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I want to replace a potentiometer with a force sensitive resistor (FSR.)

I’ve bought one that varies from 1kg —0g. I don’t need the middle pin of the pot, I need it to work as a resistor.

My circuit is a 30V DC motor control, and the 10K potentiometer, wired in parallel with an internal 33k trimpot where the adjustment of speed of the motor is done, controls a TIC226D triac from a 12-0-12 transformer. After the triac, the AC voltage gets transformed to DC through a diode bridge and an electrolytic capacitor, and powers the motor.

I want to implement the foot pedal of my motor, which acts today as a switch, to also vary the speed. So, I'd bypass the 10K potentiometer with the FSR inside the pedal, when the pedal is activated.

I had to open the control box of the motor. I did this controller maybe 15 years ago, so I don't have the schematics anymore. I could try to draw it completely if needed.

In this one I bought, its resistance varies from 1M to 2K, but I need its resistance to be close to zero when I press it at maximum “force." I’d also like its max resistance to be close to 10K.

Is there any way, a circuit, that could allow this to happen? A workaround to “set” its top and lower resistances to a defined value?

Attached goes the circuit schematics (corrected II). I didn't draw the fuse and on/of switch on the primary side of the transformer. The Triac is a TIC226D

Corrected Diagram - I guess :)

Rodrigo
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  • 5 kΩ really isn't anywhere close to "infinity", so I changed your question title. Especially, since the original title asked for something that was impossible. – Marcus Müller Mar 20 '20 at 11:16
  • So, how did you use the potentiometer? Was it used to vary a voltage, with an externally fixed current flowing through it, or was it used to vary a current, with an externally fixed voltage? Or something else? That's what is important to know here – whenever I'm replacing something by something else, I'll need to know what it was used for before. – Marcus Müller Mar 20 '20 at 11:18
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    Smells of XY to me. – Andy aka Mar 20 '20 at 11:24
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    @Andyaka yeah, a lot. Rodrigo, [you're asking for a solution to a problem that's probably only a problem because you decided to solve another problem in a strange way](http://xyproblem.info); I don't think anyone can give you a sensible answer if you don't fully explain what that potentiometer was used for, very likely requiring a full schematic. – Marcus Müller Mar 20 '20 at 11:26
  • @MarcusMüller, more details added. I can draw the schematics, but I would need some hours to do it.. – Rodrigo Mar 20 '20 at 12:00
  • wow, yeah, that changes things *a lot*; not only with possible solutions: It's way easier to explain things for someone who's built motor controls than some kid who's playing with their first LED blinker... So, this is a bit hairy, because Triac control effectively means we need to be a bit careful w.r.t. safety. Hm. Hard problem! – Marcus Müller Mar 20 '20 at 12:03
  • @MarcusMüller: It sounds like the triac is on the low side. But, a schematic would really be a good thing here. – JRE Mar 20 '20 at 12:16
  • @MarcusMüller Ok. Will reverse engineer the schematics.. eheeh. I'll post it later! – Rodrigo Mar 20 '20 at 12:17
  • @Rodrigo looking forward to it :) – Marcus Müller Mar 20 '20 at 12:18
  • @MarcusMüller Done! Ops. Forgot one capacitor. Will add and correct the diagram. Sorry – Rodrigo Mar 20 '20 at 14:20
  • ah interesting! how linear do you want the adjustment to be? – Marcus Müller Mar 20 '20 at 14:29
  • @MarcusMüller. Corrected it I guess... I wanted it from 0-100% speed as linear as possible... if possible.... – Rodrigo Mar 20 '20 at 14:34
  • Will that triac do anything on negative half-cycles? Quick simulation suggests the LED bypasses it on negative half-cycles. – Hearth Mar 20 '20 at 14:34
  • ([the simulation i mentioned](http://tinyurl.com/s9ftvwe). Don't worry about the horrendous motor model; it isn't relevant to the LED bypass problem anyway) – Hearth Mar 20 '20 at 14:41
  • @MarcusMüller Is the simulation right? I'm trying to vary the potentiometer value and the voltage and current remains the same in the motor... I'll recheck if I forgot something. You forgot to add the small electrolyctic capacitor I added afterwards in the simulation.. – Rodrigo Mar 20 '20 at 15:05
  • @Rodrigo Ah, that cap certainly [improves things](http://tinyurl.com/vw92g4x) a good deal; the triac can actually turn on now. I don't think you had added that capacitor when I made the simulation. – Hearth Mar 20 '20 at 17:01
  • @Hearth Sorry! I answered to Marcus and not to you! Yes, I forgot the capacitor. But still, when adjusting the pot value in the simulation, the voltage doesn't change in the motor. Is that right? – Rodrigo Mar 20 '20 at 17:06
  • @Rodrigo Bear in mind that I have no idea what the parameters of your triac are. I just used some random numbers that made it appear to work. – Hearth Mar 20 '20 at 17:13
  • @Hearth Sorry! The Triac is a TIC226D. Will update the original post! – Rodrigo Mar 20 '20 at 17:48
  • @Andyaka What does XY mean? – Rodrigo Mar 21 '20 at 12:23
  • @MarcusMüller Do you think there´s way to solve my question? To adapt a FSR to my circuit? I could make some changes in the original circuit if needed! – Rodrigo Mar 23 '20 at 13:14
  • @Hearth Same question I did to MarcusMuller: Do you think there´s way to solve my question? To adapt a FSR to my circuit? I could make some changes in the original circuit if needed! Are you guys in lockdown? We in Brazil are told now to stay at home, and shops and stores and gyms are closed for 15 days for now... – Rodrigo Mar 23 '20 at 13:15
  • Yes, I think it's possible to solve your problem, but I don't know how hard it will be. Might be easiest to use a microcontroller to convert the signal from the force sensor into firing pulses for the triac. And to answer your question to Andy: An XY problem is when you have a problem that you think could be solved by method A, but instead of asking "how do I solve this problem", you ask "how do I implement method A". The former question is better, because someone can then tell you that, perhaps, method B is a much simpler way to do it. – Hearth Mar 23 '20 at 13:39
  • @hearth Thanks. Got it. I do have small arduinos, I could read the FSR signal as an analog input on the arduino, but I wouldn´t know how to "send" correct pulses to the triac .I would need help in the electronics part to perform this...Thanks for studying this for me. – Rodrigo Mar 23 '20 at 13:51
  • @Rodrigo you'd need some form of gate driver, since the arduino itself probably can't handle the current needed to turn on a triac, and definitely can't do a negative pulse on its own. – Hearth Mar 23 '20 at 13:52
  • @Hearth, could you help me in this idea of yours of this gate driver, to connect to my circuit? :) – Rodrigo Mar 31 '20 at 00:16
  • Comments are not for extended discussion; this conversation has been [moved to chat](https://chat.stackexchange.com/rooms/109060/discussion-on-question-by-rodrigo-is-there-a-way-to-make-a-force-sensitive-resis). – Voltage Spike Jun 08 '20 at 17:12

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