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thermistor

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mat337d

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I have several ntc thermistors 100 150 200 10000 ohm. I am trying to use the thermistor to control power to a led. So when the thermistor cools slightly
the led flickers. (just like blowing on a candle). I have tried wireing in many different configurations with no sucess. It was sugested that I was saturating the thermistor with current but I have not been able to get it to work. I am a
beginner when it comes to electronics. any help would be appreciated.

PS 9V 300mA
white led ~ 4V 20mA
 
matt using a LM139 is a good place to start ..
someone could probably draw you a complete circuit..
but the idea is, set up your reference input with voltage divider to say 5V
then use your thermister to vary the resistance in the leg connected to the Vin input..
when your thermister encounters heat the resistance changes and turns on the comparitor; with the comparitor on you would have to drive a transistor gate of a FET becaue the comparitor cant drive the LED directly , actually it probably could since it has short circuit protection..but in any case it can only provide 16 mA and you would like 20 mA..
 

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Im not sure but wouldn't the comparator give the led a on off (digital) effect.

If the thermistor is blown on the temp goes down and I would like the brightness to also go down. Kind of an exponential decay rather than a on off effect. just like a candle.
 
Hi Mat,
Use your 100 ohm thermistor in series with a 82 ohm resistor. With your battery across both the current will be high and the thermistor will selfheat. Connect their junction to the base of a transistor wired as an emitter-follower. The LED and its current-limiting resistor connects from the emitter of the transistor to either ground or the positive supply, depending on whether it is an NPN or PNP transistor respectively.

The LED's brightness will change with the thermistor's resistance change, which might not be much.
The LED won't flicker because the thermistor can't change its temp quickly.

What you need is a white noise generator with its output level contolled by the voltage at the junction of the thermistor and its series resistor. You might have to blow on the thermistor for many seconds for it to cool enough to increase the level of white noise driving the LED.
You can amplify the voltage change with an opamp.
Now the circuit eats batteries because of the power required to heat the thermistor, the circuit is getting complicated and it still might have too much delay.
 
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