The principle of operation
The principle of operation of this detector is based on a comparison of the two frequencies generators, one of which with a stable reference frequency, and the frequency of another (search) is changed under the influence of closely spaced metal items.
Schematic diagram
A schematic diagram is shown in Fig. 2.24, and. Reference oscillator assembled on the element DD1.1. Through a resistor R1 and an inductor L1 is negative feedback DC between output and input element. Thanks to this element enters linear region of the transfer characteristics. This creates conditions for the initiation of a cascade of frequency approximately 100 kHz. This frequency is determined by the parameters of the circuit L1C1C2C3.
Fig. 2.24. The metal detector on a chip series C, K, K: a - schematic diagram; b - circuit Board; - additional matching cascade
Gate IC has a high input impedance, so the factor of the circuit and the stability of the oscillator frequency is relatively high. The resistor R1 weakens the shunting effect of the output resistance of the element on circuit. The form of oscillations in the circuit is sinusoidal, and the output element rectangular. The oscillation frequency can be changed within narrow limits the variable capacitor C2.
Search the generator is assembled on the element DD1.2 in a similar way, but the coil the inductance L2 is - external encased in a shielding metal tube. Square wave reference and search generators arrive at the inputs element DD1.3 operating the mixer signals.
At the output of the element will be as a signal of fixed frequency oscillators, and difference and sum frequencies (including frequencies of harmonic components). One of the most powerful will be the signal of the difference frequency - they are allocated on the resistor R4. Other signals are suppressed by the filter R3C6. The amplitude of the output the signal of the element DD1.3 is large enough, amounts to several volts. So the need for additional amplifier 34 no.
To the output connector XS1 connect high impedance head phones, for example, TONE-2 connected in series with the fuses. The volume is adjustable a variable resistor R4. When using low impedance phones metal detector should complement cascade transistor VT1 (Fig. 2.24 in) by setting the resistor R3 resistance of 10 ohms, and the capacitor C6 with a capacity of 1000 pF.
Components and recommended replacement
The detector can be used chip series C, K, K, containing at least three logical elements OR-NOT AND or-NOT, for example, CLE, CLE, CLE, CLE. AC condenser from radiokonstruktor "Youth CP" or other small-size with a maximum capacity not less than 150 pF. The remaining capacitors - KLS, km, KT, and capacitors C1, NW-C5 should be with TKE no worse M, M1500. This will increase the thermal stability of the device.
Variable resistor R4 - CP3-3V resistance 68, 47, 33, 22 and even 10 ohms, but mechanically coupled to the power switch SA1, the remaining resistors - MLT with a capacity of 0.125 watts. Coil L1 is made on the three-section frame of the inverter circuit radio Sokol-403", placed in armored core diameter of 8.6 mm ferrite NN with podstroechnik diameter of 2.8 mm and a length of 12 mm from the same ferrite. It should contain 200 turns of wire sew-2,0,09.
The coils of
The coil L2 carry out so. In thin-walled aluminum tube with a diameter of about 7 mm and a length of about 950 mm threading conductors 18 MGTF-0,07. Then the tube to bend mandrel and the coils be connected in series with each other.
The inductance of the coil should be equal to about 350 µh. The ends of the tube to leave open, but to one of them to connect the conductor connected to the common wire.
Design
Connector XS1 - socket for connection of headphones. Power source - battery "Krona" or the battery. Details of the detector, except the coil L2, battery and connector should be placed on a printed circuit Board (Fig. 2.24, b) from foil fiberglass thickness of 1-1,5 mm from the printed conductors.
Unused input the findings of the fourth element of the chip should be connected to common wire. The circuit Board preferably placed in a metal case (preferably aluminum). It is necessary to penetrate the window handle of the resistor R4 and the capacitor C2. To the upper part of the housing to attach the coil L2, and to the bottom - stick, which is placed inside the power source, and the outside connector XS1.
Adjustment
With proper installation and proper details of the establishment is to install the desired frequency of the reference oscillator. The handle of the capacitor C2 should set approximately in the middle position. The podstroechnik coil L1 preferably to achieve in phones zero beat (disappearance of sound).
If your setup is correct with a slight turn of the knob capacitor in any side in the phone, you will get a low pitch sound. This configuration need to spend at least one metre away from solid metal items.
The use of a metal detector
Use a metal detector. The capacitor C2 may establish a lower the beat frequency. This will increase its sensitivity, as will be noticeable even small changes in the frequency tunable oscillator. To unfortunately, a very low frequency set will fail because it the volume on the phone drops sharply.
When approaching the coil L2 to a metal object, its inductance will be to change, and therefore will change and the frequency of the search oscillator. If detected object made of magnetic material (iron, ferrite, Nickel), the inductance will increase and the frequency will decrease. If the detected object from non-magnetic material (aluminum, copper, brass), the inductance will decrease, and the frequency will increase.
Following the above rule, when searching for magnetic materials the frequency of the reference the generator should be set higher than the frequency of the search oscillator. Then when the approach to such material, the frequency of the search oscillator will decrease and the beat frequency will increase.
When searching for non-magnetic materials the reference oscillator frequency should to set below the frequency search. If to set the frequency of the reference generator above the frequency search at 400-500 Hz, the increase in beat frequency will signal the coming of the metal detector to the subject of magnetic metal and reduce it to non-magnetic.
Author: Nechaev I.