OK Foxy.
I have a bunch of resistors, mostly 0.5 ohm 5%, a few wire wounds, some ICs, some caps, scope leads. I also have a 100 MHz real time Tektronix oscilloscope and a few 1:1 and 10:probes.
I have a clip on current probe. It’s a HEME 1000 Ammeter, a fine piece of kit. Probably not a use for you but if you want it that’s OK.
Before setting about building the circuit it was necessary to construct the search coil. I had already made a coil winding apparatus for construction of the mark 1 detector, so with a little modification was able to use it for the MK2 coil as seen in the photo…
This coil is very different to the ones I have wound previously. It is larger (200mm dia) and must be covered with aluminium foil. As you can see, the coil has now been wound and is being covered with an insulating material before the application of the foil. I will figure out how this can be tidied up and attached to a supporting ring fitted to the end of the shaft later.
There are some specialised items required for this model, and rather than forage around in my extensive collection of recycled components, I decided to remove any doubt as to their ability by purchasing some new bits…They arrived yesterday…
These is not a complete list, and there are still some capacitors and assorted hardware required that I might already have in stock, but if I have, the whole lot has only come to less than twenty quid…
They sound great Besoeker, although the 1000 Ammeter might be a little overkill for small electronic circuits. How much do you want for the items, because the oscilloscope won’t have come cheap…
Thanks I sort of understand that …well, maybe as much as anyone can who hasn’t looked into metal detectors before. 
The pot looks as though it sets the feedback for the op amp and therefore the sensitivity of the detector. Is that right?
Edit: How many turns does the coil need to have?
You are welcome to it.
There is also a CEN-TECH multi meter. Volts, Amps, Ohms, NPN/PNP meter.
Yes, I didn’t think the Ammeter would float your boat…
Nor the 4.5 kV scope probes…
I’ve still got my dual-beam scope but no probes. I was offered £200 for it when I stopped work. I thought I might need it but never have. Should’a took the money.
Yes, I suppose. Various things took my toll. So I passed the scope on.
I don’t know if mine would even work now. It’s been up in the loft for over 20 years. It’s a Hameg 203-5 20Mhz
I think you are right about VR1 adjusting the sensitivity Mart, it appears to provide feedback from output ‘A’ to the inverting input ‘B’… I’ll post the ‘Pinouts’ for IC3 and you can have a look.
The coil was made up of 25 turns of 24 SWG enameled copper wire, and should be covered with Aluminium Foil.
Here are the pinouts for IC3 Mart…
After removing the coil from the winding machine we can attach stronger leads to the fragile coil wire and finish fully insulating the coil and we end up with this…
Notice that I decided to make the coil hexagonal, this was to make it easier to insulate the coil to give it some rigidity before removing it from the coil winding machine. It shouldn’t affect its performance…
Before proceeding with covering the coil with Aluminium Foil, I shall start work on the coil body.
Just a few words on ‘IC1’ the NE555…
I have lots of these in stock because they are the beating heart of many a circuit in the past.
They are described as ‘The most popular and versatile chip ever produced’ …
They contain…23 transistors, 16 resistors and 2 Diodes in such a small package…
This is just one circuit I built previously containing five NE555’s.
It’s a five stage light chaser. At just a few pence each, they don’t break the bank either…
I know nothing about the subject myself but I was talking to a bloke in a private museum in the back of beyond who was a prospector who also sold metal detectors.
Apparently these days they are clever enough to be set to only find certain materials or ignore others. This means that prospectors are not constantly having to dig up nails, aluminium pull tabs etc. I thought it was brilliant.
You occasionally see the odd person combing the local beaches where it seems a very solitary hobby, though come to think of it I have met a few couples in caravan parks with their detectors in the back of their ute.
555 chips are a timer/oscillator chip, they have been around since Noah was a boy
I’ve never built much myself. Only a guitar amplifier back in the 60s …and that was valves. I haven’t come across or don’t remember the IC names mentioned here. I can’t say if they were or weren’t used in the stuff I mended. I might only remember the names if they gave trouble and needed changing.
Logic gate theory, FETs, MOSFETs op amps ICs of various types were all involved in the job, so some rusty knowledge about those remains. I can still recognise common resistor values at a glance.
I thought about ordering a kit to build something when I stopped work but think the trouble was that by that time, I’d more or less lost interest in electronics. However, when OGF writes about a metal detector project, I find I still have some left. 
Why is that?
There are some brilliant discriminator detectors on the market Bruce, and not too expensive for a mid range machine. My detector will be no where near the power of these models, and money has never been the driving force behind my electronics hobby, some of the things I build can be purchased for just a few pounds from the local store, and they are probably a lot more efficient and reliable. But I have a learnt a lot about electronics over the years, inventing stuff that has already been invented…It’s the design, draw, adapt and build that keeps me interested, even more so if it works, and once you get hooked on the smell of hot solder in your nostrils it’s a hobby for life…
Ah yes, the humble 555 Bruce, I didn’t realise how long they had been on the market, but I did a job for work using the 555 back in the early nineties, almost forty years ago. Hell! where does the time go…
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After running and other outdoor pursuits took over my life back in the eighties mart, I flogged a lot of my electronics stuff, and gave some away, but I knew that one day I would return to the hobby, so I kept some of my most important tools and components, and I mothballed my workshop in the loft. I never lost interest in electrics and electronics and although I was a machinist and mechanical engineer, my electrical knowledge spilled over into my job and I ended up repairing, installing and rewiring as part of my working day.
After retirement I started pawing through some of my old electronics books and realised that the excitement of building something had never gone away, so I opened up the lab and started pottering around. I have always been self taught concerning electronics, but the passion has driven me on to read and learn but mainly through practical experience. Some work and some don’t, but sometimes you learn more from the ones that don’t work…
I’m glad it has encouraged you regain your interest in electronics mart, you are a mine of information and it’s good to have someone like you to bounce my ideas off…
Many years ago (early eighties I think) a distant member of the family asked if their son could come and have a look round the lab. He was about sixteen and keen on electronics. We cobbled a few bits together and produced an amplifier and a couple of oscillators that played interesting tones and sounds.
He was so impressed, and so was I, most people glaze over and get so bored when I rant on about electronics… I heard later, that he went to university and secured himself a career in electronics. The last I heard he was some ‘big shot’ in a well known electronics company in America…I like to think that I had contributed to his interest in all things electric…
The BFO detectors I’ve built in the past work on RF signals and the coil remains unhindered so as to pick up the disturbance from metal objects. By beating together the signals from two oscillators - one determined by the frequency of the search coil, and the other a fixed frequency - The tone would be greatly affected by the search coil altering frequency, a kind of ‘Harmonics’? After searching around the web, I found a couple of detectors based on the 555, and in most cases the coil must be covered with foil. I believe it must work on magnetics rather than RF, the foil would not present a barrier to a magnetic field as it would to RF waves. Doesn’t the MOSFET detect a magnetic field?
I’m not sure of the MOSFET part but can read (on the Internet) MOSFET and magnetic field being mentioned in the same sentence. The name seems to change to MAGFET.
I was curious about the aluminium foil because it would seem the coil would detect metal all the time (the aluminium) and how could it work then? I had a look around and found this about metal detector coils wrapped up in aluminium…
…and the quote below from here…
Using a Faraday Shield around the search coil will reduce the effect
of the ground altering the frequency if the ground has a large amount
of iron in the rocks
Sort of desensitizes the coil to interference basically… or stops the detector responding to metal outside of the coil?
