The following metal detector circuit uses two RF oscillator with 5.5MHz frequency. The first oscillator used as a reference frequency produced by the ceramic filter. The second oscillator produced by transistor T3. When there is no metal detected, the value of frequency difference will be zero and the speaker will not produce sound. When the metal is detected, there will be difference between reference frequency and the reflection frequency, and the speaker will sound.

Metal Detector Circuit Diagram

Components list :

Diode D1-D2 : OA79
Resistor R1,R5,R6,R9,R10 : 330k ohm
Resistor R2 : 330 ohm
Resistor R3,R4,R7,R8,R11 : 1k ohm
Resistor R12,R13 : 10k ohm
Resistor R14 : 4.7 ohm
Resistor variable VR1 : 10k ohm
Polar capacitor C1,C14 : 100 uF/25 V
Capacitor C2-C4,C7,C11,C13,C16 : 10 nF
Capacitor C5 : 100 pF
Capacitor C6 : 470 pF
Capacitor C8,C12 : 0.1 uF
Capacitor C9-C10 : 15 nF
Polar capacitor C15 : 1 uF/16V
Capacitor C17-C18 : 100 nF
Transistor T1-T3 : BF494
IC1 : TDA282
Ceramic Filter : 5.5 MHz
Trimmer VC1 : 0-22 p
Inductor L1 : 20 rotation 25 SWG 4mm diameter
Speaker
9V power supply