Question about circuit for Low fuel warning LED

[MEZ]

I need to ask a question not related to flasher units but it is an electronics issue. Are there any electronics boffins on here..?? Frankenduck that could be you Sir..???

[rbm]

Give me a try.

[MEZ]

Give me a try.

Hi RBM,
Thanks for replying. On my special build I have a custom auxiliary fuel pod mounted low down which is the last point to run dry. I have a sender unit mounted in there to detect the last 3-4 litres sourced from a CB500 and basically is a variable resistor which draws current through a bulb and illuminates. (I want to install a LED lamp sourced from good old China that has the petrol symbol) I'm guessing I will need to parallel a resister across the LED or maybe a small pcb like the one here....
Screenshot (81).png (48.58 kB . 768x512 - viewed 563 times)

[rbm]

Although a LED acts like an incandescent light bulb (in that it emits light when a voltage is applied), the similarity ends there. These are two different types of devices with different characteristics.  Putting a parallel resistor across the LED won't accomplish your goal.

Yes, you will have to build a threshold driver circuit like the one you show above.  You might be able to use an Opamp-based circuit, but that  one you posted seems pretty simple and elegant.  Check the range of the CB500 sender to make sure it varies from 0 Ohms at one extreme to 135 Ohms at the other.  As mentioned in the text, if the sender you have differs from that standard, you'll have to adjust the value of the 330 Ohm resistor.

[MEZ]

Thank you again. I have found some bumf on this topic and should I use this calculator below to calculate the required resistor..??

Screenshot (80).png (48.17 kB . 768x512 - viewed 532 times)

[rbm]

Sorry, that calculator's not appropriate to what you are trying to accomplish here.

The transistor is basically a switch that will allow current to flow through the 470 Ohm resistor and the LED when on.  The string of three resistors (100, 330 and 135) forms a voltage divider.  To turn on the transistor, the voltage at the base of the transistor has to reach about 2 Volts.  This voltage is determined by (1) the forward voltage for the B-E junction of the transistor of 0.7V and (2) the forward voltage of the LED of 1.3V.  If you choose a LED with a different forward voltage, then you may have to adjust the value of the 330 Ohm resistor.

To calculate the voltage at the base of the transistor, you do this calculation (I'm assuming that the 100 Ohm trimmer pot is midway in its travel, or 50 Ohms):   12 * (90/(50+330+135))

Be aware that there is no integration delay in this circuit so as the fuel level gets to a point where the LED turns on, the sloshing of the fuel in the tank will cause the LED to flash on and off instantly in sympathy with the sloshing.  It could be distracting.

I split this topic into its own thread.

[MEZ]

Sorry, that calculator's not appropriate to what you are trying to accomplish here.

The transistor is basically a switch that will allow current to flow through the 470 Ohm resistor and the LED when on.  The string of three resistors (100, 330 and 135) forms a voltage divider.  To turn on the transistor, the voltage at the base of the transistor has to reach about 2 Volts.  This voltage is determined by (1) the forward voltage for the B-E junction of the transistor of 0.7V and (2) the forward voltage of the LED of 1.3V.  If you choose a LED with a different forward voltage, then you may have to adjust the value of the 330 Ohm resistor.

To calculate the voltage at the base of the transistor, you do this calculation (I'm assuming that the 100 Ohm trimmer pot is midway in its travel, or 50 Ohms):   12 * (90/(50+330+135))



Be aware that there is no integration delay in this circuit so as the fuel level gets to a point where the LED turns on, the sloshing of the fuel in the tank will cause the LED to flash on and off instantly in sympathy with the sloshing.  It could be distracting.

I split this topic into its own thread.

Great input thanks, I now have a starting point...

[MEZ]

This is the sender unit below...

IMG_3052.JPG (132.97 kB . 640x480 - viewed 558 times)

I'm getting ohm readings much higher than displayed on the wiring diagram. Submersed in cold water : 1677 ohms. Out the water drops to around 1610 ohms and decreases as it warms up in the not so ambient temperature of a British kitchen with central heating on full chat. Conclusion is it ranges around 60 to 70 ohms and I have no idea how to proceed from this point. Do them ohms readings sound high to you RBM..???

[rbm]

Normally, a fuel sender is a potentiometer, a variable resistor with a wiper that is attached to a float.  This is a negative temperature coefficient thermistor, whose resistance changes inversely proportional to its temperature.  These are two different devices.

It is possible to use this device for the purpose you need but you will have to calculate different values for the resistors in the voltage divider.  The range of resistance you published seems normal.  You'd be advised to find the manufacturer and part number for the device and see if you can find the specs.  In the absence of specs, you can measure the resistance at various temperatures (like you have done), but the problem becomes that you don't know what appropriate temperatures will be in service.  For example, when not immersed in fuel, what temperature will the thermistor assume?  Ambient?  Internal temperature of the tank?  This device will have largely different values if immersed in cold fuel versus hot fuel.  You will have to experiment to find a circuit that does what you want.

I just thought that you would need to create a circuit similar to that used on early K-bikes.  A description is here:
http://www.motobrick.com/index.php/topic,10566.0.html (see "Early Fuel Level Sensor (4L/7L lights)")
BMW gets over the problem of managing readings in cold/warm fuel by requiring a 95mA constant current to be driven through the NTC resistors in the unit, that would cause them to heat up in free air and change their resistance.

[DEcosse]

... Although a LED acts like an incandescent light bulb (in that it emits light when a voltage is applied), the similarity ends there. These are two different types of devices with different characteristics.  Putting a parallel resistor across the LED won't accomplish your goal.

