Simple Metal Detector

[THATguy]

Hello, I would like to make this:



Components:
9v Battery
speaker
5 capacitors
11 resistors
2 coils
1 potentiometer
1 NPN transistor
2 PNP transistors
2 polarised capacitors

However I would like to make some changes:
The total diameter of the DD coil will be 20mm.
If possible I would prefer to make this from SMD components.

Some of the equipment I have:
basic multimeter
LCR tester (https://youtu.be/7Br3L1B80ow.)

I'll be using scrap components.
I'M NOT ABLE TO PURCHASE NEW COMPONENTS, please don't tell me otherwise, it's just my current situation.
I can test the components, and have a multitude at my disposal if some are faulty.

I have all components, though still trying to ID some SMD ones.
Here are some transistors I've found, do you think they will be suitable?
c3311 NPN (http://www.datasheet.hk/view_download.php?id=1496756&file=0249%5Cc3311_71850.pdf)
a124 e PNP (https://alltransistors.com/transistor.php?transistor=57650)

This circuit diagram seems to be simpler:
http://www.zen22142.zen.co.uk/Circuits/Testgear/ccomd.jpg]http://www.zen22142.zen.co.uk/Circuits/Testgear/ccomd.jpg]http://www.zen22142.zen.co.uk/Circuits/Testgear/ccomd.jpg

Though it does use 12v, which would not be a problem for me.
I do not see a potentiometer in there for tuning, do they use the FM radio for this?
Could I just add a pot & speaker?

[Codebird]

I've seen this design before.

You can see two oscillators in there, identical. Their outputs are mixed together. If they are in tune then the mixing produces no audio or a very low tone, but if one is thrown out of tune - by the presence of metal near to its inductor - it'll shift in frequency, and their mix will produce a high tone. No radio required. There is a small 'fine adj' pot in there that lets you tune them initially, to compensate for the unavoidable variation in component values.

You can aubstitute any of the resistors or capacitors for a replacement of more-or-less the same value. 2k7, 3k, 4k, close enough. So long as the two oscillators match: Anything you change in one, you must change in the other too.

The right-hand side of the circuit is nothing but an audio amplifier, and so is very easy for you to replace with whatever output amplifier is most easily constructed from your available scrap components. Tear about any cheap-and-nasty set of powered speakers, there's an audio amplifier board inside just begging to be repurposed. The amplifier design shown in that circuit offers high voltage gain but little power output - it'll drive a pizo, but not a full-size speaker.

[THATguy]

The FM tuner I was refering to the second circuit image - that you need to click on to see, image wasn't displayable.

Thanks for your other info, that is very helpful.