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Need advice on choosing an oscillator

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m12lrpv

New Member
Hi
I am building a hobby project which involves using an induction coil as an eddy current sensor. I'm not an EE or an EE student; i'm just doing this for my own purposes.

I have a basic functioning circuit which pulses the coil at using a 555 circuit and some pots. The driving frequency range is currently adjustable between approx 32KHz and 87KHz which seems to work well with the various coil sensors that I have tested. The duty cycle is presently unknown as I don't have a scope.

The ability to tune the frequency to match the coil circuit seems to be critical as it's the variation in resonant frequency that I am using to measure the variation in inductance. It's presently working although in need of amplification.

I'm confident that I will be able to work my way through many of the challenges associated with the circuit itself but I have realised that choosing the oscillator mechanism is something that I should seek advice on.

What would be the best approach for creating a stable oscillating wave (of whatever shape) for driving a inductor at frequencies adjustable through my current range from 30KHz to 90KHz. Is the current 555 circuit my best choice or is there something out there?

I am expecting to have to wind my own coil sensor at some point and it may not have the same inductance as the commercial coils I am presently using. That's not a major problem except that it may raise the frequency required to be more than what the 555 is able to generate reliably.

What alternative would I use to create a stable adjustable frequency with a 50% duty cycle above 100KHz? I have had success generating a oscillating square wave of unknown duty cycle using a LM339 comparator which gave me up to approximately 150Khz. That was in a separate experiment and I have yet to revisit it.

Are crystals a better option? Or the ceramic resonators?

The current 555 oscillator adjustment is via pots but I would like to have it adjustable via micro controller so that I can do things like put in a temperature sensor and use that to adjust for temperature changes to further create stability (not asking how to do that because I think I can work that out myself).

Regards,
David
 

alec_t

Well-Known Member
Most Helpful Member
If wave shape doesn't matter then IMHO it will be easiest to meet your requirements by generating a square wave. A suitably programmed micro with a crystal/resonator-controlled clock source could do that easily, and drive a buffer stage to supply the coil.
 

carbonzit

Active Member
Just another idea to throw out there: if you want a really simple, yet reliable square-wave oscillator, consider using an astable multivibrator. Just two transistors and a few passives. The linked page has a handy table to size the RC components to give a desired frequency.
 

m12lrpv

New Member
Thanks for the reply's.

I conducted some tests last night and modeled up my current 555 circuit (basically the same as what riccardo posted) in LTSpice.

I realised then that my current duty cycle was not 50% and that was the reason that I can see a voltage variation in the smoothed output from the coil. It is that voltage variation that I am currently using to easily detect the variation in the coil resonance. So I have learnt that a non 50% duty cycle is important to what I am doing. I hope that doesn't mess up some of the suggestions.

I will have a play with the circuit carbonzit linked to. I'm hoping that will work with some simple transistors like the 2N2222

I think i'll avoid the microcontroller at the moment although I appreciate the suggestion. I've been playing with one (an arduino) to make my frequency and voltage measurements of this project and from the documentation getting it to provide a managed frequency without making that the sole purpose of the micro is a bit hard.

Does anyone have an opinion on the VCO's like the LM331?


Thanks again.

Regards,
David
 

carbonzit

Active Member
I will have a play with the circuit carbonzit linked to. I'm hoping that will work with some simple transistors like the 2N2222

Yes. I've built it with exactly that transistor, and it works fine.

The nice thing is that it's easy to get pretty close to exactly 50% duty cycle simply by matching the R-C components. (Of course, you really can't verify this without a 'scope, but you can be pretty well assured of even mark/space ratio.)
 

m12lrpv

New Member
I tried it and couldn't get it to work or more likely couldn't observe it working as I don't have a scope. Thought I might have cooked the transistors but a test circuit marked them as ok.

That link also had an oscillator based on a Schmitt Inverter so I tried that and got it working but it was very inconsistent whereas the current 555 circuit is proving to be very stable <0.005% so I think I'll stick with that

I did some reading up on VCO PLL's so I may give them a go if I hit the limits of the 555.

I was talking to a rather knowledgeable person when it comes to induction sensors and he said that they typically drive them at very high frequencies so I'll just have to see how things go.

Thanks for the advice.
 

carbonzit

Active Member
I tried it and couldn't get it to work or more likely couldn't observe it working as I don't have a scope. Thought I might have cooked the transistors but a test circuit marked them as ok.

Then it probably worked just fine. There's really no way to detect such high frequencies (well, relative to audio, anyhow) without a 'scope or similar device.

Can you borrow a 'scope? or build a frequency divider to verify operation of the oscillator?

I really like the multivibrator because it's so simple a circuit (and gotta love that symmetry too!).
 
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