-1

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
  • 17,231
  • 5
  • 37
  • 58
johny adv
  • 55
  • 4
  • 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
  • 2
    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](https://whatis.techtarget.com/definition/logic-level) and [that](https://learn.sparkfun.com/tutorials/logic-levels/all) are the first and second pop-up-s. – jay Aug 23 '21 at 18:36
  • 2
    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

2

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
  • 49,832
  • 2
  • 47
  • 138