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Thread: My Project Near Completion

  1. #1
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    Aug 2008
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    Decatur Illinois
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    My Project Near Completion

    Hello,

    I am nearing completion of my project, an engine monitoring system for my light sport aircraft. I have a question that I hope someone can help with. I am using the LM2907 tach chip to develop the RPM input. I have followed the instructions on the data sheet to minimize the output ripple (it is a trade off between response and ripple) I have the ripple quite low, less than 1%. However since the digital display shows the RPM down to 1 rpm increments, the last digit bounces around due to the ripple. Is there a way to read in a min of 10 rpm increments? I am using MDX for the display.

    I have attached a couple of pictures of my project. The one picture shows me heating the Exhaust Gas Temp thermocouple in a candle flame. Note the display is indicating 797 degrees. The second shows the hardware ready to slide into my project box. I am using a 25 pin Sub D connector for the inputs and outputs. Comments or questions are welcome.

    Fred
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  2. #2
    Newbie
    Join Date
    Aug 2008
    Location
    Decatur Illinois
    Posts
    41
    Here is how I accomplished input over voltage protection. I quickly found you just cannot connect a zener across the input. A zener will have enough conduction well below the zener voltage to seriously impact the input signal accuracy. The circuit shown is for my fuel level. Simple voltage divider consisting of the 540 ohm resister to the level sensor (the level sensor is the variable resistance type with a maximum resistance of 270 ohms). Note that without the input protection, if the fuel level sensor were to be disconnected, the input to the fusion brain would go to 12 volts.

    The over voltage protection portion consists of the 4.7 volt buss, BAT85 diode, and 10k resistor. As long as the voltage to the fusion brain is less than 4.7 volts the BAT85 diode is reversed biased and since it has very very low reverse leakage, there is no impact on the signal. If input voltage goes above 4.7 volts (plus 0.2 volts across the the BAT85) then the BAT85 becomes forward biased and any additional voltage will be dropped across the 10k resister. Net result is that input the the fusion brain will exceed 4.9 volts.

    I choose a BAT85 diode because it has a low forward voltage drop (0.2 volts) and has very low reverse leakage. You can find them cheap on Ebay.

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