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Thread: New Low Fuel Alarm Sensor

  1. #11

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    Default Re: New Low Fuel Alarm Sensor

    Quote Originally Posted by avidflyer View Post
    Am I wrong?

    A bit, yes. When there's no fuel detected, both outputs are high impedance, or floating (OFF). When fuel is detected, both outputs are active (ON). The active high output (yellow wire) provides 12V (or whatever voltage the sensor is powered with), and the active low output (white wire) pulls to ground. That's the important point: with fuel present, both outputs are ON; with fuel absent, both outputs are OFF.

    If you want to light an LED when fuel is present, simply connect it to the active-high output with an appropriate current-limiting resistor (R):

    R = [supply voltage - LED forward voltage] / desired current

    For example, a typical green LED will have a forward voltage of 2V at 20mA. So, for a bus voltage of 14.4V...

    R = (14.4 - 2) / 0.02 = 620Ω (a standard value!)

    LED.jpg

    This will indeed confirm that the sensor is working. However, when fuel is absent, both outputs are OFF, so it won't send current to your red LED.

    If you want to light the red LED when fuel is
    absent, you need to invert the active-high output to obtain 12V when it's OFF. The simplest way to do this is with two components: a resistor and a P-channel MOSFET. That circuit looks like this:

    Inverter.jpg

    When
    the active-high output is ON (fuel present) the MOSFET's gate (labeled "G") is driven to 12V by the sensor, and when the gate is at the same voltage as the source (labeled "S"), a P-channel MOSFET is off. When the active-high output is OFF, the 10kΩ resistor pulls the MOSFET's gate down to ground, which turns the MOSFET on, allowing current to flow, lighting your LED (you still need the current-limiting resistor above). The 10kΩ gate resistor value isn't critical. It just needs to be high enough to limit current through the sensor output when it's ON; anything from 1kΩ to 100kΩ will work fine.

    So, if you want two LEDs, green turned on when fuel is present and red turned on when fuel is absent, the circuit looks like this (again, LED current-limiting resistors not shown):

    Inverter_2_LEDs.jpg

    Now, with all of that said, I should point out that virtually all aircraft manufacturers these days adhere to the "dark cockpit" concept, which means that no indicators should be lit when systems are operating normally. So, your green LED would not be installed. In any case, the fact that the engine is running makes it kind of redundant.



    As I mentioned in an earlier post, I'm designing a small interface board to connect between the sensor and a warning indicator. It will actively hold the LED off when fuel is present
    , and flash it when fuel is absent. It may also have a press-to-test function to confirm that the flasher circuit is working. If you press the test button and the LED flashes, then goes off when you release it, both the sensor and the flasher are working. Using an illuminated pushbutton switch would incorporate both the warning indicator and the press-to-test function in one panel hole.

    It may turn out that the sensor outputs are OFF when it's first powered up, and it takes a couple of seconds to self-calibrate and detect fuel in the tank. In that case, the press-to-test feature would not be necessary because the warning indicator would flash briefly each time the master switch is turned on. This would confirm that the flasher and LED are working, then when it stops, you would know that the sensor works. The switch could instead be used to de-power the circuit, eliminating distraction during a genuine fuel emergency.

    Anyway, that's enough middle-of-the-night thinking out loud. More testing to be done. Stay tuned...
    Eric Page
    Building Kitfox 5
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  2. #12
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    Default Re: New Low Fuel Alarm Sensor

    Thanks Eric for the information you provided. The LED lights I have run on 12 volts, so they must have a built in resistor. Because I'm electronically challenged, If you don't mind, how do I wire up one of these for a low fuel alarm in the header tank? That would be one red led that would light up on the panel if the sensor sees no fuel at it's height in the tank. Thanks, JImChuk
    Last edited by avidflyer; 11-20-2020 at 07:16 PM.

  3. #13

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    Default Re: New Low Fuel Alarm Sensor

    Quote Originally Posted by avidflyer View Post
    The LED lights I have run on 12 volts, so they must have a built in resistor. Because I'm electronically challenged, If you don't mind, how do I wire up one of these for a low fuel alarm in the header tank? That would be one red led that would light up on the panel if the sensor sees no fuel at it's height in the tank.
    If your LED came with a resistor, then you won't need to add one. In my last post, above, I explained how to light the LED when the sensor doesn't see fuel. Do you have a specific question about the circuit that I can help you with?

