NTC Thermistor to turn LED on / off

[MrAl]

Hello,


Testing at 12v or so can easily be done in distilled water, and even some tap water wont conduct too much and will allow fairly good testing. Distilled water is sold in gallon jugs at supermarkets that sell food. Remember water does not conduct it's the minerals in some water that can cause conduction.

Probably the best bet here is to use a comparator circuit where you set it up to detect the resistance of the thermistor, while at the same time biasing the thermistor with enough current to allow self heating as the old bulb did.

A funny solution would be to leave the bulb circuit intact and use a photo cell to detect the light which would then turn on the LED. The circuit would have no choice but to work exactly the same except hide the bulb and show only the LED

 

[EricZ]

The led install isn't just to look cool. I have removed the stock gauge cluster which held the indicator lamps. I am waiting on the components that second286 recommended and will build and test that setup.

 

[Reloadron]

For what it may or may not be worth this link provides a pretty good overview of the use of a thermistor as a low fuel warning light driver. If the resistance of the original bulb was known a small circuit could be possibly built around that. Maybe a resistor to replace the bulb and the drop across the resistor to enable or inhibit a 555 running astable to blink a LED as a low fuel warning?

Ron

 

[MrAl]

The led install isn't just to look cool. I have removed the stock gauge cluster which held the indicator lamps. I am waiting on the components that second286 recommended and will build and test that setup.

Hello again,

Ok sure, it's up to you what circuit you want to try first. I always try to recommend a circuit that will definitely work even if it involves a few more parts. A comparator chip is very low cost too these days so that's not a problem.

The only question i have with that transistor and zener circuit is will it provide enough self heating for the thermistor. The thermistor change in resistance depends a lot on the change in temperature when the thermistor looses it's main heat sinking (the fluid in the tank) and it will not heat up much without the proper current flow.

 

[EricZ]

Could you explain how to use a comparator chip for this? I am very inexperienced with specific electronic devices and how to design circuits. I would really benefit from and appreciate any info on these various components and how they function. Thank you!!

 

[Reloadron]

Could you explain how to use a comparator chip for this? I am very inexperienced with specific electronic devices and how to design circuits. I would really benefit from and appreciate any info on these various components and how they function. Thank you!!

This link is a good read on comparator circuits. It should afford a basic understanding of how they work.

We know in the original circuit a NTC thermistor was used to illuminate a light bulb for a low fuel status. We also know how it did that, as the fuel level went down there was no liquid to remove the self generated heat of the thermistor. The resistance decreased and the overall current in the loop increased lighting an incandescent bulb. If the bulb is replaced with a resistor we know the current flow through the resistor will change on a low fuel status. Since the current through a fixed value resistor changes so will the subsequent voltage drop (the voltage developed across the resistor) will change. That voltage developed across the resistor could in turn be fed to an input of a comparator.

Something to consider using a comparator would be to add hysteresis (mentioned in the link) for stability on a low fuel indication.

Hope That Helps
Ron

 

[KeepItSimpleStupid]

OK guys, I had to replace a lamp in a tuner with similar wierd problems that I think we are seeing here. What I would suggest is Ground + thermister + optional resistor+ Zenier diode + 12 V or however the circuit is arranged.

I missed the empty and full values for the thermister resistance.

I bet I could figure something out given the empty and full resistances of the thermister.

 

[MrAl]

Could you explain how to use a comparator chip for this? I am very inexperienced with specific electronic devices and how to design circuits. I would really benefit from and appreciate any info on these various components and how they function. Thank you!!

Hi again,


See Ron's post right after your last one. If you still have questions just post back here when you are ready.

 

[tempest411]

Hello,

I have this schematic which has worked very well in the past for this use, so I thought I'd post it here. I've used it for a low voltage warning lamp system so far, but it should also work as a switch for a thermistor to an LED. I'll just put a 12V regulator at the input to insure a steady voltage supply (barring a charging system malfunction).

Rick