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I am trying to build a circuit based off of a trigger I took apart and recreate it in LTspice to simulate how it operates.

I am having trouble simulating a rheostat for this circuit.

The basic premise is that the contact slides with the trigger across the rheostat which is between two pads which are connected to an opamp. I have set up two rheostats since as the contact slides, the resistance to each of the opamp's inputs is altered. I have two questions with this.

  1. Can I get a resistor to have a constant value for a set time then have a variable value? The purpose of this would be to show each state of the trigger, that being fully unpressed and fully pushed, and then with the varying value based on how far the trigger is pushed.

  2. Is there a better way to set up a rheostat to increment and decrement to its limits? I would like to have the rheostats climb to its max value on one, and decrease from its max value to one on the other.

Below is my current LTspice setup and results.

enter image description here

The voltage is measured at the source of the n-enhancement MOSFET, since I assume that is where the load would be hooked up to. The first 30s are set to have the trigger fully pushed, from 30s to 50s the trigger state is set to fully unpressed, and the last 10s I was looking to show the affect of the rheostat. The periods for these are irrelevant and if they should be adjusted to optimize the simulation, it's no issue.

R3 and R6 are the ones I was looking to have act as rheostats.

enter image description here

This is a picture of the component I am trying to replicate, minus the IC. The black strips are the rheostats.

enter image description here

I believe I may be missing a few other key aspects of the circuit as a whole, but want to address the rheostat if possible.

JRE
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JTH828
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    See [this question](https://electronics.stackexchange.com/questions/229545/is-there-a-potentiometer-model-for-ltspice), it may help you. – rdtsc Nov 09 '21 at 17:34
  • Is it possible to make a rheostat out of a potentiometer? If that works, I'll take that approach and see what I can make up. – JTH828 Nov 09 '21 at 21:04
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    A rheostat *is* a potentiometer; just a higher-power version of it. LTspice is only modelling the resistance (Ohms), not taking into consideration power (volts * amps.) For this sim, it won't matter. – rdtsc Nov 09 '21 at 21:48
  • What @rdtsc ssays is true, I'll just add that in the SPICE world you can generate 1 kA over 1 Ohm and nothing will burn. Not even overdriven transistors & co. It doesn't mean that it will not burn on the breadboard, just that it's simply a numeric solver, and it will throw whatever results it finds (it's up to the user to detect the anomalies). – a concerned citizen Nov 10 '21 at 09:14
  • That makes sense, I installed the library and symbol from the LTspice group. I set each to be a a linear potentiometer with a max resist of 412k. Would I set the wiper to be a negative value in order for it to decrease over time, or would I need to look for an alternative symbol? Also, does the wiper pin on the model need a source voltage tied to it? I looked through the readme quickly and couldn't find anything that explained that part. – JTH828 Nov 10 '21 at 14:03

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