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I am in the process of modeling a trigger circuit and have been running into some trouble, both conceptually and in the modeling process. Below is the circuit I came up with in Circuitlab. I had a similar circuit in LTspice, but had trouble with simulations where it would hit a critical value and the voltage would oscillate at an unexpected amplitude. I know my rheostat in LTspice was not correctly defined and that could have caused the issue with the oscillations; however, there were some other issues I could not account for.

  1. In my LTspice model, I have it set so that SW2 activates on startup for 1s, then deactivates. After that there is a 1s pause before I activate SW1 which stays on for 1s then deactivates. This produces the second image below. I understand why the voltage drops to 0 at the source as a result of the trigger being pushed in terms of the simulation. I don't believe the states should be the same for when the trigger is pulled and when it is not. Practically, when the trigger is pushed completely, the voltage should be able to pass freely. I am guessing I am missing a load setup or am taking measurements at the wrong point in the diagram.

enter image description here

  1. R7 in the below circuit gets shorted upon SW2 being closed, but has minor oscillations when SW1 is closed, I assume this is due to the capacitor being discharged, but wanted another opinion.

These are a couple of other threads related to this project:

IC Identification Thread - Pictures of Physical Components

LTspice rheostat simulation

I am just looking to understand how a basic variable speed trigger operates. I am sure there are some/many issues with how this circuit is designed. I am fairly confident about the IC layout and the SW2 connection, though I am probably missing a connection for a load I should be measuring across.

schematic

simulate this circuit – Schematic created using CircuitLab

JTH828
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    ‘Trigger’ circuit? That doesn’t tell us much. Trigger on what condition? – Kartman Nov 10 '21 at 21:33
  • It's a general replacement trigger for an electric battery operated drill in this case. The trigger would be someone physically pulling a contact across the rheostat. Updated the title for clarity. – JTH828 Nov 10 '21 at 21:35
  • I wouldn’t call that a trigger circuit! The ‘trigger’ in this instance is a on/off switch. The real fun happens with the speed controller. – Kartman Nov 10 '21 at 21:37
  • Ah, that makes sense. So, aside from the two complete states, that being completely off or completely on, it should be more considered a variable controller. Edited title again for even better clarity. – JTH828 Nov 10 '21 at 21:39
  • So you want to build a speed controller? A 555 timer ic set up as astable with variable pulse width might be what you want. – Kartman Nov 10 '21 at 21:45
  • I think showing this ideally would have a switch for the button being pushed, a switch for the button not being pushed, and then a switch for operations that fall in between. And for that timer ic, I'm not too familiar with how they operate, so I apologize in advance, would I use a rheostat or some other variable controller to modulate the frequency of the output signal? I was trying to reverse engineer an off brand replacement trigger in hopes of understanding it. – JTH828 Nov 10 '21 at 21:52
  • Your circuit does not make much sense. Having a floating input to an opamp is bad juju and a capacitor in series with the mosfet gate is also strange. I think you’ve made some very wrong assumptions. – Kartman Nov 10 '21 at 22:54
  • Yeah, I had some concerns about the second opamp. The other two links in the post have pictures of the board itself as well as some pins. Would the capacitor traditionally get tied into the source? I think that would make the most sense, but could be wrong. As for the opamp, I think it would just act as an amplifier in this case. – JTH828 Nov 11 '21 at 13:29

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