Simple Single LED 'Low Fuel Light'

Discussion in 'The Projects Forum' started by nizzkid, Sep 8, 2009.

  1. nizzkid

    Thread Starter New Member

    Sep 7, 2009
    3
    0
    Hi, I've been looking for weeks and its doing my head in! :confused:
    All I'm after is a VERY simple circuit, just one led that lights up when the fuel gets low on my bike. (making a custom dash using the "Veypor")

    I'm usually pretty good with electronics ect, but i need some help with this one!
    I've been searching for ages and only come across crazy complex (for a noob) circuits with time delays and 100,000 led light bars! :D

    Is this anywhere near a working cercuit?!

    What i've "tried" to do is have the led switch on when the fuel drops below 90Ω this should be @ 6.2v that the zener is at. So when the fuel drops the zener has more voltage and switches the led on.

    Dont realy need any sort of 'time delay' to combat fuel sloshing around and continualy ficking the light on and off. This is because as it stand the fuel guage sits at 'full' 90% of the time anyway, its not till the last 30km or so that needle moves down. (and when it does its a rapid decent!)

    Also any feedback on the resistor ratings would be very apriciated.

    Cheers Steve.


    Click on pic below

    Low Fuel Warning help.jpg
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Your looking for a comparator circuit if all you want is a low fuel indication. The problem is your wasting your time if the sensor is defective in your vehicle.
     
  3. Gustav180

    Member

    Aug 25, 2009
    17
    0
    Hello

    Your problem is not the circuit, it´s OK. It´s the sensor. I have not test this, but I should test with a NTC.

    If you use a NTC with low value, i.e. 100 ohm, it will bee wam of the current from the 12 V supply. When it is in the fuel, it will bee cool of the fuel. When it comes over the fuel, it will bee warmer and the output of the OP will switch.

    If you want the LED to light up when fuel is low, you have either to change + and - on the OP-amp or put the LED from output to GND by 560 ohm.

    Best regards
    Gustav
     
  4. nizzkid

    Thread Starter New Member

    Sep 7, 2009
    3
    0
    Hi, thanks for the help I haven't built the circuit as yet I just wanted a little opinion/help to see if it 'will' work before I waist any time on it!

    The level indicator is a float style not a thermistor.
    Which explains below.


    The sensor works fine, the fuel tank design/shape is the culprit.
    Its a 5 liter tank mounted under the bike with @ 1+ of those liters being in the fuel filler pipe (above the sensor). And @ a 1 liter remaining on empty.
    So it's only got 3 liters to play with in a long thin tank.
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    That circuit will energize the LED when the sensor is about 98 ohms ... which is within the specification for empty.

    The best thing you can do is fill the tank with the amount of gas you want for the low level alarm, then measure the sensor. Then design the circuit to illuminate the LED at that resistance. At least you won't be pushing the bike to the next fueling point.
     
  6. nizzkid

    Thread Starter New Member

    Sep 7, 2009
    3
    0
    Good point! lol
    My final design has two LED's (so much for the simple one LED effort!)
    First one being solid Red coming on at 67 ohms.
    Second being flashing Red coming on at 93 ohms.
     
  7. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,159
    Why not a solid yellow .... then a solid RED?

    They make bi-color LEDs and you can get dual comparators on one chip.
     
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