A simple metal detector with seven silicon transistors
The metal detector allows detecting any metal object at a distance of up to 20 cm. The detection range depends solely on the surface area of the metal object.
For those who find this distance insufficient, such as treasure hunters, it is recommended to increase the size of the frame. This should enhance both the detection depth and range.
The schematic diagram of the metal detector is assembled using transistors operating in the micro-current mode. It consists of a radio frequency (RF) generator (100 kHz) at VT1, which is tuned by resistor R1 for maximum sensitivity to metal objects.
Fig. 1. Schematic diagram of a simple homemade metal detector using transistors.
Two coils (L1 and L2) are used as coils, as shown in Fig. 2. Transistors VT2 and VT3 are connected as diodes and provide stabilization of the auto-generator modes (VT1) and active detector on VT4 when the power supply voltage and temperature change.
Resistor R6 sets the sensitivity of the metal detector. A sound auto-generator is assembled on transistors VT5 and VT7, which is activated by transistor VT6. To ensure a loud sound from the piezoelectric transducer HF1, coil L3 is connected in parallel to it.
This increases the voltage on the piezoelectric transducer due to resonance between the internal capacitance of HF1 and the inductance of L3. When a metal object enters the field of coils L1—L2, the generator frequency changes, leading to a decrease in the voltage amplitude at the input of the detector (VT4). It locks, and transistor VT6 opens, allowing the sound generator to operate.
Compared to similar devices using the beat frequency principle, this scheme provides greater sensitivity and is simpler to manufacture.
Details and Construction:
As a power source, a “Korund” or “Krona” type battery (9 V) is used, but any stationary power source with a voltage of 6–10 V can be used as well. The standby current consumption is not more than 1.5 mA, and during the operation of the sound signal, it is 7 mA.
Fig. 2. Construction of a simple metal detector using transistors.
All components of the circuit can be placed on a single-sided fiberglass printed circuit board (Fig. 3). The frame’s casing should be made of any dielectric material, such as laminated glass.
Coils L1 and L2 should be identical, each containing 40+40 turns of PEV wire with a diameter of 0.25 mm (perimeter of the coils is 340 mm).
Coil L3 is wound on two bonded ferrite rings with dimensions of 10x6x3 mm, grade 400—1000НМ, and contains 250—300 turns of PEV wire with a diameter of 0.1 mm.
Variable resistors R1 and R6 should be of type CP5-16V, while the others can be any compact type. Capacitors C7 are of type K50-35 at 16 V, and the rest are of type K10-17.
Printed circuit board of the homemade metal detector.
“Fig. 3. Printed circuit board of the homemade metal detector.
Diode VD1 can be replaced with any pulse diode. Microswitch SA1 is of the PD-9-2 type.
During the device tuning, if it is not possible to obtain generation at VT1 by adjusting resistor R1 (monitor the voltage on this resistor with an oscilloscope), it may be necessary to change the phase connection of the leads of coil L1.
When adjusting the circuit for maximum sensitivity to metal objects, it may be necessary to change the overlap distance of coils “A” (Fig. 1). After that, the coil frames should be fixed with glue.
