# NTC Thermistor Trigger LED

I'm wondering if anyone would be willing to help me with the basics here. I'd like to create a circuit, as small and simple as possible, with off-the-shelf components, that will allow me to turn on an LED when the temperature of this particular thermistor (MA100BF103A) exceeds 37 degrees C.

The resistance vs. temperature values for this thermistor are online here.

Is this even possible without something like an arduino? What parts would I need, besides the thermistor and an LED? How would I wire them together?

I'm a complete novice. Thanks!

• Solved on this site already, I think. Commented Mar 1, 2020 at 17:19
• for this particular thermistor? link please? Commented Mar 1, 2020 at 17:28
• Please edit your question to explain what part of the referenced circuit you didn't understand or you think doesn't suit your application. Commented Mar 1, 2020 at 17:31
• Opamp and some precision resistors.
– user16324
Commented Mar 1, 2020 at 17:47
• @BrianDrummond - I'm curious why you choose an op amp (lm358) over a comparator (say and lm393) for this? I'm trying to learn... Commented Mar 4, 2020 at 20:32

You won't get much simpler than this.

simulate this circuit – Schematic created using CircuitLab

As the temperature rises, the voltage on IN+ risis; at 37C it crosses the threshold and the LED switches on.

You posted the data for 5 thermistors, I'm assuming the correct one is the right and column based on the numbering (103) where 37C corresponds to 6.2k.

So I made all the other resistors in the bridge the same value, so that they are likely to be from the same batch, reducing error : 1% tolerance is probably good enough, go for 0.5% if you can afford it. Supply is not critical, a 9V battery is OK but 5V should work too. Or 2 Lithium coin cells, a bit smaller.

It appears that assumption is wrong; for the thermistor labelled "10K" the correct resistance is 6.016K. Simplest fix is to bring R4 down to (about) 6.016K by adding 200K in parallel with it, so that's what I have done. (6K resistors are a bit less readily available).

The LM358 is a pretty standard opamp (datasheet here) - you probably want the LM358P or another variant with the DIP-8 package. On Digikey, eliminate "surface mount" from the search and it's easy to find.