Hey!
I recently found this circuit in which an LED is being turned on when the resistance of the NTC goes below a given value. I was wondering if something similar could be made with more LED's (and transistors?) in order to sense several temperature levels.
Does anybody have an idea on the topic?

 

Welcome to AAC!

Look up LM3914 LED driver IC.

 

Thanks very much. However, I'm still interested whether it's possible by using only transistors.

 

Yes, in theory it will work, but...

It will have a very "soft" threshold that depends on BOTH the thermistor and transistor temperature.
AND it will be very sensitive to the battery voltage.

Not usable in the real world.

 

Hey!
I recently found this circuit in which an LED is being turned on when the resistance of the NTC goes below a given value. I was wondering if something similar could be made with more LED's (and transistors?) in order to sense several temperature levels.
Does anybody have an idea on the topic?

When the resistance of the NTC goes below a given value - what will happen?
I think, the voltage Vbe goes down - and with it the collector current through the LED.
An this effect will turn on the LED (as you think)?

 

Ooh. I made a huge mistake. In my version the NTC and the potentiometer are swapped. So as the NTC-resistance gets lower the LED lights up.

 

whether it's possible by using only transistors.

Not with the simple transistor circuit you show.
Each transistor circuit would need a difference voltage sense level and that would make the circuit significantly more complicated.

One easy way is to use a comparator, such as the quad LM339, for each sensed temperature.
Each comparator can be biased to sense a different voltage level, and thus a different temperature.

 

Thank you very much!

 

You can use the NTC thermistor and a string of resistors in series.
Thus the voltage at each junction will be progressively higher which can then drive the base of a transistor. I would prefer to use a digital IC with six inverters such as 74HC14 for better defined switching thresholds and to buffer the signal to the LED or use darlington transistors.

 

I would suggest dividing down the voltage (if necessary) to something suitable for a microcontroller, which can then do whatever you want...turn on LED's, etc. You won't need multiple sense levels this way.