Quartz metal detector on two transistors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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Schematic diagram
Schematic diagram of the metal detector is shown in fig. 2.42. The reference oscillator of the metal detector is assembled according to the capacitive three-point circuit on the transistor T1. Loop coil L1 is the instrument's sensor. Capacitors C3-C6 are designed to tune the generator to a frequency of 50 kHz. Through the coupling capacitor C7, the sinusoidal voltage from the generator is supplied to the quartz filter.
The capacitance C7 is chosen small, thus the influence of subsequent stages on the operation of the generator is practically excluded. The threshold device is assembled on a field-effect transistor T2. The threshold voltage is set by the divider R5-R7. Capacitor C8 smoothes the pulsations on the IP1 indicator.
Rice. 2.42. Schematic diagram of a quartz metal detector on two transistors
The R4C1 filter provides AC decoupling between the threshold and master oscillators.
metal detector sensor
The metal detector sensor is a rigid annular frame made of plexiglass, on which 65 turns of the PEL 0,2 passage are wound. The winding is enclosed in an aluminum foil screen and filled with epoxy resin. The sensor is connected to the measuring unit with a RK-75 coaxial cable.
Particular attention should be paid to the quality of the metal detector frame. It should be light in weight, tough and resilient. Otherwise, even with light shocks, inevitable when working with the device in the field, the frequency of the generator "leaves" - the metal detector is upset. The basis of the frame can serve as an annular frame made of plexiglass or polystyrene d = 300 mm. The winding must be shielded with aluminum foil 0,05 mm thick.
It is impossible to connect the ends of the screen to each other, otherwise a short-circuited coil is formed. The winding leads should be connected to the PK-75 cable 0,3-1 m long (connect the coil screen with the cable braid as well). This place then needs to be filled with epoxy. The connection between the sensor and the electronics unit must be permanent.
Opportunities and features of using a metal detector
The metal detector has a very high sensitivity. The indicator needle deviates by one division when the frame of the device approaches the iron disk with a diameter of 13 cm at a distance of 80 cm. And objects with a diameter of 3-5 cm give the same deviation of the arrow at a distance of 10-15 cm.
The device reacts almost equally to any metal. So, for example, steel, aluminum and brass disks give the same deviations of the arrow at equal distances. They do not depend on whether the object is solid or hollow.
When working with a metal detector, background noise must be taken into account.
Sandy and peaty soils, black soil, wood, water do not give a background signal. Therefore, the device works well in fresh water, in wooden buildings and on non-stony soils. A strong background gives a brick (baked clay has magnetic properties) and some minerals.
Changes in temperature also affect the readings of the instrument. Therefore, it is better to place the frame in a case made of a heat insulator, for example, foam. To work underwater, the metal detector sensor must first be held in water for 10-15 minutes and then set up the device.
Authors: Zakharov A., Kshikin V.
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