute temps are not especially important, so I'm just setting it with sensor pot when I start up with a cold engine and that gives a solid lower point to the temp readings.

    Of course an inlet air temp sensor is designed to have airflow through it, and just plonking it in the engine bay does not equal airflow. I found the sensor was just heating up and staying at that temp, so obviously some sort of airflow was required.
    I thought of the old broken laptop's fan and making up a little enclosure, but after pulling it apart found it was 5v, so no good for now.

    It took a while for the penny to drop that there is already a great source of airflow......the engine.

    I've now got the idle valve inlet on a hose I can move around the engine bay, with a slightly modified air temp sensor shoved down its end.

    Bingo....the response of the gauge is very fast now.

    With an almost standard front end/radiator system, on a 14 C night, and with the air temp sensor mounted near the throttle plate(but on the outside ), the air temp stayed at about ambiant whilst driving at any speed.

    As I slowed down to a crawl they rose slightly, maybe a degree C or two, but the temp stayed very low until I stopped, where they shot up to 45. (this 45 is just a number hotter than the 14 the gauge was set at. Its ball park but could be under by 10).

    What was remarkable though was that at a slow walking speed the air temp around the throttle area returned to almost ambient temp.
    I never gave much thought to airflow at walking speed but my little sensor showed it was a massive thing through a fuego engine bay.

    In fact on this cool night, there is more difference between 0-3 km/h air temps than there is 3-100 km/h.
    Doesn't make me feel so bad about some of those quick and nasty pod induction systems I've had in the past.

    Jo

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