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NTC Thermistor to turn LED on / off

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osphoto

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I have a NTC Thermistor rating of about : .75 - 1.1k Ohms @ 25C

This thermistor is used in a motorcycle gas tank.

Typically, a 12v lamp is used to indicate the fuel is low, Thermistor is at it's lest resistance.

I want to replace the 12v lamp with and LED. Seems simple enough since my knowledge of using an LED is using the resistor that comes with the LED in line with a power source.

This doesn't work since the thermistor has constant ground backed by 12v's and even at the highest resistance on the thermistor, the LED stays on.

What is the simplest way for me to get this to work?

I just want the LED to come on when there is no gas in the tank and OFF when there is gas in the tank.

Thank you in advanced.

I'm not sure what other info is needed.
 
The ntc thermistor is used by various manufactures for fuel level indication.

Since gas doesn't conduct, the thermistor is bare and submerged in the fuel and is at it's fullest resistance. Thus, the lamp indicator is off. No gas, thermistor heats up, less resistance lamp comes on. he concept is simple.

I need to replace the 12v lamp with a LED.

How does one go about doing that?

I can't just put a LED in place with a resistor because there is enough amps flowing to turn the LED on even when the thermistor is at it's fullest resistance.
 
osphoto said:
The ntc thermistor is used by various manufactures for fuel level indication.

Since gas doesn't conduct, the thermistor is bare and submerged in the fuel and is at it's fullest resistance. Thus, the lamp indicator is off. No gas, thermistor heats up, less resistance lamp comes on. he concept is simple.

I need to replace the 12v lamp with a LED.

How does one go about doing that?

I can't just put a LED in place with a resistor because there is enough amps flowing to turn the LED on even when the thermistor is at it's fullest resistance.

hi,
Connect a resistor in series with the thermistor, use the voltage drop across the resistor to switch a transistor. The transistor drives the LED.

Do you follow.
 
Tried that, and that didn't seem to work.

Perhaps it may have been the components that we used?

I have an electronics guy helping me with this, he's rusty on the subject so maybe we missed something when we tried that method of the transistor?
 
osphoto said:
Tried that, and that didn't seem to work.

Perhaps it may have been the components that we used?

I have an electronics guy helping me with this, he's rusty on the subject so maybe we missed something when we tried that method of the transistor?

hi,
A couple of questions.

Whats the voltage across the thermistor.?
Can you measure the current thru the thermistor [ full/empty]

I wouldnt expect the thermistor be directly across the 12V supply.?

Is one end of the thermistor grounded.?

I have a NTC Thermistor rating of about : .75 - 1.1k Ohms @ 25C

Are you saying its 1.1K when Full [cold] and 750R when empty [hot].?
 
Try connecting about a 200 ohm, 1W resistor in parallel with the diode and a 200 ohm resistor in series with the diode. The resistor in parallel will shunt the current around the diode when the thermistor value is high. The series resistor is the normal current limiting resistor for the LED.

You may have to play with the values some to get the correct operation since I don't know the exact characteristics of the thermistor when it's hot, I'm assuming around 100 ohms.

You can also connect 1 or more regular diodes in series with the LED to increase it's threshold voltage so it's off when the themistor is cold.

Attached is the schematic for this circuit.
LED-Gas.gif
CAUTION: Don't go below 100 ohms for the parallel resistor value or it may burn out the thermistor (and cause an explosion if it's near gasoline) It's probably best to experiment with the themistor far removed from any gasoline. Use water to test its operation.
 
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The voltage remains 12v.

I believe the resistance as stated is 1k when full (Cold) .75k when empty (hot).
 
See my added schematic for clarification.
 
Thank you for the gif lay out.

I'll give that a try, as along as radio shack has the parts i need.

Thank you!
 
