Versatile Detector for Large Magnetic Objects
This detector is designed to locate relatively large objects made of materials with high permeability. Additionally, it can discern whether the magnetic object inside the detection coil possesses good or poor conductive properties.
Identifying Materials with Magnetic and Electrical Properties
Materials such as ferrites pressed from metal oxides, combining excellent magnetic properties with relatively high electrical isolation, are effectively detected by this device. It may not be sensitive enough for coin detection, but it reliably locates more unique items like bombs or treasures left by pirates.
Symmetrical Power Supply and Oscillator Operation
The metal detector operates with a symmetrical power supply from two 9-V batteries, each drawing approximately 15 mA. The detection coil, L1, is a part of a sine wave oscillator centered around transistor T1. Under normal circumstances, the central frequency of the Voltage-Controlled Oscillator (VCO) in the Phase-Locked Loop (PLL) within IC1 matches the oscillator frequency of T1. However, when a metal object, whether ferrometallic or non-ferrometallic, enters the field induced by L1. The sine-wave oscillator is detuned. The voltage difference between pins 6 and 7 of IC1 reflects the variance between the sine oscillator frequency and the VCO frequency. This discrepancy causes the moving-coil meter M1 to deflect. The extent of needle deflection signifies the frequency change, while the needle’s direction indicates the type of material detected by the coil. The meter employed here is a center-zero ±50 μA type.
Designing the Detection Coil
For the detection coil, wind 40 turns of enameled copper wire around a plastic former with a diameter of approximately 10 cm (4 inches). This specific inductance ensures the sine wave oscillator operates at a frequency roughly equivalent to the Voltage-Controlled Oscillator (VCO) frequency in the Phase-Locked Loop (PLL).
Calibrating the Oscillator and Adjusting Potentiometers
Use an oscilloscope to verify that pin 2 of IC1 produces a sine wave signal at around 75 kHz. Then, fine-tune P1 until the edges of the rectangular signal at pin 4 align with the positive peaks of the sine wave at pin 2. Following this, null meter B is balanced using potentiometer P2.
Monitoring and Maintenance of Null Adjustment
It’s crucial to note that the null adjustment may experience slight drift due to fluctuations in battery voltage. Therefore, periodic rebalancing is necessary during device use to maintain accurate functionality.
Ri = 6.8 lid/R2, R3 = 4.7 k.C2 R4 = 680 t2 R5, R6 = 5.6 Id/P1 = 10 ki.(2 multiturn presets) P2 = 470 S2 linear potentiometer
C1, C2 = 100 p.F., 16 V radial C3 = 68 nF C4 = 15 nF C5:C8 = 10 nF C6:C7 = I nF
L1 = details in text
T1 = BC547B T2, T3 = BC557B