Combined metal detector is a high class device, consisting of transmitter and receiver.
Principle of operation. Generator RF transmitter works on the scheme with a parallel circuit (L1 C1 to D1, thyristor frequency 120 kHz) Frame aerial (transmitting and receiving) are placed relative to each other at right angles so that the receiver can receive signals of the generator. Metal object distorts the magnetic field generator, an antenna and the receiver picks up the vibrations. Mass of the metal acts as an additional probe, sending some of the energy of the generator to the antenna of the receiver.
The transmitter also includes a sound generator T1 (KT117) on the frequency of 200 Hz. D1 thyristor, power- -1 0.25 Watts. When a metal object in the phones you can hear sound with a frequency of 200 Hz. The power of sound depends on the mass of the metal and the distance to it.
Receiver (diodes D1 and D2)-Crystal set. His resonant LI C1 is configured to 120 kHz frequency. The amplifier can be any BASS amp radio. It is sufficient for it to be at least 5-6 transistors. At the output of the connected loudspeaker or incremental dynamic head phones.
Fig. Receiver scheme
The transistor T1-amp DC (DCA) to switch the appliance (0.1 MA), which measures the level of received signal. Thus, there are Visual and audible indicators. Operating frequency 120 kHz Finder that does not interfere with the radio, because the intermediate frequency (if) of most receivers 465 kHz.
Details. Coil L1 (transmitter antenna) consists of 74 turns of wire 0.4 PEV (fig. b).
Fig. Coil L1
Withdrawal is made from the second revolution from below (diagram); coil reel up first with two turns, then in the same direction with 72 turns and connect them in series. Capacitors in the device applied to the paper operating voltage 200 v, except Cl, C2 and N to be film. Better to take the phones with resistance 2 coils … com 4.
Housing transmitter and antenna coil bracket is made from plastic. Housing dimensions 160 x 95 x 50 mm. Power source: two 4.5-volt battery included, point, or two 9-volt, connected in parallel.
Receiver antenna L1 contains 68 coils wires or PAL PEV 0.4. Transistor BASS amplifier is convenient to assemble together with the receiver. It can be worn on a belt over his shoulder.
Switch the measuring instrument on the body have a receiver. You can take a millimeter with less sensitivity (0 … 5 … 10 MA). The transmitter with the receiver are at the ends of a wooden pole with a diameter of and a length of 600 20 … 900 mm. Epoxy paste receiver (ED-Ed 5-6), and the transmitter is placed on a tripod head (1 in Fig. Coil L1) for easy setup, zero signal. Both coils wrapped around with a layer of insulating tape to protect it from moisture.
Building. Input circuit receiver transmitter frequency adjust selection size of capacitance C 1 If this operation has a transmitter at a distance of 3 metres from the receiver. Gain control set to maximum volume, the receiver frame and orient it so that the volume and milliampermetra readings were highest. Then switch off the receiver and is connected parallel to the condenser C1 ceramic capacitor mica or 500 PF.
If you repeat the previous tests will show that the sound volume or testimony will milliampermetra the secondary capacitor need to remove. If the volume increases, the capacitor or connect another leave. Setting is done once. Then the transmitter is fixed on the end of a pole tripod head turning, and find that the receiver will not get a signal. Transmitter lock in this position. Before you start searching for items once again check the fitting on zero.
The Application. Metal detector in hand. The receiver antenna must be located near the Earth’s surface and parallel to it.
The device effectively detects metal objects large and medium sizes. Start with things like canning jars, dug to a depth of 1.5 m in normal soil.
If you want to repeat the pattern be attentive, despite the simplicity of this scheme is not for beginners. The book “Electronic toys” has always been beautiful that inspired, but contains a bunch of bugs and simplifications. In my opinion, at least, in the scheme of the receiver, there is one fundamental mistake there between the capacitor and the resistor “3000”, “270” should be the gap, and it turns out that “BASS amp” takes the signal from the power-this is wrong.