It's been a while since I've shown one of my electronics designs here. This one is Copyright © Thomas O. Scarborough, but feel free to copy it with acknowledgement (click on the image to enlarge). This is a self-contained Beat Balance
(BB) metal detector. It is not the same as Induction Balance (IB). As will be seen, the concept lends itself to extreme simplicity. It also offers considerably better performance than Beat Frequency Operation
(BFO) -- in this case about twice the sensitivity. This is I think my best design for simplicity and sensitivity combined (see BB Metal Detector
for an even simpler design and a few tips on construction). Details of the coils are at BFO Metal Detector 
. Place the coils for greatest sensitivity -- some patient experimentation will be required. VR1 may be a multi-turn
potentiometer for more precise control. The "mirror" design ensures good stability. Note that this 556 is a CMOS
device (e.g. the 7556), not the bipolar device.
i have one question, maybe its stupid but i am new to this, what is work voltage on capacitors?
The capacitor voltage: typically this is anything above the supply voltage. So if the supply voltage is 12V, the capacitor may be, say, 16V or 25V. In this design, you ideally need a non-polar capacitor. This can also be made of polar capacitors in series, wiring the negative terminals together.
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