Fuel meter with thermistor

[augustin87r]

Hello,

I have a BMW F650 that doesn't have any indication about the fuel level, not even a reserve warning light. A reserve light is very useful , you never know when you run out and have to switch to reserve while riding, which is not very comfortable nor safe...

So, Africa Twin uses a NTC thermistor to light a bulb (look at the first attachment). When a current flows through a thermistor, it will generate heat which will raise its temperature above that of its environment; so, when they are submerged into a fluid (gasoline in our case) they are getting cooled, maintaining high resistance; when the level of the gasoline is getting lower and the thermistor is not immersed into the gasoline, their temperature is getting higher, roughly decreasing their resistance.

Because the bulb lights with 'fading' effect somebody made a schematic with 2 comparators. I don't need the relay, because i'm not going to use a bulb. It's too big for my dashboard. I'm going to use a LED in the collector of the transistor.

Questions:

1.Pls look at the second attachment. Do you think is it going to work?
2. Is it OK if i replace the bulb with a resistor of a few ohms 1 or 2 W? What value shoulf the resistor have?

The article that i read :

**broken link removed**

 

[JimB]

1.Pls look at the second attachment. Do you think is it going to work?

Yes, probably.
It is curious that he has used two comparators to make a "window discriminator", so that when the voltage at the input of the two comparators is between two values the output of the comparator circuit is high.
I would have thought that one comparator would have been enough.

2. Is it OK if i replace the bulb with a resistor of a few ohms 1 or 2 W? What value shoulf the resistor have?

Difficult to say without knowing the temperature/resistance characteristics of the thermistor, and the heating/cooling effects of air/gasoline on the mounted thermistor.

JimB

 

[augustin87r]

Thank you!

The original dashboard bulb for Africa Twin is 12v 3W. If i want to replace it with a resistor, i need to know the value of the bulb's resistance.

P = V x I
3w = 12 x I => I = 3/12 = 0,25

V = I x R
12 = 0,25 x R => R = 0,25 x 12 = 3 Ohm

Is this correct?

 

[JimB]

Thank you!

The original dashboard bulb for Africa Twin is 12v 3W. If i want to replace it with a resistor, i need to know the value of the bulb's resistance.

P = V x I
3w = 12 x I => I = 3/12 = 0,25

V = I x R
12 = 0,25 x R => R = 0,25 x 12 = 3 Ohm

Is this correct?

Not quite
12 = 0,25 x R => R = 12/0.25 = 48 Ohm

But also, the resistance will vary as the lamp filament heats up.
When the filament is cold, its resistance is much lower.

JimB

 

[KMoffett]

Since you're dealing with a motor vehicle fuel tank that can experience -40°F to +130°F environments you need a way to compensate for fuel and air temperatures. Since your sensor is part a voltage divider just use two matched thermistors for the divider. One always above the fuel, and one in the fuel at the fuel-empty-level. One comparator should serve the purpose.

Ken

 

[JimB]

Since you're dealing with a motor vehicle fuel tank that can experience -40°F to +130°F environments you need a way to compensate for fuel and air temperatures. Since your sensor is part a voltage divider just use two matched thermistors for the divider. One always above the fuel, and one in the fuel at the fuel-empty-level. One comparator should serve the purpose.

Ken

An excellent suggestion!

However, I would suggest that the two thermistors are in series with a resistor so that when both thermistors are in air and "hot", there is not a thermal runaway situation where the thermistors get hotter and hotter and short out the supply.

JimB

 

[KMoffett]

An excellent suggestion!

However, I would suggest that the two thermistors are in series with a resistor so that when both thermistors are in air and "hot", there is not a thermal runaway situation where the thermistors get hotter and hotter and short out the supply.

JimB

Or switch to PTC thermistors.

Ken

 

[MikeMl]

Every Toyota I have ever owned has the low-fuel warning lamp. Go to a car junk yard and extract the thermistor from the fuel tank, and a dealer will probably tell you what the lamp in the dash is.

 

[augustin87r]

Thank you. very much. What an embarassing mistake i made

Thank you KMoffett for the wide view. Luckily, i don't have to make such a complex circuit because I use the motorcycle only in spring-summer, so i don't need it to be that accurate... Now i'm going to experiment with 100 -400 ohm resistors...I'll let you know ...

 

[JimB]

Every Toyota I have ever owned has the low-fuel warning lamp.

Mike, unlike the motor bike your car will also have had a real analogue fuel gauge, I guess that the low fuel light would be driven from the tank level sensor which drives the gauge rather than having a separate sensor.

JimB

 

[MikeMl]

Mike, unlike the motor bike your car will also have had a real analogue fuel gauge, I guess that the low fuel light would be driven from the tank level sensor which drives the gauge rather than having a separate sensor.

JimB

Nope, it is a really simple circuit consisting of just two components: the lamp and the thermistor. The Negative tempco of the thermistor, and the positive tempco of the lamp create a bistable circuit, one state when the thermistor is submerged in fuel and the other when it isn't.

 

[JimB]

Nope, it is a really simple circuit consisting of just two components: the lamp and the thermistor. The Negative tempco of the thermistor, and the positive tempco of the lamp create a bistable circuit, one state when the thermistor is submerged in fuel and the other when it isn't.

OK Mike, just that I would have expected a bit more sophistication when there is an analogue guage there.
Cheap and simple, there is a lot to be said for it.

JimB