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Thermistor Troubles

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circuitfish

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Hi Everyone,

I've built a Schmitt trigger which is connected to a thermistor. Trouble is I want the Schmitt trigger to set off an astable when i heat the thermistor with my fingers but because of the small change in resistance I can't get the astable to be set off without manually turning the variable resistor.

I've tried an LDR and it works fine. Any ideas would be much appreciated.
**broken link removed**
**broken link removed**
 
The feedback resistor is far too low, I'd replace it with a 100k fixed resistor and use the variable to adjust the reference.
 
Hi Everyone,

I've built a Schmitt trigger which is connected to a thermistor. Trouble is I want the Schmitt trigger to set off an astable when i heat the thermistor with my fingers but because of the small change in resistance I can't get the astable to be set off without manually turning the variable resistor.

I've tried an LDR and it works fine. Any ideas would be much appreciated.

hi,
Whats the OPA also what is the specification of the thermistor.

As 'hero' points out, you have far too much positive feedback.
 
If you know the on/off temperatures and the resistance of the thermistor at those temperatures, you can calculate the resistor values so there's no need for trimmer resistors, unless you want it to be adjustable of course.

If you use a dual comparator such as the LM393 it's possible to use one comparator as the astable so you don't need to use another IC such as a 555 timer.
 
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