Can pinpointer detecting thin chain?

[Nupi]

I Can't Weigh that Small of an amount.

However,......................

I can even weigh that, but the jeweler does.


You weigh the chain without closure.
You count the number of links.
Your divided weight by the number of links.
Now you know the weight of each link.

Tell it to anyone else.


Post #86

[Davor]

It is a great pinpointer and very nice documented project. I think also it needs a notes with explanation how electronic part works to let users to understand it better and fix it more easy if problems occurs. Thanks!

Basically it is an absorption detector.
Tr1 forms a Hartley oscillator and it is biassed so that it operates in linear regime that is prone to amplitude changes. When a metal object is introduced to the coil, the amplitude of oscillation is reduced because of the eddy current losses, but more importantly, the transistor power consumption is enlarged, and thus the emitter voltage gets shifted. This arrangement is not new, and most pinpointers work at this or similar principle. The change in the oscillator working conditions is translated into some sort of indication.
What is new and very good here is the way this working point gets detected
It is the role of 4024 that forms a relaxation oscillator with narrow strip of input signal levels that brings it from non-operation to saturation, and in a process it provides a very sensitive indication. It operates on the oscillator's envelope rather than the DC level, pulse durations are set by pulse counting method, and in a process it provides pulsed audio. Brilliant.
Attached is the LTspice solution with selected traces for almost no target (lower pane), and a close by target (upper pane). I'll place the Spice files soon.
Enjoy

[Nupi]

Basically it is an absorption detector.
Tr1 forms a Hartley oscillator and it is biassed so that it operates in linear regime that is prone to amplitude changes. When a metal object is introduced to the coil, the amplitude of oscillation is reduced because of the eddy current losses, but more importantly, the transistor power consumption is enlarged, and thus the emitter voltage gets shifted. This arrangement is not new, and most pinpointers work at this or similar principle. The change in the oscillator working conditions is translated into some sort of indication.
What is new and very good here is the way this working point gets detected
It is the role of 4024 that forms a relaxation oscillator with narrow strip of input signal levels that brings it from non-operation to saturation, and in a process it provides a very sensitive indication. It operates on the oscillator's envelope rather than the DC level, pulse durations are set by pulse counting method, and in a process it provides pulsed audio. Brilliant.
Attached is the LTspice solution with selected traces for almost no target (lower pane), and a close by target (upper pane). I'll place the Spice files soon.
Enjoy

An excellent study and explanation, genius. Thank you very much.

I do not know if it is still possible with the LTspice to tell
something on the role of the coilloop on the circuit and the filtering
of the voltage on the sensitivity at the time of detection?

[Davor]

It is possible, but it is often more rewarding to do some makeover for the specific purpose of improvement.
I rearranged your detector's front end to a Colpits type, and it is far more interesting to me because it uses a single coil, and in this configuration it operates in near saturation mode. I guess it will have better stability, but I'd have to test it in real life. If you are up to it, please let us know if it works and how it performs.
I only made a few small changes against the original layout, but the mode of operation is quite different.
A single coil is much easier to make, and it would lend itself nicely if you ever wish to make a grown-up detector with a large air coil and a stem out of this project.
Here is the schematic, lower pane is with no target and the upper with a target near by:

[Thomas]

Thanks Davor for the Colpitts front end version, and the explanation for the original Nupi. What do you think about using a 4060 with the internal OSC ?

[porkluvr]

Davor, this may be a little premature because you have not yet posted your simulation file, but you did post a screen-shot... so would you please give a quick explanation of how "click" works in your .param and K2 statements? I don't know what you did and I can't seem to figure it out! (I'll bet I'm not alone.)

Thanks!

[chemelec]

I can even weigh that, but the jeweler does.


You weigh the chain without closure.
You count the number of links.
Your divided weight by the number of links.
Now you know the weight of each link.

Tell it to anyone else.


Post #86

Weighing my Whole Small Chain, "150 Links", it works out to .0095 Grams Per Link.
So Slightly heavier than Nupi's chain.

[Qiaozhi]

Davor, this may be a little premature because you have not yet posted your simulation file, but you did post a screen-shot... so would you please give a quick explanation of how "click" works in your .param and K2 statements? I don't know what you did and I can't seem to figure it out! (I'll bet I'm not alone.)

Thanks!

The parameter named "click" is the coupling coefficient between L1 (the detector coil) and L2 (the target). The .STEP command causes the simulator to make 3 runs, first with K=0 (no target), K=0.001 (weak target), K=0.1 (stronger target).

[Davor]

@Thomas, CD4060 might be handy for it's access to the input stages (pins 9,10,11) but because the bias is set in the oscillator, this feature does not seem essential for this kind of detector. Pulse counting approach takes care of the analogue response. BTW, I did not test the Colpitts version in real life, and I'm confident it should work only because I used two inverters simulated with MOSFETs to mimic the real life conditions.

@porkluvr, "K" statements handle coil couplings. In a Hartley version i used a dirty trick and stated only L1 and L3 coupling at .1 ... which is not correct in spice, but works for very weak couplings without triggering any error messages.
In Colpitts version I simply copied the K2 statement and did not go into tidying it.
The "click" parameter's name is inspired by a clicker that some people use to make some professional pinpointers more sensitive. It was an arbitrary choice. It sets coupling value (presented as {click} in the K statement). ".step param" command makes LTspice go for two passes, first with coupling at 0.001 (negligible) and then 0.1. The goal was to find a "potentiometer" value at which the first pass does not trigger beeps on neither of the "pin#" outputs after some initial settling period, while it should beep wildly in a second pass.

Now, I have a model of mine of variably coupled coils that I wish to try here, because I can supply it with a varying coupling and see how the pinpointer copes with it. I'll make all these soon and post the Spice files here.

There is a hidden detail that might prevent you from mimicking the device properly, and that is the coil Q. There is always some resistance of the coil, and I just left it hidden.

[Nupi]

It is possible, but it is often more rewarding to do some makeover for the specific purpose of improvement.
I rearranged your detector's front end to a Colpits type, and it is far more interesting to me because it uses a single coil, and in this configuration it operates in near saturation mode. I guess it will have better stability, but I'd have to test it in real life. If you are up to it, please let us know if it works and how it performs.
I only made a few small changes against the original layout, but the mode of operation is quite different.
A single coil is much easier to make, and it would lend itself nicely if you ever wish to make a grown-up detector with a large air coil and a stem out of this project.
Here is the schematic, lower pane is with no target and the upper with a target near by:

I'm not a technician like you and my expertise is limited, but I think with a colpits we will not be able to improve. I've tried it with multiple oscillators, but I did not succeed.
There is another component that played a role, maybe something "with regenerative loops."
I think LTspice not told everything.
Maybe we should LTspice "hacking".

[Davor]

LTspice is a great learning tool, and a great way of improving circuits. It is as good as the models you use, and more often than not the models are simplified in a way that simulations converge faster. Some things are very difficult to simulate, say comparators with positive feedback.
The Colpitts oscillator I suggest here has a few interesting details and I'll most certainly try it in a real circuit. It detects coil loading by excess current flowing through a transistor base during saturation. As such shallow saturation stabilises the oscillator amplitude, I expect some more stability with this design than the strictly linear ones. At the same time the detection voltage change largely exceeds coil amplitude change in target presence.

[Davor]

Here you go, both versions in LTspice and only slightly edited.

[Thomas]

Don't use the layout I posted, Nupi Maybe Final. The components C5, C6 are wrong size.

[Thomas]

Corrected

[Nupi]

Corrected [ATTACH]29117[/ATTACH]

Have you already made the pinpointer on your layout?