    I would recommend that you test your LED in bright sunlight to see if it will be bright enough to get your attention immediately. Most small indicator LEDs like are shown in your picture are meant to be used on a control panel, indoors. They're not very bright, and their small size makes them even less likely to be noticed, especially if they're not flashing.
    Eric Page
    Building Kitfox 5
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  4. #14
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    Default Re: New Low Fuel Alarm Sensor

    Well I ordered some of those Mosfets. Not sure how to tell which wire connects to what leg on the Mosfet, but maybe that will become apparent when I see them in person. Like I said, I'm very much electronically challenged, and not at all afraid to admit it. Thanks again Eric, JImChuk

    VP2106N3-G Microchip Technology | Mouser


  5. #15

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    Default Re: New Low Fuel Alarm Sensor

    The product page on Mouser has a link to the datasheet. The first page shows the pinout.

    It's not my place to tell you your business, Jim, but I think you're jumping the gun a bit. As I've said a couple of times, I'm working on an interface for the sensor that will take care of flashing an indicator and will be able to confirm that the sensor, flasher and indicator are all working before each flight. It'll be built on a printed circuit board that can be neatly packaged in heat shrink for protection and easily tied into a wiring loom.

    I'm going to post links to sources for everything so that anyone can buy the parts, including a custom circuit board, for around $50 total, and potentially less if you don't like my choice of switch/indicator. The circuit will be all through-hole parts, so assembly will be no more difficult than an electronics starter kit. I'm looking at two versions, for EFIS and non-EFIS equipped aircraft.

    If there's enough interest, I might even assemble a batch for forum members as a one-time deal, for the cost of parts and postage.

    But, hey, it's your airplane!
    Eric Page
    Building Kitfox 5
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  6. #16
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    Default Re: New Low Fuel Alarm Sensor

    Thanks Eric. Not unusual for me to jump the gun. But I guess I missunderstood the whole works right from the beginning. I'll keep an eye on what you come up with, but of course in the meantime, I've got these parts.....JImChuk

  7. #17

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    Default Re: New Low Fuel Alarm Sensor

    I've got a little more testing data to share. This time I placed the sensor where it would see fuel from the moment power is applied, and measured the delay from power-up until the output reflected the presense of fuel.

    In this image, the yellow trace is the 12V supply to the sensor and the blue trace is the active high output. Each vertical division is 2V and each horizontal division is 0.5 second. At the left edge, both traces start at 0V. Power is applied where the yellow trace rises to 12V, then 2.5 seconds later, we see the output rise to 12V as well. This result is very repeatable; it looks exactly the same every time I run the test.

    DS1603NF_Startup.jpg

    This is the result I was hoping for, because it means that the sensor's output will briefly simulate a low-fuel condition at power-up, allowing an automatic self-test of the whole system. When the master switch is turned on, the indicator will flash for 2.5 seconds, then normal sensor output will begin and the indicator will go dark. If any of that doesn't happen, you'll know there's something wrong.

    More to follow...
    Eric Page
    Building Kitfox 5
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  8. #18

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    Default Re: New Low Fuel Alarm Sensor

    Looks like you posted more then deleted the post. If you want, I could go over what you have and review it. I am an electrical engineer and specialize in this sort of thing.

  9. #19

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    Default Re: New Low Fuel Alarm Sensor

    Yeah, I did. I guess the server must have sent out email notifications instantly. My apologies to anyone who came here looking for the full story. As soon as my post went live, I saw a bone-headed mistake and took it down. Then I didn't have time to sort it out yesterday. Figures!

    Thanks, CoClimber. I'll write up what I've got so far and send you a PM. I'd appreciate a second set of eyes; it might prevent further embarrassment!
    Eric Page
    Building Kitfox 5
    Member: EAA, AOPA, ALPA
    ATP: MEL / Comm: SEL, Glider / ATCS: CTO
    Map of Landings

  10. #20

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    Default Re: New Low Fuel Alarm Sensor

    I think it's great what you are doing. I believe that fuel starvation is the leading cause for engine outs and anything we can do to improve that situation is worth the time. If you want a PCB knocked out, I'd be glad to help.

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