How to implement a variable inductor coil?

I am creating a project that will replicated a car sensor that is not made anymore. The sensor is mechanical and detects vacuum pressure and an inductance coil is adjusted that changes the frequency of the input signal. You can see an image here:

Now I have a vacuum sensor breakout board that gives digital readings based on vacuum. Now I need an inductance coil that can be changed via the arduino based on those readings. All coils I have found are in the style of a pot that can be adusted with a screw driver. I am looking for something that can be adjusted via code. Any ideas on how to go about that? Thank you!


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So adjust the screw with a servo.

You may use stepper motor, but this is a quite weird way.
You may use DC magnetization coil, which will vary core mu due to saturation , but this is a qute wonky.

Maybe you better need not coil, but sin or square signal of the frequency proper on the board input?

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I was thinking about not using a coil also. But I think the problem is the incoming signal has to be modified (through some mechanism), think of tuning. I cant just generate a signal and send it back. So if there is another way to modify the incoming AC signal in Hz (Henry), I would be good with that.

Personally, I would check voltage, freq and the waveform on car's board with connected inductance and try to reproduce it with Arduino PWM or something like that.
Maybe it will work.

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Once you know the actual inductance range of the original sensor, perhaps someone can suggest something. Without that specific information, it is hopeless.

I have the original sensor in my car, so I can get that information. I have it hooked up to an oscilloscope to get the readings. Now I just need a way to replicate the behavior.

Ok, what is the inductance range you need?

... or you may provide variable-inductance gyrator with spi-controlled R-s in it... :crazy_face:

How is inductance used?
It may be possible to made with it by gyrator using an operational amplifier and a digital potentiometer.

@Paul_KD7HB @chrisknightley the inductance changes the frequency based on the amount of vacuum (see diagram in first post). The range is 2.55mH/70 kHz and the other extreme, inductance is 3.66mH @ 25 kHz.

Just thought ….

You could replace the ignition controller with something that uses a more conventional sensor ? It might be easier and more reliable

Megajolt maybe ??

Agreed, however, more conventional sensors use simple voltage as the indicator. Voltage goes up and down based on vacuum (see MAP sensor). Unfortunately this 80's car uses variable frequency to determine engine load.

Thank you! The diagram seems to use a ferrite pot core moving to change the inductance. You can do exactly the same design and use a servo to move a pot core to change the inductance, which is a very small range.

I envision one day someone using electromagnetic levitation to move a steel bar through coil thereby creating a programmable variable inductor.

Variable Inductor
Arduino Repulsive Electromagnetic Levitation

@Paul_KD7HB exactly this ^. I am a bit surprised this does not already exist. I thought this project would be a piece of cake with a "programmable variable inductor" breakout board. But as usual, it turns out to be way more complicated than I had hoped :).

Except for your project, what other use could there be for a production run of 10,000 per year?

I would try to inject the desired frequency into the oscillator input, without any coil. I don't know how exactly the circuit works but it should be possible to force it to accept the external frequency source.

I think I am going to try that. It just seems odd that instead of modifying the existing signal that I am jamming a new signal into it. Seems like they might fight each other? But worth a shot.

I agree, is it possible to "over power" the signal from the vehicle? I.e. if you sent back a square wave (perhaps with some filtering) of the appropriate frequency would the vehicle be able to interpret it?


Seems like they might fight each other? But worth a shot.

I think the vehicle is not creating the frequency but has some circuit that will generate a specific frequency based on what the coil inductance is at any given time.

If you find you must use a variable coil I would suggest you consider taking a screw adjustable coil apart and operating the core with something like this

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