Metal detector for finding archery arrows

[Mothough]

The 50Hz I'm seeing does not seem to come from the coil its also present with shorted coil.
Tried another opamp which seems to behave itself. It has a more reasonable 4Vpp output swing but lacks offset trim functionality.
Nevertheless, this one should allow me to continue building the integrator stage until I can get my hands on some genuine NE5534's.

Moving my iron test chunk over the coil I get the following response: https://daten-transport.de/ansicht.php?id=aCJhNF6PraMz
CH1: 555 output
CH2: NE5534 output
CH3: Sampling pulse
CH4: 1st LM358 output

Looks at least promising or what do you guys think?
Still need to wire up second integrator stage, a much better coil and lots of fine tuning I think though. But hey, we are in a kinda lockdown anyways.

[kt315]

try this one. maybe will be good for you.

https://www.ebay.com/itm/GOLD-MOUNTAIN-KING-COBRA-METAL-DETECTOR-/363159916279?_trksid=p2385738.m4383.l4275.c10

[Mothough]

Very good idea! You are probably right. Wasn't a problem till now with de fake opamp having almost no gain.

Am I already at a point where I need coil shielding? Currently I just quickly wraped 20 turns of 0.5mm wire around a roughly 23cm plastic bucket to match dimensions mentioned on here.

[green]

Maybe the coil is picking up the 50Hz. Disconnect Q2 and scope IC2 output with coil connected and coil input jumpered.

[Mothough]

Looks like the NE5534's I have in stock are counterfeit. Bummer! They just behave like a bad quality jelly bean opamp with no offset trim. The pins in question seem to be NC.

Tests with a pin compatible opamp results in much more reasonable gain and allows offset trim. Sadly this opamp somehow seem to pick up huge amounts of 50Hz hum. Will have to find a proper replacment.

I think AD711, OPA134, OP27 or OPA604 may be also suitable for the job.

Will keep you posted!

[Mothough]

Exactly, thats the 0.85V I was talking about and Surf Detector's screenshots show exactly what you say. Will have to investigate further.

[green]

Looking at https://www.geotech1.com/forums/atta...7&d=1604945664 if zero is in center, should be able to adjust offset pot so amplifier out goes to zero(center screen)after signal has decayed. Don't think coil is causing problem.

[Mothough]

All channel zero markers are in the middle of the screen and stack below the CH4 marker.

Just noticed an issue while adjusting R10 offset pot for 0V DC offset (Step 5 of the Baracuda bulding guide).
Got about 0.85V on the NE5534 output which I can only compensate about 40mV up and down by turning the pot from end to end. Maybe I fried the opamp.

Will swap it when I get the integrator stage going.

Also tried with a chunk of iron and the recovery signal trace quite drastically bends. At least this looks quite promissing. Maybe my coil is still not right since the trace starts to recover after 3us and is already done in about 6us.

Surf Detector on Youtube shows much longer recovery time of about 100us using his coil:

Furthermore my delay pulse (CH2) does start way later compared to his delay pulse also CH2
His sampling pulse then coveres the recovering period of CH1 like it should?
When my recovery pulse (CH3) hits, the signal trace (CH1) is already all the way back up, which seems not right. Coil way too fast?

[green]

Oh, okay. Does a 2Vpp level seem right to you? Shouldn't it be more near the +/-5V rails. 2Vpp seems to me like almost no gain?

Don't have a Surf PI so just some thoughts. If you are reverencing reply#33. NE5534 should swing about +-2.5V with +-5V supply. Don't know where CH1 scope zero is. I'm with you, doesn't look right to me.

[Mothough]

Think you won't see stainless steel points at amplifier out. Should be able to see coins. Need to add integrator to see point's.

Oh, okay. Does a 2Vpp level seem right to you? Shouldn't it be more near the +/-5V rails. 2Vpp seems to me like almost no gain?

good. i got now. thank you. 1 question. why you do underwater PI detector?
in my opinion HH is better. also, you will dig all metal because PI does not have a discrimination FERROUS-UNFERROUS.

Just because it looked the easiest to build and I had all parts already available and there were different schematic versions I could somehow understand available.

[kt315]

good. i got now. thank you. 1 question. why you do underwater PI detector?
in my opinion HH is better. also, you will dig all metal because PI does not have a discrimination FERROUS-UNFERROUS.

[green]

I'm currently still puzzled a bit by the preamp stage. Doesn't seem quite right yet.

Think you won't see stainless steel points at amplifier out. Should be able to see coins. Need to add integrator to see point's.

[Mothough]

I would recommend the MPP E from Silverdog.

Thanks for the recommendation. I'll probably end up getting some kind of pcb in the end if I get promissing results.

You probably need to experiment a bit to find out. You may actually enjoy experimenting, learning as you go.

Yep, I'm currently trying to figure out what would be feasable with DIY detectors.

I believe the arrow heads will be more on the surface than under the ground, so one problem less

Correct, they just slip into sward. If they are shot steep enough to pierce the ground they stick up and are easily spottable.

Instead of trying to detect stainless steel; would a ring of self adhesive copper tape around the shaft help for detection purposes?

Very good point. Actually, they make arrow shafts containing carbon aswell as aluminium but they are mostly used in professional competition.
However, if detecting stainless turns out to be too difficult, copper bands or similar would be a feasable solution since all below 1-2grams are not really noticable in hobby archery. Many hobby archers mark their arrows with multi color insulation tape to disdinguish them.

He's trying to find many hundreds of dollars worth of arrows that are already lost., though obviously there will be ongoing losses in the future.
However, it's an interesting challenge , how to 'label' the arrow without compromising it in terms of weight, balance, aerodynamics, etc. How to get the maximum detectability for the minimum amount of added material.

All arrows which are in the ground for too long are practically worthless since degretation is quite an issue under wet conditions. The stainless steel head will be the only usable by then, which in itself is not worth de effort to search for them.

he does not want to waste a money on the PCBs. he wants to learn a potencial of PI making parts on breadboard.
he is just 'wife kitchen table reseacher'. i think he is still one pakistani guy. also he REFUSED to share pics with him in his hobby
because there is not hobby, just 'a wish'.

I'm not a facebook user, nor will I post any other social media nor pictures of me here, sorry.
Best I can do to ease your paranoia is a picture of my archery equipment:
And the current state of the breadboard prototype

I'm currently still puzzled a bit by the preamp stage. Doesn't seem quite right yet.

[green]

he is just 'wife kitchen table reseacher'.

my kitchen table. Fly tying one side, metal detecting the other.

[kt315]

I would recommend the MPP E from Silverdog.
---
leave it out. he does not want to waste a money on the PCBs. he wants to learn a potencial of PI making parts on breadboard.
he is just 'wife kitchen table reseacher'. i think he is still one pakistani guy. also he REFUSED to share pics with him in his hobby
because there is not hobby, just 'a wish'.