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detectors > Simple BFO Metal Detector Schematic Diagram
Simple BFO Metal Detector Schematic Diagram
This simple BFO metal detector requires only a few of components and an evening's work. The two oscillators are simple Colpitts designs using BJT transistors. The reference oscillator's frequency is approximately 370kHz, slightly tunable with the help of a silicon varactor diode. The outputs of the two oscillators are fed to a mixer made with Q3 and Q4. The signal then goes through a low-pass filter (R13, C13) and a JFET preamp. The LM386 audio amplifier has a gain of 20, more than enough for most headphones. If you need more gain, you can add a 10uF capacitor between pins 1 and 8.

Fig. 1: Simple BFO metal detector schematic diagram
Parts list: Resistors: * R1, R3, R5, R7: 22kΩ resistors * R2, R6: 1kΩ resistors * R4, R9, R12: 15kΩ resistors * R8, R10, R11: 47kΩ resistors * R13: 2.2kΩ resistor * R14: 1MΩ resistor * R15: 8.2kΩ resistor * R16: 680Ω resistor * R17: 10Ω resistor * P1: 10kΩ lin. potentiometer (Tune) * P2: 10kΩ log. potentiometer (Volume)Other parts: * L1: 10cm (4in.) diameter, 20 turns, AWG 22 * L1: 82uH inductor * SW1: SPDT toggle switch * J1: Headphone jack 1/4 or 1/8 inch | Capacitors: * C1, C6, C7, C12, C14: 100nF capacitors * C2, C8: 22nF low temp. coef. capacitors * C3, C9: 2.2nF low temp. coef. capacitors * C4, C10: 10pF ceramic capacitors * C5, C11: 4.7uF/16V electrolytic * C13: 10nF capacitor * C15: 47nF capacitor * C16, C17: 220uF/16V electrolyticActive components: * D1: NTE618 silicon varactor diode (20-440pF) * Q1-Q4: 2N2222 NPN silicon transistors * Q5: 2N5951 JFET transistor * IC1: LM386 (Audio amplifier IC) |
The coil should have 20 turns of 0.65 mm (AWG 22) enameled copper wire wound on a 10 cm (4in.) diameter form. Wrap the completed loop with strips of aluminum foil or copper shield tape, then connect the shield to the ground. Make sure the Faraday shield has