This metal detector schematic circuit is based on a transistor radio as an detector . With the radio tuned to a weak station you must adjust the variable capacitor C1 until the locator oscillator beats against the received signal . If a metal is detected the inductance of the L1 coil is changed , changing the frequency of the locator oscillator’s , resulting a change in the beat tone radio . The L1 search coil of the metal detector circuit must have 18 turns from a 0.65 mm enameled wire scrambled on a 4 inch diameter support .
This metal detector circuit needs to be powered using a 9 volts power supply ( DC) or a 9 volts battery .
The C1 capacitor is a variable capacitor with a value of 365 pF , C2 is a 100pF silver mica capacitor , C3 is a 0.05 uF disc capacitor and the C4 is a 4.7 uF capacitor .
The Q1 transistor can be RCA SK3011 npn transistor or equivalent type and all resistors need to be ½ watts .