This simple metal detector requires only a handful of components and an evening's work. Built around a cmos4011 IC, is very robust and versatile. The 250 kHz reference oscillator is built with two gates (U1/1 and U1/2), C1, R1 and P1. The search oscillator uses only one gate (U1/3), two capacitors and the search coil. The outputs of the two oscillators are fed to the fourth gate acting as a mixer and filtered with C4.
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After assembly, connect the headphones and slowly turn P1. The pitch will get lower until it disappears. Continuing to rotate P1 in the same direction will cause the pitch to rise again. The point at witch the pitch is the lowest and disappears is called "zero beat". If you can not get this zero beat frequency for the entire turn of P1 you may have to select different values for C1.
Turn P1 close to the zero beat position, then move the search coil near a metallic object. The tone should change, depending on the size and distance of the metal.
Note that this simple detector's performance is not comparable to more advanced commercial products. It will only detect relative large metallic objects at a short distance. Coins and other small objects will be much harder to find!
U1: CD4011 (Quad 2-input NAND Gate)
U2: LM78L05 (5V Regulator IC)
R1: 2.2k 5% resistor
R3: 330k 5% resistor
R4: 270k 5% resistor
R5: 1k 5% resistor
C1: 390pF NPO capacitor
C2, C3: 10nF
C5: 100uF/16V electrolytic
C6: 220uF/16V electrolytic
C7: 100nF ceramic
P1: 4.7k lin. potentiometer
L1: 22cm diameter, 14 turns, AWG 26
K1: SPDT toggle switch
J1: Headphone jack 1/4 or 1/8 inch
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