Fossicking

 I've recently been on holiday in NSW, and my uncle gave me a pile of sapphire bearing gravel which he'd added gold to. I had lot of fun sifting through it and I'd love to go back and collect more of the stuff.

I got the impression that the minerals in the gravel would glow under UV light, so I bought a UV flood light and some electronics to make a UV strobe. I could use the strobe to tell me if I'm digging in the right place. Only 1% of the bits of gravel glowed under the spotlight so that was a bust.

I'm also thinking of looking for gold. I've been thinking a lot about how I could make my own metal detector.

I purchased a solder-your-own metal detector kit, MDS-60, and it's not great. I could detect a 5c coin from around 5mm. It's a start, but I'd like to build a metal detector that can compete on sensitivity to small pieces of gold as an expensive unit.

Here is a video of the device in action 


With a bit of fine tuning I was able to achieve detection at about 1cm.

All for now, Boston 

 

Update 4/3/2025:

MDS-60 Metal Detector Kit

 


 

The MDS-60 is a very simple solder-your-own metal detector kit. It only has 3 active components: two PNP transistors and one NPN transistor (don't get these mixed up!). I'm impressed with how well this works for such a simple design.

The device shows a positive response to a 5c coin at around 2cm, and a strong negative response to ferrite.


Here is the best circuit diagram I've seen online:

I believe the circuit detects metal based on changes in the Q-factor of the resonator formed by the inductor coil (L1) and various capacitors. Positioning a conductor near the coil will lower the Q-factor and positioning a ferrite near the coil will raise the Q-factor. A resonator with a low Q-factor dissipates energy more quickly than one with a high Q-factor. Thus, more current is needed to drive the circuit. This current is detected, amplified and sent to the output.

L1 is a coil of wire etched directly into the two-sided PCB. As well as the main winding, it has an extra loop that connects to C2 in the diagram above. I have seen suggested that the coupling between the extra loop and and the main winding is used to detect pieces of metal nearby, but I like the the Q-factor explanation better. The extra loop provides feedback which is amplified and sent back into the main winding.

All for now, Boston.


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