This can be REALLY simple ........
All you need is a LED* with a 50 ohm 5W resistor across it - no transistor driver required
Ign Positive - 50 ohm in parallel with LED - Thermistor - Ground
Once the Thermistor heats up (when not immersed in gas) the LED will come on.

*Post Script Edit - note the above was using an LED already configured for '12V' (with series resistor already incorporated) - that is simplest way to go, with just the 50 Ohm in Parallel with that; if using a 'bare' LED, a series resistor will be required for the LED with the 50 ohm in parallel with the series pair of LED/Resistor.
Also the 50 ohm resistor should be at least 1W or better. 

[MEZ]

This can be REALLY simple ........
All you need is a LED* with a 50 ohm 5W resistor across it - no transistor driver required
Ign Positive - 50 ohm in parallel with LED - Thermistor - Ground
Once the Thermistor heats up (when not immersed in gas) the LED will come on.

*Post Script Edit - note the above was using an LED already configured for '12V' (with series resistor already incorporated) - that is simplest way to go, with just the 50 Ohm in Parallel with that; if using a 'bare' LED, a series resistor will be required for the LED with the 50 ohm in parallel with the series pair of LED/Resistor.
Also the 50 ohm resistor should be at least 1W or better.

Hi gents, so i'm piggy in the middle here between two very smart guys that know there electronics. I'm all for simple myself and this last piece of advice appeals to me. I would assume that a purchase of an LED warning light that was clearly advertised for 12volt application would be 'configured' for 12v...????

DEcosse dear sir, could you do a sketch of what you advised to show me, like you would on a lunch break when trying to explain to a work colleague etc etc..???

Thanks once again....

[DEcosse]

Happy to help


Thermistor_Indicator.jpg (26.73 kB . 768x576 - viewed 505 times)

Upper section as described above - yes, you would use a '12V' LED.

An alternative solution is to use a 12V incandescent bulb which negates the requirement for the resistor - you can probably find a fairly small incandescent lamp fixture, although not quite as compact as an LED - be sure it is about 3W of power though in order to ensure enough current through the Thermistor.

Both indicator components are doing the same job - they not only provide an actual indication, they also pass current to the Thermistor.
In the case of the LED, its resistance is quite high, so the parallel path of the 50 ohm resistor supplies the required current.

The current through the resistor will be 'trying' to heat the Thermistor - however as long as it is submerged in liquid, the heat is dispersed away; however once it becomes uncovered, the device WILL actually heat up and when it does that its resistance drops accordingly- That is the whole premise of a Thermistor, its resistance is governed by its temperature - similar type of device is those use for measuring Air or Coolant temp but those are being governed by EXTERNAL temperatures applied rather than being internally generated by the very current through them. .

SO - consider the series path of the illuminating device and the Thermistor in series as a voltage divider: 
(for this purpose consider the parallel combo of the LED and the 50 ohm as one segment of that chain)
e.g. if the source is 12V and IF the resistances were equal (they're not! - just for illustration)) - then the voltage across each would be 6V;
If the resistance of the Thermistor was 3 times that of the Indicator, then you would have 9V across the Thermistor and 3V across the Lamp (etc)

i.e. When the resistance of the Thermistor is much higher than that of the bulb (or the LED//50 ohm) then the voltage across the Thermistor is much higher than that of the bulb;
so in that circumstance the voltage across the bulb (or the LED//50 ohm) is quite small (relatively to source voltage) so does not illuminate.
However once the Thermistor is uncovered and begins to heat up, its resistance drops and thus so correspondingly does the voltage across it;
as the voltage across the Thermistor drops then the relative voltage across the lamp INCREASES (there will always be 12V across the series chain) - so the indicator lights!

Hope didn't over-complicate that with the explanation!!! :D

You can try this for yourself in a bath before you actually physically install it.

As far as the actual install, although the diagram shows the resistor apparently directly across the LED, it does not have to be physically there.
Indeed, the resistor can have a termination of the 'power' end at any switched source on the bike - as long as the other end is connected to the junction of the Thermistor and the LED.


Thermistor_Indicator_2.jpg (13.71 kB . 768x317 - viewed 522 times)

[MEZ]

Hi DEcosse


I get it now after reading your brief several times so many that's dear fellow Bricker...!!!

[MEZ]

Hello again DEcosse,
I am struggling to find a 2w x 50ohm resistor onlinr. Is there any room for variation on the resistor value...?? I can source a 2w x 47ohm but not a 50ohm. How crucial is the wattage value in relation to the thermistor rating..??


Regards Mez

[Laitch]

*Post Script Edit - note the above was using an LED already configured for '12V' (with series resistor already incorporated) - that is simplest way to go, with just the 50 Ohm in Parallel with that; if using a 'bare' LED, a series resistor will be required for the LED with the 50 ohm in parallel with the series pair of LED/Resistor.
Also the 50 ohm resistor should be at least 1W or better.

If you accept this part of DEcosse's previous answer (bold font added) then you find several choices of 3W resistors here.

[rbm]

The 47 Ohm resistor should be good enough.  There should also be 51 Ohm 5% 3W resistors available.

[MEZ]

The 47 Ohm resistor should be good enough.  There should also be 51 Ohm 5% 3W resistors available.

47R x 3w 5% resistor in the Xmas post....  :2thumbup:

[DEcosse]

Yes, that will be just fine

[MEZ]

IMG_3274.JPG (43.21 kB . 768x576 - viewed 475 times)
IMG_3275.jpg (63.93 kB . 768x576 - viewed 481 times)

Thank you gents, a bit of a lash-up wiring job but it totally worked giving me around 3 to 4 minutes of elapse time (none immersion in water/fuel) before it light up so a complete success I would of said and I tried it half a dozen times....


'Brickers' really are the good guys and a wealth of knowledge right at the end of your fingertips....!!!! :clap: