The principle of operation of the detector is that when approaching the metal object to the coil inductance generator-host device-frequency generator changes. The closer the subject and then it is, the stronger its influence on the frequency of the generator.
Now consider the design of a simple detector collected on two transistors. Metal detector diagram is presented in Figure 2.1. The generator is made on the transistor VT1 tanks schema. Generation is formed due to positive feedback and the basic transistor circuits. Frequency generator depends on the capacity of the capacitors C1-C3 and inductance coil L1. When approaching a metal coil inductance changes its subject-increases if ferromagnetic metal such as iron, and decreases when the metal colour-copper, brass.
But how to monitor change frequency? This is done by the receiver, which is the second transistor. This is also a generator, built as the first capacitive schema. Its frequency depends on the capacity of the capacitor C4-C6 and inductance coil L2 and not much different from the first frequency generator. Desired frequency difference picked coil. In addition, a cascade on the transistor VT2 combines the function of the detector extracts low frequency oscillation originating at the base of the transistor high frequency oscillations. Load detector are headphones BF1; capacitor C1 load for high frequency oscillations.
RLC circuit with inductive receiver associated with circuit generator, so the chain collector transistor VT2 leak currents both frequency generators, as well as current differential frequency, the frequency of heartbeats. If, for example, the frequency of the main generator frequency kHz, and 460 generator 459 receiver kHz, the differential will be 1 kHz, i.e. 1000 Hz. This signal and audible in the phones. But it is necessary to bring the search coil L1 to the metal, as sound frequency in phones will change depending on the type of metal it or fall, or become higher.
Instead the diagram suitable P401, P402 and other high frequency transistors. Headphones-high-TONE or tone 1-2, but their primers need to be parallel to the total amounted to 800 … 1200 ohms. In this case, the volume will be slightly higher. Resistors-MLT-0.25, capacitors-KLS-1 or BM-2.
Coil L1 is a rectangular frame dimensions 175h230 mm, consisting of 32 coils wires PEV-2 0.35
L2 coil design is shown in Figure 2. In two paper segments are placed 6-cylindrical rod of a diameter of 7 mm or 600 400NN ferrite: one (1) long 20 … 22 mm, fixed permanently, the other (2) -35 … 40 mm (mobile-to adjust the coil). Skeletons wrapped in paper tape 3, over which is wound coil L2 (5)-55 turns of wire (you can sew-1 or PEV-2) with a diameter of 0, 2 mm. Conclusions of the coils are secured with rubber rings 4.
Transistors, capacitors and resistors mounted on the motherboard (Figure 3) of insulating material. Charge connects with coils, battery power, switch and socket wire isolated. Fee and other details are placed in plywood glued squared case dimensions 40x200x350 mm. Coil L1 is attached to the bottom of the sheath, and the inside of the coil at a distance of 5.7 mm from its coils placed coil L2. Next to this coil fee. Connector and attach the switch to the side wall on the outside of the case. Top case (preferably on glue) the wooden handle approximately metre length.
Establishment of the detector starts with measuring the modes of operation of transistors. Include food, measure the voltage at the emitter of the first transistor (relation to common wire-plus power)-it should be 2.1. More precisely, this voltage can be matched resistor R2. Then measure the voltage at the emitter of the second transistor-it should be 1 in (more precisely identifying the resistor R4). Then a slow-moving coil core L2 are making appearance in head phones, loud low-frequency sound.