The circuit above is a very simple and easy to build BFO (beat frequency oscillator) metal detector. It is designed around 3 transistors and couple of coils .The supply voltage is 9 V DC The schematic is not mine and I’m not really sure if this thing works as it should .Test results will be posted soon .
Component list :
R1 and R4 – 47kOhm 1/4 watt.
R2,R5 and R10 – 18kOhm 1/4 watt.
R3 and R6 – 680 Ohm 1/4 watt.
R7 and R8 2k7 Ohm 1/4 watt.
R9 – 390k 1/4 watt.
RV1 – 10k mono type potentiometer (multi turn if available).
C1,C2,C3 and C4 – 100nF 63v ..
C5 -470pF 63v.
C6 – 220nF 63v.
C7 and C8 – 10uF 16v or higher .
C9 – 100uF 16v or higher .
L1 – 220uH RF core choke .
L2 – Should be connected to J1
The coil itself should be made of 26 turns of 0.25mm lacquer coated wire wound around a form with a diameter of 10.8 cm .
Q1 and Q2 – 2n3906 or similar .
Q3 2n3904 or similar.
The power leads should be connected to J2 and the piezo speaker should be connected to J3 .
To increase the accuracy I suggest using 1% carbon film resistors and powering the circuit from a 9 V battery.