That's an interesting way of detecting fuel level; this is how they do it in some OEM bikes?
I'd like to try it, except I wonder how you could do it with a single sensor without some kind of reference sensor. If you ride in temps ranging from 20 deg F to 98 deg F, it would trip just from the ambient temp, whether or not it's immersed in gas. I could see the possibility of it working if there's a second sensor left in the air for comparison though-
Do you have a schematic of the original setup?
 
osphoto said:
In looking at the schematic, I'm not sure of what size diode to use?
Just about any diode that will carry 100mA or more will do (such as a 1N4148). Since it's never reverse biased the voltage rating is not important.
 
wingerr said:
If you ride in temps ranging from 20 deg F to 98 deg F, it would trip just from the ambient temp, whether or not it's immersed in gas.
The difference in temperate that it detects is due to self heating from the current through it. When in liquid it stays basically at the ambient temperature. Even 100 deg F is not enough to turn on the light. When removed from liquid its temperature goes well above ambient, which significantly reduces its resistance and turns on the light.
 
crutschow said:
The difference in temperate that it detects is due to self heating from the current through it. When in liquid it stays basically at the ambient temperature. Even 100 deg F is not enough to turn on the light. When removed from liquid its temperature goes well above ambient, which significantly reduces its resistance and turns on the light.

hi crutschow,
I believe the OP said, 1100R cold and 750R hot, thats with a 12V supply source.
 
Tank indicator LED

Hi osphoto,

here is your required circuit. For simulation purposes the NTC is composed of RV1 and R4, both having a total resistance of 1100 Ohms when cold (submerged NTC).

If the tank is empty RV1 has no more effect and therefore the total restance is reduced to R4=750 Ohms.

Consequently the LED will be lit.

R1 is set to 56 Ohms provided the NTC resistance will reduce to 750 Ohms. If this value is not reached R1 has to be increased. Try 68 and 82 Ohms for proper function. This example triggers at an NTC value of 800 approx. Ohms.

Hans
 
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Hi crutschow,

I tried the 1W with 220 ohm and that sort of work with allowing the LED (including diode) to turn off while the thermistor is cold (full).

However, when the thermistor is hot (empty) the LED lights up but not to its full abilities.

So, i tried a few more resistors with lower ohms. The work in the sense when the thermistor is hot, the whole things gets HOT!! That being a 22 ohm, yea, I Know you said not to go below 100 but the LED won't go to full light. I bought a bunch of resistors to at least experiment.

I have noticed that that the 1 watt resister seems to be key here.

The thermistor I am using is bare, so I can't use water to cool it. Gas isn't conductive so, its a bare thermistor in gas. seems scary but that's the way these things where built. Thats also why these thermistors are on the "negative" side of the power.

I like the idea of this design because it seems compact and smaller the foot print, the better.

Thanks!
 
Hello Boncuk,

I'll give your design a try tomorrow most likely. Off to Radio Shack I go.

I'm guessing in your diagram, that Q1 is a transistor. I"ll do some searching as to the type. pnp vs npn and 2n2222 is something I can find at an electronics store.

I"m learning as I go here.

Thank you!
 
osphoto said:
So, i tried a few more resistors with lower ohms. The work in the sense when the thermistor is hot, the whole things gets HOT!! That being a 22 ohm, yea, I Know you said not to go below 100 but the LED won't go to full light. I bought a bunch of resistors to at least experiment.

The thermistor I am using is bare, so I can't use water to cool it. Gas isn't conductive so, its a bare thermistor in gas. seems scary but that's the way these things where built. Thats also why these thermistors are on the "negative" side of the power.
Did you try reducing the 100 ohm resistor in series with the LED? That will increase the current through the LED.

In general, reducing the parallel resistor value will reduce the LED current (shut it off) when the thermistor is cool (in liquid). Reducing the resistor is series with the LED will increase the ON current in the LED when the thermistor is hot (out of liquid).

As I said reducing the parallel resistor below 100 ohms may fry the thermistor.

It would be better to use oil rather than gasoline for experimenting. A light motor oil such as a 10W should work fine.
 
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osphoto said:
Hello Boncuk,

I'll give your design a try tomorrow most likely. Off to Radio Shack I go.

I'm guessing in your diagram, that Q1 is a transistor. I"ll do some searching as to the type. pnp vs npn and 2n2222 is something I can find at an electronics store.

I"m learning as I go here.

Thank you!

Hi osphoto,

yes, the 2N2222 is an NPN-Transistor. Radio Shack should have thousands of it. As I see you are not familiar with transistors. Obtain a data sheet with the pin layout at https://www.datasheetarchive.com.

In the circuit the base is connected to R2, emitter connected to GND, and collector connected to R3. Just follow the schematic when wiring the circuit. Nothing can go wrong, except you put in some extra shorts. :)

Hans
 
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