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Schematic

(Animated GIF of the simulation is here)

This is a simple circuit that makes the MOSFET turn off, as the temperature of the NTC thermistor rises.

As you increase the temperature in the diagram, the value in the voltmeter decreases.

5 volts--->4 volts--->3 volts--->2.5 volts ...

But I don't want voltage values between 5 volts and 0 volts. The value read on the voltmeter should be either logic high or logic low. How do I do this?

SamGibson
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johny adv
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  • do you want this only on Falstad? or for design purposes? you can use a comparator if it's for a design... – NeuroEng Aug 23 '21 at 17:56
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    Schmitt trigger. –  Aug 23 '21 at 18:09
  • This is from myself, so you need to interpret it by researching the key-words. Logic is a different "domain" from the "voltage level". And, those can be mapped by "symbolic" association, doesn't matter what voltage. Google search of "logic level detection" suggested many and this and that are the first and second pop-up-s. – jay Aug 23 '21 at 18:36
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    Do you want to change what you see, or do you want to change what happens? – hobbs Aug 23 '21 at 19:44
  • CircuitJS (the actual name of this simulator) has "logic outputs" that will show L or H instead of showing the voltage. – user253751 Aug 24 '21 at 08:59

1 Answers1

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Using the threshold of a FET as a comparator is a very questionable design practice. The threshold voltage can vary quite a bit, even for a specific FET. It’s impossible to guarantee a consistent input to output transition.

The reason you don’t see a sharp transition between output-high and low also relates to the FET near-threshold behavior. As the FET gate voltage crosses over the threshold, the FET is being biased in its linear mode, so the output makes a gradual transition as the temperature increases, rather than the sharp transition you’re looking for.

Strongly suggest instead that you add a comparator to the circuit, like an LM339 or similar. Falstad has a comparator model in its library. Then you can set a precise threshold that will snap the output state to a definitive level.

One more tip about Falstad: you can create a link to your simulation and add it to your description, so that other people can run it as well.

Related: Why is there a resistor in this circuit?

hacktastical
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