Matchless metal detector

[wallker]

Thanks WM6,but i know about this... I was asking you for the some document for the circuit that you posted earlier,so i would be grateful if you can post it or give me the link.
Regards.

[maikl]

At this link free download "Everyday Practical Electronics March 2012" http://www.pdfmagazines.org/magazine...arch-2012.html there is the latest design of the Rev Scarborough (55 str.)... called "LRC Beat Balance Metal Detector"...

My mistake was not in March than in February http://www.pdfmagazines.org/magazine...s-02-2012.html

[nick_f]

I see the Rev Scarborough has a new design in the latest EPE.

Hi Carl, thanks for letting us know.
I had a look at the design. This guy is the king of simplification!

Cheers,
Nicolae

[WM6]

Thanks WM6,but i know about this... I was asking you for the some document for the circuit that you posted earlier,so i would be grateful if you can post it or give me the link.
Regards.

I see, sorry. This is basically proximity detector, but can be adapted to metal detector by proper coil and by relay coil replacement by (high resistive) speaker or earphone.

I have no more project (PCB etc.) info than this:

http://coolcircuits.blogspot.com/201...m-project.html

[wallker]

Thanks WM6.
Hvala.

[Davor]

I've made a LTspice simulation of EPE Matchless circuit. It is dirty as can be, but it works. I've been toying a bit with this principle, and found that there is a lot of space for improvements, even to make it work nicely, so more to come.

As soon as I make it presentable I'll post it here. Improvements include:
- real sine output
- notch filter to silence it in absence of events
- coil compensation for a kind of ground balance
- everything in 3 555s and some kind of audio amplification I haven't decided yet

I discovered a few novel ways of harnessing 555s to my evil purposes that are so devious that you'll like them for sure.

For now only a rev Scarborough circuit... I substituted crystal earphones with something that should work OK for this simulation. Have no idea how closely it matches real earpiece.
When simulating look for FFT peak at about 1kHz.

[WM6]

I've made a LTspice simulation of EPE Matchless circuit. It is dirty as can be, but it works. I've been toying a bit with this principle, and found that there is a lot of space for improvements, even to make it work nicely, so more to come.

As soon as I make it presentable I'll post it here. Improvements include:
- real sine output
- notch filter to silence it in absence of events
- coil compensation for a kind of ground balance
- everything in 3 555s and some kind of audio amplification I haven't decided yet

I discovered a few novel ways of harnessing 555s to my evil purposes that are so devious that you'll like them for sure.

For now only a rev Scarborough circuit... I substituted crystal earphones with something that should work OK for this simulation. Have no idea how closely it matches real earpiece.
When simulating look for FFT peak at about 1kHz.

Great test Davor, thank you.

If I can suggest one more thing for improvement: stabilized supply.

What do you think about this Rev. Thomas Scarborough circuit ? :

http://thomasscarborough.blogspot.co...obbulator.html

Not trying to build it, only after visual analyzing of circuit, I guess it cannot work.
Why? Because sweep oscillator simulate target response.

[wallker]

I've made a LTspice simulation of EPE Matchless circuit. It is dirty as can be, but it works. I've been toying a bit with this principle, and found that there is a lot of space for improvements, even to make it work nicely, so more to come.

As soon as I make it presentable I'll post it here. Improvements include:
- real sine output
- notch filter to silence it in absence of events
- coil compensation for a kind of ground balance
- everything in 3 555s and some kind of audio amplification I haven't decided yet

I discovered a few novel ways of harnessing 555s to my evil purposes that are so devious that you'll like them for sure.

For now only a rev Scarborough circuit... I substituted crystal earphones with something that should work OK for this simulation. Have no idea how closely it matches real earpiece.
When simulating look for FFT peak at about 1kHz.

Nice Davor,post sam results after you finish.
Best regards.

[Davor]

Great test Davor, thank you.

If I can suggest one more thing for improvement: stabilized supply.

What do you think about this Rev. Thomas Scarborough circuit ? :

http://thomasscarborough.blogspot.co...obbulator.html

Not trying to build it, only after visual analyzing of circuit, I guess it cannot work.
Why? Because sweep oscillator simulate target response.

It doesn't. Only a brief wobble is introduced to a BFO RC oscillator, and the resulting sound becomes, say, chirpy or wobbly. In essence - it works, but annoyingly so.
Stabilised supply would not improve synchronisation locking, so the whole thing is a lost cause anyway. Both oscillators in "matchless" are dragged by supply voltage the same way, and not much so, and it is a non issue.
I remember some article about advanced BFO MDs in Radioamater magazine loooooong ago that described avoiding synchronisation lock by using harmonic detection, yet it could be even simpler. I'll show you soon.
My idea of avoiding sync lock is using a heterodyne with manageable IF. I figured the way to balance inductances of tanks resonating at quite different (!) frequencies to maintain frequency difference in order to achieve some kind of ground compensation. That should enable me to use a notch to make this kind of rig more tolerable.

Just a sneak peek to what I'm playing with, a 555 sinus oscillator. It is very efficient, and gives a very pure sine oscillation with full swing across coil ~2xVcc. There is a series R within coil of 4ohm.

[6666]

Interesting sneak peek
is it running at 20khz
the coil is 4 ohm , whats the rest of the coil specs ?

[detecto]

Matchless metal detector

Couple years ago i made this detector, only one good thing is that is wery sensitive on small items, i could not remember sensivity on gold nudget, but he can detect niddle abouth 4" in the air if i good remember. Frekvency is a little unstable but this can be resolved by adding some woltage regulators, (78L05 or other)
For building i use this detailed description for this metal detector.
He can be very good for prospecting. But i never test this detector in the ground, i built it just for fun.

Download: http://sharesend.com/zom19

[detecto]

Matchless metal detector

For all IB metal detectors coils shuld not be more the 7" in diamer.

[Davor]

Interesting sneak peek
is it running at 20khz
the coil is 4 ohm , whats the rest of the coil specs ?

Please don't mix my spiced-up imagination with reality. So far I'm exploring some interesting features that could lead to some simple and useful design. You'll know when it's ready.
The example here is designed at 20kHz simply because I wanted to observe some features, yet didn't want to wait to grow old before spice finishes. It will go higher, and coils will go down to ~1.5mH ... I guess.
The way you see it now - it eats TGSL Tx on every account.

[WM6]

It doesn't. Only a brief wobble is introduced to a BFO RC oscillator, and the resulting sound becomes, say, chirpy or wobbly. In essence - it works, but annoyingly so.

Thanks Davor. I will check (by scope too) those "Wobbulator" in real, to not stay on theory only.

[Davor]

I finally decided on audio stage, and it is a combination of notch at about 380Hz (+ and - something) and a simple buffer stage to supply simple walkman earphones. There are 3 controls altogether,

• a switch to decide between simple notch and a feedback notch;
• a 5k potentiometer to tune a notch;
• a 10k trimmer potentiometer to optimise a notch in feedback.

Feedback notch is sharper and at distance from notch frequency louder than the notch without feedback, perhaps even too loud, hence the switch. It will be easier to hit the notch frequency without feedback.

Using a notch is a substitute for low frequency BFO, but without synchronisation locking risk, and with much nicer audio. In fact, I am planing to make this frequency distance even more distant. I have already found the incredible simple and sufficiently efficient mixer for the purpose, with a flick. And a nice one. Will post it here soon.

I expect a whole shebang to comprise 3 555s, a CMOS XOR, 4 transistors and a small bunch of passives. Nice and easy.

The attached simulation is set for FFT at R2 output to indicate a notch action.

Here it goes: